ADVANCES IN CATALYSIS VOLUME 16, Volume 16

ADVANCES IN CATALYSIS AND RELATED SUBJECTS VOLUME 16 Contributors to This Volume H. P. BOEHM J. L. GARNETT JOOST MAN...

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ADVANCES IN CATALYSIS AND RELATED SUBJECTS

VOLUME 16

Contributors to This Volume H. P. BOEHM J. L. GARNETT JOOST MANASSEN MILTONORCHIN HERMAN PINES SAMUEL SIEGEL W. A. SOLLICH-BAUMGARTNER

ADVANCES IN CATALYSIS AND RELATED SUBJECTS VOLUME 16 EDITED BY

D. D. ELEY Nouingham, England

HERMANPINES Evanaton, Illinoie

PAULB. WEISZ Paulabom, New Jeruey

ADVISORY BOARD

A. A. BALANDIN Moscow, U.S.S.R.

P. H. EMMETT Baltimore, Maryland

G. NATTA Milurn, Ilaly

J. H.

DE

BOER

Deut, The Netherlands

J. HORITJTI Sapporo, Japan

E. K. RIDEAL London, England

P. J. DEBYE

Ithoea, New York

W. JOST Q6Uingen. Germany

P. W. SELWOOD Santa Barbara, California

H. S. TAYLOR Princeton, New Jeruey

-

1966

@ ACADEMIC PRESS A Subsidiary oJ Harcourt Brace lovanovich, Publishers New York London Toronto Sydney San Fraiicisco

BY ACADEMIC PRESSINC. ALL RIGHTS RESERVED. NO PART OF THIS BOOK MAY B E REPRODUCED I N ANY FORM, BY PHOTOSTAT, MICROFILM, OR ANY OTHER MEANS, WITHOUT WRITTEN PERMISSION FROM THE PUBLISHERS.

COPYRIGHT 0 1966,

ACADEMIC PRESS INC. 111 Fifth A v e n u e , New York, New York 10003

United Kingdom Edition published by

ACADEMIC PRESS, INC. (LONDON) LTD.

24/28 Oval Road, London NWI 1DD

LIBRARY OF

CONGRESS CATALOG CARD

NUMBER: 49-7755

PRINTED IN THE UNITED STATES OF AMERICA 808182

9 8 7 6 5 4 3

Contributors H. P. BOEHM,Institute of Inorganic Chemistry, University of HeideZberg, Heidelberg, Germany J. L. GARNETT,Department of Physical Chemistry, The University of New South W a b s , Sydney, Australia JOOSTMANASSEN,The Weizmann Institute of Science, Rehovoth, IsraeZ MILTON ORCHIN,Department of Chemistry, University of Cincinnati, Cincinnati, Ohio HERMAN PINES,The Ipatieff High Pressure and Catalytic Laboratory, Northwestern University, Evanston, Illinois SAMUELSIEGEL,Department of Chemistry, University of Arkansaa, Fayetteville, A r k a n s a s

W. A. SOLLICH-BAUMGARTNER, Department of Physical Chemistry, The University of New South Wales, Sydney, Australia

This Page Intentionally Left Blank

Machines and Systems A Preface The aircraft, the printing plant, and the meteorological station, are each systems containing many machines. The mode of co-action is quite complex and very specific-in each case achieving a particular over-all result. To study or describe any one of these systems, we may proceed to analyze this complexity of interaction by taking for granted the “si.mple” existence of the elements such as the aircraft engine, the printing machine, the chronometer, etc. Of course, each of these is itself a complicated mechanism, containing many “simpler” elements, like wheels, rods, pulleys, etc. To a large extent the elements are not dissimilar, but the mode of interplay is again complex, and differs in each machine in achieving its specific objective. The term “mechanism” and “mechanistic study” is quite popular in our (and all other) scientific endeavors. Presumably, we thereby mean an interest in identifying the parts that make a whole. Since we use the term so freely, it occurred to us to meditate upon the great broadness of its meaning, and therefore the desirability for recognition of the specific “machine” or “system” level to which we intend to apply it. If A transforms to A’ on catalyst X , the result of an investigation that “ A produces a complex A X A X is unstable and rearranges in a certain steric manner to A ’ X and A‘X dissociates into A’ and X” might characterize a study of the mechanism of the kinetic course of the reaction. The finding that “ A X is an ionic compound A + X -and its electronic configuration is relatively unstable compared to that of the rearranged A’ +X-” would constitute an element of information concerning the mechanism of electronic interaction (involved in a particular reaction step within the kinetic course of events of the reaction system!).Perhaps this simple example distinguishes at least two major levels of machines and systems, and therefore categories of study: the mechanism of kinetic course of the over-all reaction; and the mechanism of electronic interaction in a specific reaction step. In this volume the editors are presenting a set of what they believe are recent outstanding developments which concentrate heavily on mechanisms involved in several different classes of catalytic reactions: Isomerization of olefins (M. Orchin), dehydration of alcohols (H. Pines and J. Manassen), hydrogen exchange (J. L. Garnett and W. A. Sollich-Baumgartner), and hydrogenation of unsaturated hyvii

viii

PREFACE

drocarbons (S. Siegel). They are concerned largely with the courses of the kinetic processes, and in some cases delve into the electronic features of specific reaction complexes of the catalytic systems. In a volume otherwise heavily concerned with mechanistic questions, the editors have included a contribution (H. P. Boehm) from another sphere of investigative interest which struck us as having the quality of the imported spice to stimulate new interests within the accustomed sphere of catalytic gastronomy. It concerns the chemical nature of the surface groups on a series of materials (e.g. silica, alumina) which happen to be so frequently and universally used by the catalytic researcher. January, 1966

P. B. WEISZ

Contents CONTRIBUTORS.

...................... ... ... ..... v AND SYSTEMS:A PREFACE. . . . . . . . . . . . . . . . . . . . . . vii MACHINES The Homogeneous Catalytic lsomerization of Olefins by Transition Metal Complexes MILTONORCHIN

..... ........ . . . . . . . .. .... .... ............ . .. .... . The Carbon-Metal Pi Bond . . . . . . . . . . . . . . . . . . . . . . Stability of the Carbon-Metal Bond .......... ....... The Carbon-Metal Delocalized Pi Bond . . . . . . . . . , . . . . . . Double-Bond Isomerization in Olefins . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . .

I. Introduction

11. The Carbon-Metal Sigma Bond

111. IV. V. VI. VII.

3 4

8 12 21 43 45

The Mechanism of Dehydration of Alcohols over Alumina Catalysts HERMANPINESAND JOOST MANASSEN

.. . ....... ... ... ......... ..... .. . .... ....... ... .... ... ......... .... ........... . .. . . ..... ......... ..... . . ......... . . ........ .... ..... ...... . ........... . ... . ...... ..... ........

I. Introduction . . .. . 11. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. Early Mechanisms and Observations . .. . IV. Nature of Alumina Catalysts . . . . V. Isomerization Following Dehydration VI. Steric Course of Dehydration . . . . . . . VII. Dehydration of Aliphatic Alcohols . . VIII. Dehydration of Secondary and Tertiary Alcohols IX. Conclusions . . . . References ... . . . .

50 52 56 59 71 83 89 90

Hydrogen Exchange on Group Vlll Transition Metal Catalysts

t Complex Adsorption in

J. L. GARNETTAND w.A. SOLLICH-BAUMGARTNER

.

.

.

. . . . . . . . . 95 . . ...... .... 6 . . . . . . . . . . . 102

I. Introduction . . . . . . . . . . . . . . . . . . ..,. .. . ... . . . 1Complex Adsorption 111. Associative and Dissociative ?r Complex Substitution ... ... ....... .. . Mechanisms ix 11.

.

CONTENTS

X

IV . Experimental Evidence for R Complex Adsorption and Reaction Mechanisms ....................... V . Conclusion .............................. References ..............................

106 119 120

Stereochemistry and the Mechanism of Hydrogenation of Unsaturated Hydrocarbons SAMUELSIEGEL I. I1. 111. IV.

V. VI . VII .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Development of Some Stereochemical Concepts . . . . . . . . Variations in Stereochemistry as a Criterion of Mechanism . . . . . Conformational Analysis and the Geometry of the Pertinent Transition States in the Hydrogenation of Cycloalkenes . . . . . The Reaction of Aromatic Hydrocarbons with Hydrogen ...... Hydrogenation of Multiply Unsaturated Hydrocarbons ....... Some General Mechanistic Considerations ............. References ..............................

.."12&

125 132 144 151 160 167 174

Chemical Identification of Surface Groups

.

H . P BOEHM

I. I1. I11. IV. V. VI . VII .

-. 179 -.c Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Surface Groups on Carbon . . . . . . . . . . . . . . . . . . . . . . 225 Surface Groups on Silica . . . . . . . . . . . . . . . . . . . . . . . Surface Groups on Titanium Dioxide . . . . . . . . . . . . . . . . 249 254 Surface Groups on Alumina ..................... Surface Groups on Silica-Alumina . . . . . . . . . . . . . . . . . . 259 264 Conclusion .............................. 264 References ..............................

................................ SUBJECTINDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AUTHORINDEX

275 287

ADVANCES IN CATALYSIS A N D RELATED SUBJECTS

VOLUME 16


The Homogeneous Catalytic lsomerization of Olefins by Transition Metal Complexes MILTON ORCHIN Ilepcrrtineiit of Chcmi.qtry. (Jniiwsity of Cincinnati, Cinriiinati, Ohio

Page 2 3

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. The Carbon-Metal Sigma Bond . . . . . . .............................. 111. The Carbon-Metal Pi Bond. . . . . . . . . . .............................. IV. Stability of the Carbon-Metal Bond .................................... V. The Carbon-Metal Deloealized Pi Bond ................................. A. Ferrocene ........................................................ B. n-AllylComplexes ................................................ VI. Double-Bond Isomerization in Olefins .................................. A. Cobalt Systems; the 0 x 0 Reaction . . . . . . . . ..................... B. Olefin Isomerizations with Iron Compounds ..................... C. u + m Ligand Conversions and Rearrangements. ...................... D. Isomerization with Palladium and Other Group VIII Metals . . . . . . . . . . . . VII. Summary.. . . . . . . . . . . . . . . . . . . . ............................ References .........................................................

4

8 12 14

18 21 21 29 34

38 43 45

Many of the recent advances in the understanding of catalysis have emerged from a study of the structure of molecules adsorbed on surfaces. New experimental tools have given new insights into the bonding between the metal surface and the substrate. On the other ha,nd, recent advances in structural inorganic chemistry, especially with respect to coordination compounds, have advanced the understanding of the nature of the bonding between organic ligands and metals. Practically every issue of the most important general journals contains reports of new and exotic complexes. Indeed, there are now special journals devoted exclusively to a description of the chemistry of organometal complexes; most of these new complexes are now isolated in pure crystalline form and are well-characterized. Many of these complexes have organic ligands bonded to the metal in a manner analogous t o their bonding on the solid surface of the same metal. There is no longer any doubt that the chemistry of absorbed molecules and the chemistry of the coordination compounds are intimately related, and insight into the chemistry of either have significance for both. Part of the art of catalysis consists in 1

2

MILTON ORCHIN

arriving a t conditions favorable to the formation of selective intermediates-stable enough to provide the low-energy path to products, but not so stable that their collapse to products is energetically prohibitive. Accordingly, it behooves the more classical physical or physicalorganic chemist t o become better acquainted with the chemistry of transition metal complexes and, in particular, with the chemistry of the reactions in which the transition metal complexes have been established as catalysts for certain conversions. In this report, I have attempted to focus on one such conversion of classical interest t o all chemists concerned with catalysis, namely, that of olefin isomerization. History has shown that it is possible for the creative chemist to develop a technology based on incompletely understood scientific principles; this results from clever extensions of known reactions or from guesses or theories (however wrong) as to how these reactions proceed. I n reviewing some of the principles of bonding in transition metal complexes and in integrating many of the examples in which some of these complexes have functioned as catalysts for the isomerization of olefins, it is hoped that some chemists will be stimulated to speculate, to calculate, and to experiment in this exciting field.

1. Introduction The catalytic homogeneous isomerization of olefins by protonic acids as well as by Lewis acids is well-known and there is little doubt that such isomerizations proceed through carbonium ion transition states or intermediates. Thus, strong acids isomerize 1-hexene to all possible hexene isomers, including cis and trans isomers where these are possible. The isomerization to 2-hexene may be written as a proton addition-elimination : CH,CH&H&H&H= CH,

+[HI+ -[HI+

+

[CH,CH&H&H&HCHJ

-[HI+

d

CH&H,CH&H =CHCH,

+[HI+

The anion presumably plays only a minor role, if any, especially in aqueous systems. Now the formation of 3-hexene may be explained in either of two ways. The intermediate carbonium ion, written in brackets, can undergo hydride migration to form a new carbonium ion, which can then collapse by proton loss t o form the 3-hexene. Such a process does not require the intermediate formation of 2-hexene. The alternate explanation involves the discrete formation of 2-hexene followed by addition and elimination of a proton t o give the desired 3-hexene. There is no question but that the hydride migration occurs, and with great

ISOMERIZATION OF OLEFINS

3

speed. However, the proton addition and elimination occurs simultaneously so that both mechanisms make some contribution to the isomerization process, with the hydride rearrangement probably predominating. The acid-catalyzed isomerizations of olefins are characterized by alkyl group rearrangements (when branched olefins are substrates) and by polymerization reactions. As a matter of fact, such predictable side reactions add to the confidence in the mechanistic picture of carbonium ion intermediates. Not only can acids catalyze olefin isomerization, but strong bases can also effect isomerization. These base-catalyzed isomerizations proceed through proton abstraction of an allylic hydrogen atom followed by protonation of the allylic anion to regenerate either the original or the isomeric olefin : blow

CH,CH,CH=CH,

0

CH,CHCH=CH, 0

CH,CH=CHCH,

0

+ (CH,),COG +CH3CHCH=CH2 + (CH,),COH

+ (CH,),COH

Q

CH,CH= CHCH,

t ,

CH,CH=CHCH,

+ (CH,),COo

In the present chapter, no explicit discussion or review of the acidand/or base-catalyzed isomerization of olefins will be included. The discussion will be confined to isomerizations achieved with soluble transition metal complexes. However, it will be seen that addition and elimination reactions and allylic intermediates figure prominently in discussions of the mechanisms.

.

II. The Carbon-Metal Sigma Bond Although many compounds possessing a carbon-metal sigma bond, such as the Grignard reagent and tetraethyllead, have been known for many years, examples of nlkyl-transition metal bonded compounds have been extremely rare until quite recently. Practically all of those prepared even in very recent years have unsaturated ligands simultaneously attached to the metal, and these ligands donate a sufficient number of electrons so that the metal usually acquires the electronic configuration of a rare gas. A typical example is CH,Co(CO),. The paucity of examples of simple alkyl-transition metal compounds can be considered prima facie evidence that the alkyl-transition metal bond is both thermodynamically and kinetically unstable. I n molecular orbital (MO) terminology, the MO formed by overlap between the carbon hybrid orbital and the metal hybrid orbital is called a sigma orbital. An atomic s orbital is perfectly spherical and hence has the highest possible symmetry. It has the same sign everywhere, by

4

MILTON ORCHIN

convention plus; i.e., the orbital has no nodes. The sign of an electron wave, like the sign of a standing wave which it resembles, changes sign on passing through a node. A standing wave can be represented by a typical sine wave:

The signs of the wave change periodically on passing through the point where the amplitude is zero (the node). The s orbital has no nodes, t h e p orbital one, the d orbital two, etc. Although the u (sigma) molecular orbital does not have spherical symmetry, it does have cylindrical symmetry around the internuclear axis, and because of this s-like atomic orbital property the molecular orbital is called a u orbital. The bond that is formed by the u orbital is called a u bond. An orbital has cylindrical symmetry if, upon rotation of the orbital around the internuclear axis by any angle, there is no change in sign. Antibonding as well as bonding orbitals may be symmetrical with respect to the internuclear axis, and if this condition is fulfilled the antibonding orbital is called a U* (sigma starred) orbital. The u and u* orbitals of the H,molecule and a carbon-metal u orbital are shown schematically in Fig. 1.

0-03

- -*-

--#---+

(a)

(b)

(C)

FIG.1. Examples of a and u* MO's: (a)the u-bondingMO of H,; ( b )the a*-antibonding M O of H,; (c) the a-bonding C8pa-lLIdspa MO.

111. The Carbon-Metal Pi Bond Although the first olefin-transition metal complex, potassium ethylene trichloroplatinite, K[C,H,PtCl,], was discovered in 1827, it was not until 1953 that its correct structure was elaborated. Extension by Chatt and

ISOMERIZATION O F OLEFINS

5

Duncanson ( I ) of the molecular orbital bonding concepts of Dewar ( Z ) , which he developed to explain the structure of Agf-olefin complexes, led to the suggestion that ethylene is symmetrically coordinated t o the metal. Platinum, atomic number 78, has the electronic configuration of the xenon core ( ls2 2s22ps 3sa3p83dl04sa4pe4dl05s25p6, Z = 54)) then 6s1, 4f14 (lanthanide series), 5d9. Pt(I1) is derived from Pt(0) by loss of a 6s’ and a 5d1, and hence has a ds configuration with empty 5d 6s6p3 orbitals. However, in Pt(I1)four-coordinated square planar complexes, dspz orbitals are used for bond formation, leaving a n empty 6p orbital. I n the Pt(I1) anion [C,H,PtCl,]-, then, the three chlorides and the ethylene each occupy a single site in the square plane and each of these four ligands can be thought of as donating a pair of electrons to Pt. The electrons contributed by the ethylene are its pn electrons making up the familiar n bond of ethylene. Such electron donation is frequently denoted by the arrow shown in Fig. Za, and the expression “T bonding” has been \ /

C

(C)

FIG.2. The ethylene-platinum bonding: ( a ) ,thc common representation emphasizing electron donation; ( b ) , the. o-bonding molecular orhitat: (c). the n-bonding molecular orbital. n

used to describe this coordination. In MO terms, the bond is formed by the overlap of the T orbital of ethylene with one of the four equivalent dspa orbitals of Pt(I1);this bonding is depicted in Fig. 2b. It will be noted that the orbitals are symmetrical with respect to rotation around the internuclear ethylene-platinum axis; hence the n-bonded ethylene forms a sigma bond with Pt. At this stage of the description, i t would be reasonable to conclude that, since the two p n electrons of ethylene are now being used not only to bond the two carbons to each other but $0

6

MILTON ORCHIN

bond the ethylene to the platinum as well, the bonding between the two carbon atoms must necessarily be weakened. This in fact is true; the bonding between the carbon atoms is less in the complex than in free ethylene. Although the above explanation is correct, the major decrease in this bonding (easily verified by the shift of the infrared C =C stretching frequency to lower energy) arises from back-donation of elcctrons from the filled d orbitals of Pt into the antibonding orbitals of ethylene. Because of the importaiice of this interaction, some space and at,teiitioii will be devoted to a modest review of t.his concept. When two atomic orbitals (AO’s) are combined, two MO’s always result. The necessity for generating two MO’s from two AO’s is readily rationalized. Since every orbital can accommodate a maximum of two electrons, the comliiuation of two AO’s must generate a sufficient number of MO’s t,o accommodate a maximum of four electrons, namely two. The particular combination of AO’s that is choscw is t h o combination that rcwilts from ntItlit,ioti and snl)t,ract,ioilof tJlwAO’s ant1 this is t,hc h s i s of‘ t h inct,liotl cilll(?d thc linear comhinatioii of atomic orl)it,ds ( IAlAO) whicli is most frcqurntly usod for dcvelopiilg M O ’ s . If we w w e t o coinbincb ld two the AO’s of’,for cxamplc, two hydrogen atoms, wc ~ ~ o i i geiiorat,e Mu’s, a hondiiig M O (the addition) shown i n Fig. I n ant1 an antibontling MO (the subtraction) shown i n Fig. I b . T n the H, molecule thore are onl!, two electrons and these occupy the lowest-energy MO. I f somehow we could arrange to have two more electrons brought into the syst,em, these electrons would both occupy, with opposed spin, the antik)ondiiigorbital. Now, since the bonding and antibonding orbitals are equally occupied and since in a first approximation the bonding energy ofthc bonding MO is cancelled by the repulsive energy of the antibonding MO, there is no stability to such an electronic configuration. T h i s is the reason He,, which would have such a configuration, does not exist, and, as a matter of fact, this explains why all rare gases are monatomic rather than diatomic. The generation of MO’s by the addition and subtraction of AO’s is frequently represented by molecular orbital level diagrams. For the combination of H atoms, the diagram is shown in Fig. 3a. Now in ethylene we are concerned only with the pn electrons and so attention is focused only on the 2p orbital on each carbon atom because their addition generates the TT bond. The contours of the MO’s formed from the AO’s by addition and subtraction of the CzPxorbitals are shown in Fig. 3b and the level diagram is shown in Fig. 3c. If we examine either the T or 7r* orbitals of ethylene, we see that rotation around the internuclear axis by 180”reverses the signs of the orbitals and hence both orbitals are antisymmetric with respect to this operation. Such orbitals are called .rr-type orbitals and the bonds are called T bonds because they have the

ISOMERIZATION O F O L E F I N S

(C)

FIG. 3. Molecular orbital representations: (a),the level diagram for H,; (b), the MO's formed by the addition and subtraction of 2pn AO's; (c), the level diagram for , ,orbitals of ethylene.

symmetry of t h e p atomic orbital. The antibonding orbital having a node between the two carbon atoms is called n*. In ethylene we see from Fig. 3c that r* is unoccupied and, if ethylene is forced to accept electrons, such electrons must go into this orbital. Now if we return to the complexed ethylene in [C,H,PtCI,]-, we recall

8

MILTON ORCHIN

that the p r electrons of ethylene are donated to platinum to form the u bond shown in Fig. 2b. Such donation would place a negative charge on the electropositive Pt atom. A mechanism exists for relief of this charge. The Pt(I1) has the ds configuration with four filled d orbitals. One of these d AO’s of the Pt has symmetry (and energy) which matches that of the n* orbital of ethylene, and hence these orbitals can overlap. The bonding MO which is formed by such interaction is depicted in Fig. 2c. Examination of this orbital with respect to rotation around the internuclear axis shows i t to be antisymmetric. Accordingly, this bond, arising from the back-bonding of Pt electrons into T* of ethylene, is a r-type bond. Occupation of the r* orbital of ethylene cancels in part the effect of r bonding in ethylene, and hence we expect that the C=C stretching frequency will be shifted to lower frequency in the complex: such shifts always occur.

IV. Stability of the Carbon-Metal Bond It is desirable to examine in greater detail the reasons for the thermodynamic instability (small dissociation energy) of the alkyl carbontransition metal u bond, which appears to be so much less than the carbon-metal u bond of the nontransition metals, The reasons for the instability are: (a)the very small covalent^' energy of the metalcarbon bond; and ( b )the relatively small difference in electronegativities between the transition metal and the carbon atom, which accounts for the small “ionic resonance” eiiergy contribiition tlo the total energy of the bond. One of the early efforts to evaluate quantitatively the bond dissociation energy of particular bonds in a compound was the work initiated by Mulliken (3) in his so-called Magic Formula. Although this formula contains five terms, the two most important for the evaluation of a bond dissociation energy, D , (uncorrected for zero-point vibrational energy), between two atoms i and j,are the covalent bond energy, X , , , and the ionic resonance-energy, IRE. The evaluation of X,,takes the form: xi,

= -4&,2,,/(1

+ 8,)

where A is a proportionality constant, 8,is the overlap integral (01) bctween atoms i a n d j , and 1 is the average ionization potential (IP)of the two atoms. (In a first approximation, the IP of the atoms in the ground state can be used.) The overlap integral is a measure of the bonding between atoms and theoretically varies from zero to unity. For the Csp3-C,p3bond of ethane, for example, the value is about 0.65 and for the C-C T bond in ethylene the value is about 0.27. The first


9

ionization potential is the energy, usually expressed as electron volts, required t o remove an electron from the outer orbital of an atom, I n general, t h e overlap integrals for the C-M bond as well as the ionization potentials of the transition metals are somewhat smaller than those of the elements forming stable organometal bonds. The importance of the other major contributor to the bond energy, the IRE, can be illustrated in Pauling's (4)original terms. He showed that the reaction A-A

+ R--R

-+ 2A--B

is almost always exothermic and the explanation for this is that contributions from ionic structures such as A+B- and A-Bf stabilize the A-B bond. If atom A has a small ionization energy and B has a large electron affinity. or vice versa, one of the ionic structures will make a substantial contribution to the stability of the A-B bond, and thus it can acquire extra stability as compared with the sum of the A-A and B-B bond energies. This extra stability will increase as the two atoms A and B become more and more unlike with respect to their electronegativity, which is defined as the relative ability of an atom in a molecule to attract electrons to itself. Specifically, the I R E can be evaluated from the electronegativities of the two atoms: IRE = (XA

- XB)'

The energies are usually expressed as electron volts. The IRE for the CSpS-Cspllbond in ethane is zero and for CH,Na it is 2.56 ev. The stability of alkyl carbon-metal bonds for a variety of metals has been evaluated by Jaff6 and Doak ( 5 ) . They point out that not only is the Xii (the measure of covalent energy) for the C-M bonds of transition metals appreciably smaller (perhaps one-half) than the corresponding values for other elements, but the ionic resonance energy of the alkyl-transition metal bonds is also appreciably smaller (perhaps one-third) than that of alkyl-alkali or alkyl-alkaline earth metal bonds. It is well known that, although CH,Co(CO), is unstable, the acylcobalt

0

II

tetracarbonyl, CH,CCo(CO),, is readily prepared and is comparatively stable. I n the former compound the carbon attached to the metal is sp3 while in the acyl compound the carbon is sp2 and the carbonyl bond is highly polarized. Jaffd ( 6 )has emphasized that the state of hybridization of the carbon atom affects the following factors: the overlap integral, S; the ionizational potential of the carbon atom; and the ionic resonance energy. Increasing the s character from sp3 to s p 2increases S and hence increases X , . The valence state ionization potential t o which Xij is also

10

MILTON ORCHIN

proportional increases with increasing s character of carbon. Finally, the clectronegativity of carbon also increases with increasing s character, and, because cobalt is less electronegative than carbon (1.6 and -2.5, respectively)) the electronegativity difference and hence the ionic resonance energy increase in the acyl compound. The polarization,

n

>C = 0, further increases the electronegativity of carbon. I n addition, the n system of the carbonyl group, because it interacts with pn or dn orbitals of the transition metal, makes a substantial bonding contribution. Such interactions will be elaborated below. All factors thus combine to stabilize CH,COCo(CO), relative to CH,Co(CO),. The same kind of reasoniiig helps to explain why aryl and vinyl groups bonded to transition metals are more stable than alkyl-transition metal compounds, since the carbon bonded to the metal in such compounds has high s character and is part of a multiple bond system. Finally, the recent series of o-bonded perfluoralkyl-transition metal complexes should be mentioned. These compounds presumably owe their stability to the large ionic resonance energy contribution brought about by the substitution of the very highly electronegative fluorine atoms for hydrogen atoms on the carbon attached to the metal (7). The above discussion was concerned solely with some factors affecting the thermodynamic stability of carbon-metal bonds. I t is appropriate now to also consider factors affecting kinetic stability (reactivity) of complexes, although frequently i t is difficult to separate thermodynamic and kinetic contributions to stability, and no attempt to do so will be made here. The stable complexes invariably have uncharged ligands bonded t o metal in addition to the alkyl or aryl group. The common ligands responsible for stability are the cyclopentadienyl group, carbon monoxide, olefins, tertiary phosphines, arsiiies, and stibines. Not only does each of these ligands contribute a pair of electrons to the metal, but they all also have empty orbitals of appropriate symmetry and of appropriate energy t o bond with metal d orbitals. The transition metals are characterized by partly filled (n - l)d orbitals which are close in energy to the valence s and p orbitals. In the absence of unsaturated, electron-donor ligands coordinated to the M-C complex, it would be possible, with only a small expenditure of energy, to promoted electrons into antibonding u orbitals (u*)of the M-C bond, or to promote an electron from the bonding M-C orbital into an empty d orbital on the metal. Either type of electronic change would weaken the metal-carbon bond and cleavage of it would lead to a reactive carbanion or radical-i.e., the organometallic compound would readily go to “pot.” Now in order to minimize such electronic promotion and thus stabilize


11

the compound, it is necessary to maximize the energy difference between the occupied orbital of highest energy and the unoccupied orbital of lowest energy (a quantity which can be measured by the frequency of the longest wavelength band in the ultraviolet spectrum). I n order t o obtain a better picture of the situation, it is desirable to examine the relevant orbitals and their relative energies in somewhat greater detail. I n the crystal field theory, which neglects covalent bonding completely, it is assumed that the metal ion is surrounded (in the square planar case) by four negative ions or neutral dipolar molecules so arranged that the negative ends of their dipoles are pointed toward the central ion and the interaction between the metal ion and the ligands is electrostatic only. The energies of the five d orbitals of the metal, in the absence of an electrical field, are all equal (degenerate). But in the presence of the negative field produced by the four ligands these energies are split (8).If we assume that the four ligands in the square planar complex are on the 2 and y axes in the xy plane, the two d orbitals in this plane will be repelled more than the three other d orbitals. Furthermore, of the two d orbitals in the xy plane, the d z l - l l orbital which points directly a t the ligands will be repelled more than the dzy orbital which points along the bisectors of the angles between the bond directions. Thus the influence of the ligands is to destabilize both the dz2-yzand the d,, orbitals, the former t o a greater extent. The energy level scheme of the five d orbitals is shown in Fig. 4.

FIG.4. Energy levels of the d orbitals in a square planar complex.

Now in a ds ion such as Ni2+,Pd2+,or Pta+,there are eight electrons which will occupy the four lowest-energy orbitals. The pertinent factor affecting stability will then be the energy difference, AE, between the and the occupied dry. (AE is frequently expressed in unoccupied dz2-l12 Dq units, where 10 Dq is the energy splitting between the two sets of

12

MILTON ORCHIN

degenerate d orbitals in an octahedral field.) The greater the field effect of the ligand(s), the larger will be this split and the more stable the organometal complex. Phosphine ligands, especially triphenylphosphine, are excellent electron donors and the lone pair on the phosphorus is responsible for a strong field effect. Accordingly, AE (the ligand field splitting) is increased and the phosphines stabilize the complex. The ligand field splitting decreases in going from the heavier to the lighter metals and, as a matter of fact, alkyl derivatives of platinum and palladium having triphenylphosphine ligands are known, but the corresponding nickel complexes-have not been prepared. The above discussion has considered the stabilization of complexes in terms of the crystal field theory. It is desirable to consider the same topic in terms of modern molecular orbital theory. Although the development, and sophisticated consideration of the MO treatment is far beyond the scope of this chapter, an abbreviated, qualitative picture will be presented, focusing again on the energy levels of the highest occupied and lowest empty orbitals and again using the square planar d8 case. The metal orbitals available for bonding are the eight (n - l)d, ns, and np orbitals. Since there are only four ligands, it will be necessary to generate four bonding and four antibonding orbitals by combinations between four ligand orbitals and four appropriate metal orbitals (9).The appropriate combinations are shown in Fig. 5 . The four lowest and four highest orbitals are the bonding and antibonding orbitals. I n addition, there are four nonbonding d orbitals. The highest filled orbital is again the d,, orbital, but the lowest unoccupied orbital is now an antibonding orbital generated from an appropriate ligand orbital and the d,,-u, metal orbital. Now the better the bonding between the ligand orbital having d,.-u? symmetry and the metal d12-MIorbital, the greater the splitting between the bonding and antibonding orbitals generated from the combination; this effect raises the antibonding level and hence increases AE. But particularly important, if the ligand is capable of forming 7r bonds, particularly with the dP,, orbital, the energy of this orbital will be lowered and hence AE will increase. Ligands with strong donor properties that are also capable of forming r bonds, either because of low-lying antibonding IT* orbitals (carbon monoxide, olefins) or empty d orbitals (phosphines, arsines, stibenes), should thus be capable of stabilizing the organometal complex, and thus complexes such as (R,P),MR, are stable o-bonded M-C complexes.

V. The Carbon-Metal Delocalized Pi Bond As indicated above, in a complex such as [C,H,PtCI,]-, the Pt(I1)is assumed to have a 5ds configuration, Ethylene and each of the three

13


Complex Ligands

Metal

FIG.6. The molecular orbital scheme fords square planar transition metal complexes.

chlorine atoms are assumed to donate a pair of electrons so that the configuration of Pt becomes 6sa6p45d1°. Now usually the transition metal in a complex,tends to accept a sufficient number of electrons so that the total number of electrons surrounding it, its effective atomic number (EAN), is equal to that of the closed shell configuration of the inert gas following the metal in the periodic table. For transition metals this means that the ns, np, and (n - 1)d orbitals (n = 6 in Pt) become filled with eighteen electrons. Square planar Pt(I1) and Pd(I1) do not meet this requirement; they both have an empty p orbital. This may be one of the reasons for their very important catalytic reactivity which frequently involves nucleophilic attack and the attainment of fivecoordinated intermediates and transition states.

14

MJLTON ORCHIN

A. FERROCENE The bookkeeping on electrons in ferrocene, dicyclopentadienyliron, can be done in either of two ways. The compound may be thought of as Fe(I1) with 2 = 24 and two cyclopentadienide ions, [C,H,]-, donating six electrons each to give the 4s24p03d1° configuration, or as Fe(0) with 2 = 26 and two cyclopentadienyl radicals,C,H,. ,donating five electrons each. No decision can or need be made between these alternate methods of bookkeeping, In neutral complexes such as ferrocene, it is usually most convenient for electron counting to assume the metal to be in the zero oxidation state and the ligand t,o be a neutral radical or olefin. Then n complexed ligand having n carbon atoms in its x system may be considered an n-electron donor, each carbon contributing one p x electron to the metal. It is desirable to analyze the bonding in the ferrocene molecule so that important concepts affecting bond strength and stability may be

(d)

(el

(0

(g)

(h)

Pro. 6. Molecular orbital and resonance structure representation of cyclopentadienyl anion.


15

brought out. Since the MO treatment of ferrocene, n-Cp,Fe, can be found elsewhere (10,11),only certain features which emphasize the difficulty of writing an accurate but concise representation of the structure will be discussed. Consider first one isolated cyclopentadienyl ring. Although as mentioned above, the ring may be considered as a five-electron donor for electron-counting purposes, for structural purposes there is some Again this is a advantage in assuming it to exist as the anion [C5H5]-. matter of convenience, since in the molecular orbital treatment all the appropriate molecular orbitals are first generated and finally all electrons are placed in these orbitals in the order of increasing energy of orbitals. The p v AO’s of the five carbon atoms in the cyclopentadienyl group are combined to give five MO’s. The lowest-energy MO would look like two five-sided doughnuts separated by a nodel plane and having opposite signs (Fig. 6 a ) . If attention is focused on the sign of the orbital, this MO can be represented, looking down from the top, by a plus sign everywhere, as is shown in Fig. 6a’. One realizes of course that there is a second portion of this one orbital, not shown in Fig. 6a‘, which is identical to the top half but which has the opposite sign. Figure 6a‘ is usually said to have no nodes, the one in the plane of the molecule being disregarded, since all the MO’s are generated from pr atomic orbitals, which of course have a node in the molecular plane. The four other MO’s of cyclopentadienyl are made up of two pairs having different energies; the MO’s of each pair however have the same number of nodes and identical energy (doubly degenerate), The degenerate pair with one node (Fig. 6b) is lower in energy than the degenerate pair with two nodes (Fig. 6c). I n [C,H,]there are six electrons to account for, the aromatic sextet. These will occupy, two in each, the three MO’s (Figs. 6a and b). Orbitals ( 6 c )which are antibonding will be used only if more electrons are forced on [C,H,J(e.g., by back-bonding from a metal) or if electrons in lower-energy orbitals like b are excited (e.g., by ultraviolet radiation). In the ground state of [C,H,]-, the three MO’s are equally occupied and hence the electron density is uniform around the ring. It has become increasingly popular to represent [C5H,]- by the structure shown in Fig. 6d. This representation is a valence bond structure notation ; it is intended to represent the five equivalent resonance structures (three of these are shown as Figs. 6e, f,and g ; the other two are similar), in which the negative charge is located a t each carbon in turn. Each double bond is thus only a partial double bond and, if the usual notation of writing a partial double bond by a dashed line were applied, the structure would be written as 6h. The solid circle ( 6 4 is a rapid way to write the 6h structure. In a completely analogous

16

MILTON ORCHIN

(b)

(a)

(C)

FIG.7. Resonance structures of benzene.

manner, the two resonance structures of benzene (Figs. 7a and b ) can be represented by the single structure shown in Fig. 7c. Figures 6d and 7c are not MO representations; they are not intended to represent the familiar doughnut-shaped orbitals of lowest energy. The solid circle inside the ring is a notation first suggested by Sir Robert Robinson t o represent the so called C+H H3C

Fe(CO),

In support of this mechanism, it was shown that allyl alcohol on treatment with Fe(CO), is isomerized to propionaldehyde. The identical isomerization of allyl alcohol has been demonstrated (23)to proceed by HCo(CO), catalysis and evidence secured for a similar 1,3 or allylic hydrogen shift [Eq. (1l)]. Certain conjugated iron diene tricarbonyl compounds have been

32

MILTON ORCHIN

isomerized to conjugated dienes with a different structure (32). The conversion of 2,6-dimethyl-2,4-hexadieneto trans-2,5-dimethyl-l,3hexadieneiron tricarbonyl may be written as proceeding through r-allyliron hydride intermediates :

The complete shift of a, conjugated diene to a new conjugated diene sygtem has also been observed in the !-ionone system (33).Such diene rearrangements require a stoichiometric quantity of Fe(CO),. The rearranged diene ligand is conveniently liberated from the Fe by oxidrttion of the complex with FeCI,. Nonconjugated dienes are rearranged to iron carbonyl complexes of conjugated dienes when treated with Fe(CO), or Fe,(CO)I,. Thus 1,4-pentadiene gives trans-l,3-pentadieneirontricrtrbonyl (34) possibly by the route:


33

/ HFe(CO1,

Fe(CO),

1,4-Dihydromesitylene likewise gives a 1,3 complex (34) and 1,4cyclohexadiene gives 1,3-~yclohexadieneirontricarbonyl. 1,a-Cyclooctadiene on treatment with catalytic quantities of Fe(CO), gives 1,3-cyclooctadiene(35),as the iron tricarbonyl complex is probably not very stable and is continuously displaced by fresh 1,b-diene until isomerization is complete. The n-pentadienyliron tricarbonyl cation is an important grouping. It can, for example, be generated by treatment of cyclohexadieneiron tricarbonyl with triphenylmethyl fluoroborate. This salt is a powerful hydride abstractor (36) and the reaction gives rise to x-cyclohexadienyliron tricarbonyl cation (37):

The ionic complex may be formally regarded as a carbonium ion complexed to the Fe(CO), moiety. The diene moiety before reaction is obviously a four-electron donor. After hydride loss the four p n electrons are distributed over the five spa carbon atoms, all of which must be in a plane and all simultaneously bonded to iron. It would be expected that considerable back-donation from the metal d orbitals occurs in view of the positive charge, and this is substantiated by the abnormally high C-0 stretching frequency of the carbonyls in the complex. When uncomplexed cyclohexadienyl cation is treated with q5,CH, hydride transfer occurs and 4,C+ is formed; i.e., cyclohexadienyl cation is a stronger acid than +&However, I+. as seen above, the c m p l w e d cyclohexadienyl cation is a weaker acid (more stable, less electrophilic) than q&C+ since it will not abstract the hydride from +,CH. The difference in electrophilicity is reasonably ascribed (32) to the extensive electron transfer (back-bonding) from the metal to the cyclohexadienyl moiety, thus giving further support to the suggested structure of the complexed cation. It should be noted that the n-cyclohexadienyl cation can

34

MILTON ORCHIN

theoretically be obtained from the protonation of benzene and this fact suggests certain interesting possibilities. It is important that the five carbon atoms making up the n-pentadienyl fails to abstract a hydride system have the cisoid arrangement. Thus from trans- but does from cis-l,3-pentadieneiron tricarbonyls (38):

@cH3

-+-no reaction

+ @,c+

(284

FdCO),

However, treatment of cis-l,3-pentadiene with Fe(C0) leads to formation of the trans-l,3-pentadieneiron tricarbonyl, probably via a nallyliron hydride intermediate as shown for the conversion of 1,4pentadiene to the trans-l,3-pentadieneiron tricarbonyl [Eq. (26)]. The cis-1,3-pentadieneiron tricarbonyl [Eq. (28)] could be prepared only indirectly by borohydride treatment of x-pentadienyliron tricarbonyl cation (see below). The n-pentadienyl cation system can also be generated by perchloric acid treatment of complexed alcohols and an interesting rearrangement of a primary t o R secondary alcohol can thus be achieved (39): H3C

c%oH +

Fe(co),-

H , c q L c b o H Fe(CO),

(29)

c. a $ x h l A N D CONVERSIONS AND REARRANGEMENTS I n 1961, Green and Nagy (40) reported that treatment with HC1 of a a-ally1 group coordinated to iron gave a n-propene complex cation :

35


The cation can be isolated as the chloroplatinate, [T-C~F~(CO),C,H,], PtCl,. Similar cations had been prepared from ethylene by Fischer and Fichtel(41). Green and Nagy also found (42)that it was possible to form the same complexed propene cation by hydride abstraction with +,C+-BF,- from the n-propyl complex as indicated above. Thus the identical cationic rr-propene complex can be generated either by proton addition to the a-ally1 or by hydride abstraction from the o-n-propyl complex. They then discovered that NaBH, reduction of the cationic 7-propene perchlorate complex in THF gave the a-isopropyl complex T-Cp(CO),FeCH(CH,), in good yield. Thus it is possible by successive hydride abstraction and addition to isomerize the normal propyl t o the isopropyl group :

Studies by Green and Nagy (43) on the hydride removal from the isopropyl complex -Fe-CD<EE:

showed that the carbon atom beta

to the metal is removed, and they suggest that the removal is an SN2type reaction, e.g., with the ethyliron complex:

It is unlikely that the hydride addition to the cation proceeds via an isolable iron hydride, ?r-CpFe(CO),H,plus CH,CH =CH,, followed by readdition of the hydride to the olefin, because it was not possible to transfer the iron to either 1-hexene or butadiene during the reduction. The conversion of u + 7r by external removal of a hydride (with qi3C+) and the subsequent conversion of T + u by external supply of hydride (with NaBH,) has been suggested (43) as a model system for the doublebond isomerization of chemisorbed olefins on metal surfaces in which the metal can act as the internal banking site for hydride: R\

P' C$,CH2

R\

___t

CH I -M-

R\ C!$

/R'

CH=CH I H

-d-

Ck

C% ,R 'CH I

(33)

-M-

In this connection, a recent article by Phillipson and Wells (44) dealing with the isomerization of butenes is of interest. The Group V I I I metals catalyze both the hydrogenation and isomerization of butenes, cobalt

36

MILTON

ORCHIN

being the most selective for isomerization and iridium and platinum for hydrogenation. It might be expected that, since an adsorbed C,H, species is a necessary intermediate for the hydrogenation, i t is also a reasonable intermediate for the isomerization. However, the authors demonstrated that butene isomerization on a cobalt surface, after adsorption of the butene, proceeds via abstraction of hydrogen as a first step to a C,H, species, even in the presence of a hydrogen atmosphere. With the good hydrogenation catalysts, C4H, is probably the first species formed or it is formed in competition with C,H,. The protonation of a o-allyltungsten complex to the cationic n-propene complex and the NaBH, reduction of this to the isopropyl complex in complete analogy to the corresponding iron compounds has been demonstrated by Green and Stfear( 4 5 ) :

I

W / I\\CH&H=Cri,

cococo

(34)

The interconversion of u- and n-ally1 complexes has been observed by Kwiatek, Mador, and Seyler in the interesting homogeneous catalytic system of potassium pentacyanocobaltate(II), K,Co(CN), (46). This solution absorbs molecular hydrogen to form the active hydride species, K,[Co"'(CN),H]. Addition of butadiene to the hydride results in the formation of a a-methallylcobalt complex :

+ CH,=CHCH=CH,

[(CN),CoH]

+ [(CN),CoCH,CH=CHCH,]

(35)

An analogous u-allylcobalt complex is formed by treating ally1 halide with K,Co(CN),: 2[Co(CN),]a-

+ CH,=CHCH,X

-+ [(CN)sCoX]S-

+ [(CN),COCH,CH=CH,]~-

(36)

Loss of a cyanide group converts the o-ally1 complex to a n-ally1 and when excess CN- is added reconversion occurs (47): /

CN

CH=CHR

CN CN

37


Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentacyanocobaltate(11) catalyst or when the u butenyl complex is reduced with the hydride complex [HCo(CN),]3-. Thus 1-butene is the major product in the presence of excess CN-, and trans-2-butene is the major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a u complex, and the 2-butene via an intermediate wallyl complex. The r-ally1 complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO), (48)or from methallyl halide and NaCo(CO), (49). H I

CH*=CHCH=CH,

+

-

HCo(CO), - -

.

or CH,CH=CHCH,X

+ NaCo(CO),

The protonation of the o-allylic cyanocobaltate complexes has been reported by Kwiatek and Seyler (50)to proceed with the liberation of the corresponding olefin. Thus the complex prepared from butadiene [Eq. (35)] on treatment with aqueous HC1 liberates 1-butene. The carbonium ion which probably forms first can cleave directly to 1-butene or it may first rearrange to a n-olefin complex, from which the olefin is then displaced with either H,O or chloride: [ (CN),CoCHzCH=CHCH3]

3-

+ H+

-

[ (CN),CoCH,6HCH2CH3 I s-

/

[(cN),coJ~- + CH,=CHCH,CH,

-

(39) 2-

The n-olefin complex is analogous to the complex which Green and Nagy (40) reported for the protonation of the a-2-butenyliron complex:

Kwiatek and Seyler (50) also showed that, if the R group in the cobalt

38

MILTON ORCHIN

u complex is saturated, protonation results in rearrangement of the R group and the formation of the nitrile, RCN, on further treatment with alkali. The postulated sequence is:

9-

-

(41)

[OH]-

CH,CH,C&CN

The rearrangement is reminiscent of the well-known carbonyl insertion in the alkylmanganese pentacarbonyl system (51): 0

This CO insertion occurs under the influence of nucleophiles other than carbon monoxide, e.g., triphenylphosphine. The independence of the rate on the concentration of ligand suggests (52) a rate-controlling dissociation of the octahedral complex assisted by a nucleophilic (ether) solvent.

D. ISOMERIZATION WITH PALLADIUM AND OTHERGROUPVIII METALS In 1934, Anderson (53)reported that hydrolysis of Zeise’s salt, K[PtCl,C2H,], gave some acetaldehyde. For some time this fact was partly responsible for the assumption that ethylene was unsymmetrically However, Chatt and coordinated to the platinum as -Pt=CHCH,. Duncanson (54)showed that the ethylene was symmetrically coordinated to platinum. One of the suggestions (55) made t o explain the formation of acetaldehyde was that the ethylene ligand was first hydrated to ethanol, which in the presence of Pt(l1)is then oxidized to the aldehyde. However, Joy and Orchin (56)showed that, although such an oxidation proceeds very well, its rate is considerably less than the formation of acetaldehyde from Zeise’s salt under identical conditions. Interest in the ethylene oxidation skyrocketed when Smidt, Hafner, and co-workers at the Consortium for Electrochemical Industries in Munich reported (57) the commercial process for converting ethylene to acetaldehyde via a palladium complex analogous to Zeise’s salt. The stoichiometry of the reaction is: C,H,

+ PdCI, + H,O + C,H,O + Pd f 2HC1 Pd + 2CuC1, + PdCl, + 2CuCl

(43)


39

The oxidation of elementary palladium formed as a n intermediate in the process was best achieved by CuCl,, since CuCl is more easily oxidized by oxygen than is palladium (58). I n view of the commercial importance of the reaction, considerable effort has been expended on elucidating its mechanism and extending the reaction to a variety of other synthetic processes based on olefins. The formation of acetaldehyde from the sr-olefin complex was shown to involve intramolecular migration of a hydrogen atom from one carbon of the ethylene to the other, rather than OH- attack on a vinyl group -OH --f generated by hydride abstraction with Pd (CH,=CH+ CH,=CHOH) followed by rearrangement of the vinyl alcohol to acetaldehyde, since hydrolysis in D,O yielded acetaldehyde free of deuterium (59). One possible mechanism for the reaction may be written as follows:

+

H

H

[A1 The complex [A] is essentially the intermediate inferred in the mechanism proposed by Stern (60)t o explain his results from the reaction between propene and PdCl, in the presence of acetic acid. The kinetics of essentially the same reaction have been carefully investigated by Moiseev et al. (61)and a thorough study of each step in the reaction was recently reported by Henry (62). Henry, assuming that PdCl, in an aqueous system exists in the form of [PdC1J2-, proposed that the steps in the reaction were as follows: [PdCI,]z-

+ C,H, + [PdCI,C,H,]- + C1P I

[B]

+ H,O

i.? [PdCI,(H,O)C,H,] $. C1-

[CI

[C]

+ H,O

$

[PdCl,(OH)C,H,]-

+ H,+O

[Dl slow

[D] --+CIPdCH&H,OH [A1

+ CI-

(49)

40

MILTON ORCHIN fnst

[A] ---+

HCI

+ PdO + CH,CHO

The hydroxo 7~ complex [D] is assumed to rearrange to the a-bonded compound [A] since only a secohdary isotope effect is observed with C,D,, and if the 7~ complex went directly to acetaldehyde Henry claims that the hydride shift involved would have produced a primary deuterium effect. Although the hydroxo complex [D] would have been expected to be trans,i t was suggested that kinetically significant amounts of the cis isomer are present, Henry chooses to write complex [A] as an activated complex in which the palladium somehow is assisting the migration of hydride [A']:

[A' 1

Stern showed rather conclusively that the palladium does not depart to leave a carbonium ion but that both hydride migration and collapse to an aldehyde proceed simultaneously. The removal of the 9, hydrogen in a complexes by the heavier Group VIII metals has been documented. Thus Chatt and Shaw (63) showed that a platinum hydride complex could undergo the reversible addition of ethylene: (Et,P),Pt(Cl)H

+ CH,=CH,

+

t (EtsP)2Pt(CI)H*C,H, (Et,P),Pt(Cl)C,H,

(51)

The loss of ethylene from the a-bonded ethyl group most reasonably proceeds by migration of hydrogen from a carbon of the ethyl group t o platinum. This mechanism again represents reversible addition of H-M to a double bond. It is worthwhile noting that in the preparation of the unusual hydride [IrHCI,(PPh,),], in which the hydride hydrogen is captured from ethanol, Vaska and DiLuzio (64) showed that when CH,CD,OH was used the iridium was coordinated to deuteride. Thus it is likely that the hydrogen alpha t o oxygen (beta to metal) is abstracted by the iridium: CH,CD,OIr

--f

IrD

+ CH,CDO

(52)

I n an extensive study of the isomerization of hexenes and heptenes by platinum, palladium, and ruthenium, Harrod and Chalk (65) found that in many cases the equilibrium mixture of isomerized olefins was obtained. Isomerizations were achieved with: Pt(I1) (as 1,3-bisethylene-2,4dichloro-p-dichlorodiplatinum(I1) with alcohol as cocatalyst), Pd(I1)


41

(as 1,3-bisbenzonitrile-2,4-dichloropalladium(II), Rh(II1) (as trichlororhodium(II1) dihydrate with alcohol as cocatalyst), and Ir(II1) (as trichloroiridium(II1) in the presence of alcohol as cocatalyst). The authors conclude that in all cases isomerization may be explained by the formation of the metal hydride, coordination with the olefin, reversible addition of H-M to the olefin to form the o-bonded intermediate, and dissociation again t,o the isomerized olefin. I n an interesting example of isomerization of a conjugated diene to the nonconjugated position, Rinehart and Lasky (66) demonstrated that 1,3-cyclooctadiene is rearranged to the 1,5 isomer via a rhodium(1) complex. Treatment of the 1,3 diene (1 ml) with RhC1,.3H,O (2 gm) in ethanol (20 ml) a t 50" for 24 hours gave [1,5-C8H,,RhCl],. Treatment of this complex with aqueous KCN liberates the nonconjugated diene. The square planar configuration of this dimer complex originally suggested by Chatt and Venanzi (67) was confirmed by an X-ray structure analysis by Ibers and Snyder (68):

The preferred mechanism (66)for the isomerization involves n-complexing followed by hydride abstraction to give a n-allylrhodium hydride intermediate, e.g.,

A variety of n-ally1 complexes are possible, including the transannular ones. It should be noted that, although 1,3-~yclooctadieneis thermodynamically the most stable diene, the stability of the 1,Fi complex provides the driving force for the rearrangement. The 1,4 diene complex does not appear to be an intermediate, suggesting that the hydride abstraction-addition sequence is not straightforward. Davies (69) has carried out a series of isomerization experiments in a medium consisting of acetic acid, palladium(I1) chloride, and sodium chloride, the latter in a 1:l mole ratio. I n this system sodium 1,2,3,4p,p'-dichloropalladium(I1)was assumed to be present:

42

MILTON ORCHIN

When 1-octene was the substrate and the reaction was carried out in CH,C02D, no D appeared in the mixture of octene isomers. I n order t o determine whether the 2-octene formed was obtained via a 1,3 shift [Eq. (54)l or two cocurrent 1,2 shifts [Eq. (5511, the isomerization was carried out with 1-octene labeled in the 3 position, C,H,,CD,CH=CH,. The 2-octene isolated was not labeled in the methyl group. Hence, Eq. H

C

T

C,H,,CH,CH-CH,-C~H,,HC~~+CCH, Pd Pd H / -

A c,H,,cH,cH-CH, L/ J d

-

C,H,,CH-CHCH, T Pd

\

C,H,,CH-CHCH, f Pd

(54)

C,H,,CH=CHCH,

(55)

(54) is improbable but Eq. ( 5 5 ) is consistent with the data. Positive direct proof of deuterium incorporation at carbon-2 was not obtained, C

I

however. Isomerization of C-C-C-C=C was also attempted with the palladium catalyst under conditions which isomerized 1-octene. The fact that no isomerization occurred was taken as additional evidence for the correctness of Eq. ( 5 5 ) , since the above compound has no hydrogen on the methyl-substituted carbon but allylic hydrogens are available for reaction by Eq. (54). No precise mechanistic path for the isomerization was suggested. The suggestion that the palladium chloride-catalyzed isomerization of a 1-olefin to a 2-olefin is achieved by migration of hydrogen from C-3 t o C-2 and from C-2 to C-1 has been challenged by Harrod and Chalk (70). These authors prefer a reversible metal hydride additionelimination mechanism and contend that if a large isotope effect, k,/k,, is operative, Davies’ results are consistent with the reversible metal hydrogen mechanism. They studied the isomerization of CH,CH,CH,CH,CD=CHD and found that it proceeded 3 times as fast as I-hexene. I n the metal hydride addition mechanism, the reaction leading to isomerization involves metal-carbon u bond formation a t C-2: RCH,-CD(M)-CH,D. Reversion to l-olefin would require elimination of either MH or MD while isomerization requires elimination of MH only. Hence, isomerization of the deuterated species proceeds more rapidly than that of the nondeuterated species, since in the latter the analogous o-bonded intermediate obviously can revert to

ISOMERIZATION O F OLEFINS

43

1-olefin without a deuterium effect. Harrod and Chalk also report that no deuterium migration occurs as would be required by the Davies mechanism. Davies' retort ( 7 1 ) to these arguments is that the metal hydride mechanism requires an appreciable concentration of MH and such species have not been detected. Recently Roos and Orchin (72) studied the isomerization of allylbenzene with DCo(CO),. They found that isomerization to the propenylbenzenes proceeded very nicely but that no significant quantity of deuterium had been incorporated into either recovered allylbenzene or its isomerized products. These authors propose an interval 1,3hydrogen shift.

VII. Summary Transition metal hydrides play a key role in the catalytic homogeneous isomerization of olefins. The pure hydrides such as HCo(CO), can function as the catalyst, or transition metals complexed t o stabilizing ligands can function as catalysts; the catalysis almost certainly proceeds through hydride intermediates in many cases. The most attractive mechanism for olefin isomerization consists of initial complexing between olefin and metal, addition of H-M across the double bond (via a four-centered transition state) t o generate a u carbonmetal bond, and then elimination of H-M in the opposite direction with eventual release of the isomerized olefin. The evidence for such a mechanism is compelling, especially with the heavier transition metals. However, when some a-alkyl metals of cobalt and iron have been treated with olefins under conditions appropriate for isomerization, no transfer of the metal t o the olefin occurred. An alternate mechanism in which an allylic hydrogen and the metal hydrogen are exchanged with simultaneous bond migration has been suggested, and there is now evidence (72) that interval 1,3-hydrogen shifts also occur. Fe(C0) isomerizes simple olefins, nonconjugated dienes to conjugated dienes, and i t can also catalyze the migration of both double bonds in a conjugated diene t o give a new conjugated diene. An attractive mechanism for the isomerizations involves wallyliron hydrides :

H

44

MILTON ORCHIN

B:

H

That the stability of the final complex is of great importance is dramatically illustrated by conversion of the conjugated 1,3- t o the nonconjugated 1,5-cyclooctadiene by a rhodium catalyst. One of the most intriguing recent developments involves the skeletal rearrangement of alkyl groups a-bonded to the iron in a cyclopcntadienyliron dicarbonyl group. Though not an olefin isomerization, the rearrangement is of much significance for the mechanism of such isomerizations. The skeletal rearrangement proceeds by conversion of the a-alkyl to n-alkene complex by abstraction of hydride from the carbon beta to the metal and then hydride addition to the n-alkene complex :


Q

@ c+ s

45

1

CH,

The abstraction and addition of hydrogen by a metal without the metal and substrate ever being separated is of particular significance for heterogeneous catalytic isomerization of olefins and hydrocarbons on metal surfaces, as well as for certain biological systems. Aqueous cobalt cyanide systems that activate hydrogen and catalyze hydrogenation of dienes produce different olefin isomers, depending on the cyanide concentration, and a reasonable case for a + rr interconversions can be made t o explain the stereoselectivity observed. Such mechanisms may well have a bearing on the steric control obtained in certain transition metal catalyzed polymerization of dienes. Practically all the heavy transition metals can be made to catalyze olefin isomerization, presumably through transient formation of metal hydrides. A stable platinum hydride has been shown t o react with ethylene to form a o-C,H,Pt complex which can eliminate ethylene to regenerate the hydride. The commercially successful processes for the conversion of ethylene to acetaldehyde and ethylene to vinyl acetate via PdC1, catalysis have stimulated enormous interest in the mechanism of these reactions, their application to other conversions, and their extension to other catalytic systems. The various stages in the conversion of ethylene are quite well-understood and an important step in the reaction involves hydride migration. The exact role of Pd in the migration has not yet been elucidated. It seems almost certain that the phenomenal interest in the whole area of transition metal isomerization in the last several years will be more than matched by the wealth of work that is certain to pour out of research laboratories in the next few years. ACKNOWLEDGMENT The author is grateful to Drs. W. Clement, J. Kwiatek, I. Mador, and L. Roos for criticisms end helpful discussions and to Sharon Butryrnowicz for help with the manuscript.

REFERENCES 1. Chatt, J., and Duncanson, L. A., J . Chem. Soe. p. 2939 (1953). 2. Dewar, M. J. S., Bull. SOC.Chim. France 18, 71 (1951).

46

MILTON ORCHIN

3. Mulliken, R. S., J. Phys. Chem. 66, 295 (1952). 4. Pauling, L., “Nature of t h e Chemical Bond,” 2nd ed. Cornell Univ. Press, Ithaca, New York, 1940. 5. Jaff6, H. H., and Doak, G. O., J . Chem. Phys. 21, 196 (1953). 6. Jaffb, H. H., J. Chem. Phys. 22, 1462 (1954). 7. Treichel, P. M., and Stone, F. G. A., Adwan. Organomelallic Chem. 1, 143f (1964). 8. For a review of this subject, see almost any modern text on inorganic chemistry or coordination chemistry. A particularly attractive account is contained in Orgcl, I,. E., “An Introduction t o Transition Metal Chemistry,” Wiley, New York, 1962. 9 . A full treatment of the bonding involved can be found in Coates, G. E.,“OrganoMetallic Compounds,” 2d ed. p. 311. Wiley, New York, 1960; also in Gray, H. U., and Ballhausen, C. J.,J. A m . Chem. SOC.85, 260 (1963). 10. Coates, G. E., “Organo-Metallic Compounds,” 2d ed., p. 237ff. Wiley, New York, 1960. 11. Richardson, J. W., in “Organometallic Chemistry” (€1. Zciss, ctl.), pp. 22f. Reinholcl, New York, 1960. 12. McClellan, W. R.,Hoehn, H. H., Cripps, H. N., Muetterties, E. L., and Howk, B. H., J. A m . Chem. Soe. 83, 1601 (1961). 13. Orchin, M., Kirch, L., and Goldfarb, I.,J . A m . Chem. Soe. 78, 5450 (1956). 14. Wender, I., Sternberg, H. W., and Orchin, M., J . A m . Chem. Soc. 75, 3041 ( 1 953). 15. Kirch, L., and Orchin, M., J. A m . Chem. SOC. 80, 4428 (1958). 16. Kirch, L., and Orchin, M., J. A m . Chem. Soc. 81, 3597 (1959). 17. Heck, R . F., and Breslow, D. S., Chem. h Ind. (London) 17, 467 (1960). 18. Karapinka, G. L., and Orchin, M., J . Org. Chem. 26, 4187 (1961). 19. Heck, R. I?., J . rim. Chem. SOC.85, 651 (1962). 20. Brown, H. C., and SubbaRao, B. C.,J. A m . Chem.Soc. 81,6428,6434 (1959);Hennion, G. F., McCusker, P. A., Ashby, E. C., a n d Rutkowski, A. J., ibitl. 79, 5190 (1957); Brown, H. C., and Zweifel, C . , ibitl. 82, 1505 (1960). 21. Brown, H. C., and Rhatt, M. V., J. A m . Chem. SOC.82, 2074 (1960). 22a. Heck, R.F., and Breslow, D. S . , J . A m . Chem. Soc. 83,4023 (1961). 22b. Heck, R. F., and Breslow, D. 8.. J. A m . Chem. Sor. 85. 2779 (1963). 23. Goetz, R. W., and Orchin, M., J. A m . Chem. Soc. 85, 1549 (1963). 24. Emerson, G. F., and Pettit, R., J. A m . Chem. SOC.84,4591 (1962). 25. Takegami, Y., Yokokawa, C., Watanabe, Y.,and Okuda, Y., Bull. Chem. Sor. Japan 37, 181 (1964). 26. Whitlock, H. W., Jr.,J. A m . Chem. Soe. 85,2344 (1963). 27. Krumholz, P., and Stettiner, H. M. A., J. A m . Chem. Sor. 71, 3035 (1949). 28a. Sternberg, H. W., Markhy, R., and Wendcr, I., J . A m . Chem. S o r . 78, 5704 ( 1 956). 2Rb. Sternberg, H. W., Markby, R., and Wender, I., J. A m . Chem. S o r . 79, 61 16 (1957). 2!). Asinger, F., and Berg, O., Ber. 88, 445 (1955). 30. Manuel, T.A., J. Org. Chem. 27, 3941 (1962). 3 1 . Emerson, G. F., and Pettit, R., J. A m . Chem. Soe. 84, 4591 (1962). 32. Pettit, R., and Emerson, G. F., Advan. Organometallic Chem. 1, 16 (1964). 33. Cais, M., private communication. 34. King, R. B., Manuel, T. A., and Stone, F. G. A.,J. Inory. Nurl. Chem. 16,233 (1961). 35. Arnet, J. E., and Pettit, R . , J . A m . Chem. Soc. 83, 2954 (1961). 36. Dauben, H. J . , and Honnen, L. R., J . A m . Chem. SOC.80, 5570 (1958). 37. Fischer, E. 0..and Fischer, R. D., Angew. Chem. 72,919 (1960). 38. Mahler, J. E., and Pettit, R., J. A m . Chewa. Soc. 85, 3955 (1963). 39. Mahler, J. E., Gibson, D. H., and Pettit, R., J. A m . Chern. SOC.85, 3959 (1963).


47

40. Green, M. L. H., and Nagy, P., Proc. Chem. SOC.p. 378 (1961). 41. Fischer, E. O . , and Fichtel, K., Chem. Ber. 94, 1200 (1961). 4 2 . Green, M. L. H., and Nagy, P. L. I., J. A m . Chem. SOC. 84, 1310 (1962). #3. Green, M. L. H., and Nagy, P . L. I., J . Organomet. Chem. 1, 58 (1963). 44. Phillipson, J. J., and Wells, P. B., Proc. Chem. Soc. p. 222 (1964). 4 5 . Green, M. L. H., and Stear, A. N., J . Organomet. Chem. 1, 230 (1964). 46. Kwiatek, J., Mador, I. L., and Seyler, J. K., Advan. Chem. Ser. 37, “Reactions of Coordinated Ligands,” p. 201, (1963), J . A m . Chem. SOC.84, 304 (1962). 47. Kwiatek, J., and Seyler, J. K., Proc. 8th Intern. Congr. Coord. Chem., L’ienna, 1964 p. 308. 48. Jonassen, H. B., Steams, R. I., Kenttgmaa, J., Moore, D. W., and Whittaker, A. G., J . A m . Chem. SOC.80, 2586 (1958). 49. Heck, R. F., and Rreslow, D. S., J . A m . Chem. SOC.82, 750 (1960). 50. Kwiatek, J., and Seyler, J. K., to be published. 5 1 . Coflield, T. H., Kozikowski, J., and Clossen, R. D., J. Org. Chem. 22, 598 (1957). 6 2 . Mawby, R. J., Basolo, F., and Pearson, R. G., J . A m . Chem,. Ror. 86, 3994 (1964): Calderazzo, P., and Cotton, F. A., Inorg. Chent. 1, 30 (1962). 53. Anderson, J. S., J . Chem. SOC.p. 973 (1934). 54. Chatt, J., and Duncanson, L. A., J. Chem. SOC.p. 2939 (1953). 55. Chatt, J., and Duncanson, L. A., J . Chem. SOC. p. 2942 (1953). 56. Joy, J. R., and Orchin, M., 2. Anorg. Allgem. Chem. 305, 236 (1960). 57. Smidt, J.,Hafncr, W., Jira, R., Sedlmeier, J., Sieber. R., Ruttinger, R., and Kojer, H., Angew. Chem. 71, 176 (1959). 58. Anonymous, Chem. Eng. News April 1961, p. 52. 59. Hafner, W., Jira, R., Sedlmeier, J., and Smidt, J.,Chem. Ber. 95, 1575 (1962). 60. Stern, I secondary > primary (3" > 2" > l o ) .Accordingly, the isomerization of cyclohexene (I)involving the rearrangement of a secondary to a less stable primary carbonium ion can be expected to proceed with greater difficulty than that of 3,3-dimethylbutene (IIa), which involves the rearrangement of 2" to 3". The 2,3-dimethylbutenes formed according to (IIa) can in turn form 2-methylpentenes (IIb) by steps which involve an unstable primary carbonium ion. Therefore, (IIb) should proceed more slowly than (IIa) or require stronger acid sites. For similar reasons the conversion of 2-methylpentenes to hexenes (IId) should also proceed slowly. For that reason not only the total conversion of 3,3-dimethylbutene is important, but also the depth of isomerization. Rearrangements of olefins proceeding through 1 O carbonium ions [steps (I),(IIb), (IId)] are believed to occur with reasonably fast rates only on relatively strong acid sites, whereas those involving 2" and 3" carbonium ions take place on both strong and weak acid centers. The relative activities of different alumina catalysts for the above-described reactions were used as a measure of their relative acidities. The extent of isomerization, or the total amount of 3,3-dimethylbutene consumed, will be defined by the amount of acidic sites. The depth of isomerization, or the occurrence of reactions (IIb) and (IId), will give information as to the strength of the acidic sites. This method gives excellent relative information for comparing and characterizing different aluminas. Aluminas, which were prepared from sodium aluminate and which retained about 0.1yoof sodium ions, had a large amount of weakly acid sites, and were therefore excellent dehydration catalysts. At the same time these aluminas did not isomerize cyclohexene, owing to the absence of strong acid sites, which were neutralized by the alkali metal ions. Pines and Haag (36)determined that the upper limit of the total number of acid sites, capable of dehydrating butanol, and of the number of strong acid sites, capable of isomerization of cyclohexene, was 10 x 10l2 and 8 x 10l2sites per cm2, respectively. The Lewis acidity of the dehydrated surface of alumina could best be explained by not fully coordinated aluminum atoms and its formation during calculation could be pictured by a model suggested by Hindin and Weller (43):

The adsorption of moisture by the surface of the catalyst deactivates the

56

HERMAN PINES AND JOOST MANASSEN

Lewis acid sites and therefore inhibits the color change of the Ieuco base indicators:

The Lewis acid sites can thus be converted into Bronsted acid sites. The Lewis base sites of the aluminas also participate in the dehydration of primary and secondary alcohols by the removal of a proton from either the 8- or y-carbon atom in relation to the OL carbon containing the hydroxyl group. The nature of the hydroxyl group on alumina was studied by Perri and Hannan (44) by means of infrared spectroscopy and they found that the attachment of the hydroxyl groups is largely ionic. The hydroxyl groups exchange hydrogen easily, but the rate is significantly slower than the rate of isornerization of 1-butene into 2-butene on the same catalyst. Consequently they doubted that the hydroxyl groups, which are visible by infrared techniques, are catalytically active for the isomerization reaction (45). To account for the surface hydration and catalytic properties of y-alumina a model for the surface of y-alumina was proposed by Perri (46).He also studied, by means of infrared techniques, the OH- and appear to sites which chemisorb ammonia to form NH,include those sites which isomerize olefins (47). These sites are ion-pair or acid-base sites. Lippens (48) has studied the texture of the catalytically active aluminas by means of diffraction and adsorption techniques. Hie concluded that the structure of q-alumina formed from bayerite consists of lamellae with an average thickness of about 15 A and a distance of about 25 A. y-Alumina prepared from gelatinous boehmite is composed of fibrillar-shaped particles of about 30 x 30 A. Both structures can easily account for the “pseudosolvent” effect of alumina, which will be referred to in more detail in the forthcoming discussion.

+

V. lsomerization Following Dehydration A. CYCLOHEXANOL Pines and Haag (49) studied the dehydration of cyclohexanol over various alumina catalysts. Over alumina containing about 0.4% of sodium or potassium ions, cyclohexene was the only product, in agreement with numerous reports in the literature. However, the high-purity

MECHANISM OF ALCOHOL DEHYDRATION OVER ALUMINA

57

alumina prepared from aluminum isopropoxide gave a mixture containing up to 60% methylcyclopentenes.

020 (I)

and

(n)

a (III)

The relative proportion of (111)in the unsaturated product increased with increasing temperature. Two mechanistic pathways may be considered by which methylcyclopentenes could be produced from cyclohexanol. I n the first, (11) and (111) are formed from (I)in parallel reaction with or without consecutive interconversion of the cycloalkenes:

The second possibility is that of a consecutive reaction with cyclohexene on a desorbed intermediate: (I) -+ (11) + (111)

(2)

The product composition varies as a function of contact time (Fig. l ) ,

Tima vorioMa, HLSV-'

FIG. 1. Dehydration of cyclohexanol over pure alumina (P) at 410". Influence of contact time.

58


which strongly suggests that cyclohexanol is dehydrated to cyclohexene, which in turn undergoes a slow isomerization to methylcyclopentenes [Scheme (2)]. It was independently shown that cyclohexene is converted to methylcyclopentenes under the same conditions over pure alumina, while catalysts containing alkali gave only 0.9% of isomerization. To further test Scheme (2), the composition of unsaturates was plotted against total amount of unsaturates produced (Fig. 2). Extrapolation to

0

20

40

€0

80

100

Olefins produced (%I

Fro. 2. Dehydration of cyclohexanol over pure alumina (P)at 410”.

zero conversion indicates that the primary dehydration product on the “acidic” alumina consists of pure or nearly pure cyclohexene. The above study indicates that kinetic investigation should be made in order not to confuse the primary product with subsequent products of reactions.

B. ~-BUTANOL Results similar to cyclohexanol were obtained with 1-butanol. Again alkali-containing catalysts gave a high yield of the expected dehydration product, 1-butene, especially a t lower temperature. It was accompanied, however, in all cases by some 2-butene. With the alkali-free high-purity alumina the proportion of 2-butene was much higher and approached equilibrium values under more vigorous conditions. The available data again indicate that the primary dehydration

MECHANISM OF ALCOHOL DEHYDRATION OVER ALUMINA

59

product from 1-butanol on all the alumina catalysts was the expected 1-butene. Dependent on the nature of the catalyst and the reaction conditions, this may then undergo double bond shift or even skeletal isomerization into isobutylene. I n agreement with this view is the observation that the 2-butenes produced during the dehydration of 1-butanol have a similar &/trans ratio as those obtained from the isomerization of 1-butene over the same catalyst. It will be noted that the 2-butenes are not formed in their selective equilibrium concentration, but in a stereoselective way favoring the cis isomer.

VI. Steric Course of Dehydration One of the most fruitful approaches to the elucidation of reaction mechanisms in organic chemistry is the study of the effect of structure on the reactivity and the course of the reaction. This approach is used extensively in homogeneous reactions and found to be equally rewarding in the study of the mechanism of dehydration of alcohols over alumina catalysts. Much information was obtained by changing the configuration of the alcohols.

A. MENTHOL AND NEOMENTHOL Much evidence supports the conclusion that the elimination of the group HX from alkyl halides by bases is a trans elimination reaction. This means that the atoms H and X leave from the opposite site of the incipient double bond. It is mostly explained by assuming that the electrons which are left by the leaving proton and which will form the double bond prefer to attack the leaving group X- from the rear ( 5 0 ) . The transition state for the elimination, if it is concerted, is most stable if H, X, and the carbon atoms 1 and 2 lie on one plane, which in most molecules is best realized in the trans position (51).*

I n order t o determine whether trans elimination may occur also in the removal of elements of water from alcohols, the dehydration of menthol *Throughout this review, A represents acid sites, and B represents proton-accepting sites.

60


and neomenthol was studied. These alcohols are ideally suited for the study of the stereochemistry of the dehydration. The pyrolysis of esters and xanthates (52),of trimethylammonium hydroxides (53), basecatalyzed elimination of menthyl and neomenthyl chlorides ( 5 2 ) ,and decomposition of the amine oxides (53)follow the expected steric course. Pines and Pillai found that 2-menthene is the preponderant product of dehydration of menthol over alumina catalysts at 280-330” (54). The general picture also shows that 3-menthene is also formed in all experiments even when the extent of dehydration is small. Also revealing is the fact that traces of 1-menthene are formed at all times even though 1-menthene is not t o be expected from a simple 1,2 elimination of the elements of water. The preferred formation of 2-menthene is a clear indication of trans elimination. This was further supported by the results obtained from the dehydration of neomenthol which yields 3-menthene aa the preferred compound.

(2- menthene)

(3- menthene)

compoeition,

(I 1

*

(n)

-

(1 menthene)

Q (XI)

80-90

18- 10

tertbutylbenzene) or with increasing number and symmetry of methyl substitution (e.g., toluene > p-xylene). Similar observations with respect to steric hindrance in the adsorption of alkylbenzenes have been reported by Rader and Smith (17)in competitive hydrogenation studies (Table 111). 3 . Monohalogenated Benzenes and Naphthalenes Trends in aromatic toxicities (Table IV) are somewhat obscured in these two series by the toxicities of the individual substituents (2). Fluorine and chlorine substituents are reasonably inert; however, the bromine and iodine atoms, particularly the latter, are extremely toxic. I n the case of inert substituents (F, C1) the results resemble those of the alkylbenzenes in terms of steric hindrance to adsorption. The above data indicate considerable differences in adsorption strengths of aromatic molecules. This observation is difficult t o interpret by either the classical associative or dissociative chemisorption theories, particularly by the latter, since this fails to explain why different aromatic hydrocarbons, possessing the same carbon-hydrogen bond strengths, should on chemisorption form carbon-metal bonds of differing strengths. The difficulties with respect to the associative 'theory are not as severe, since T electron energies differ greatly for individual members of the above series. Consequently differences in chemisorption strength *See ortho deactivation effect in Section IV,B.

TABLE I1 Deuterium Ode-Alkylbenzew at 120Q

Run

1A 2B 3B 4B 5B

6C 7C 8B 9B 10B 11D 12D 13D

Substance

Benzene Ethylbenzene o-Xylene Isopropylbenzene (cumene) Toluene Benzotrifluoride Toluene (reference sample for Run 6C) m-Xylene p-Xylene tert-Butylbenzene tert-Butylbenzene (reference sample for Run 12D) Hemimellitene ( 1,2,3-trimethylbenzene) Mesitylene

Reaction time (hours)

D,

Percentage of D in active aromatic hydrogens

9

k; (hour-')

Active aromatic hydrogens

Retarded reactivity

(X)'

Ionization potential (ev)

1 3.0 6.1 6.7 9.3 13

9.24 8.76 8.55 8.68 8.82 9.68

2 4 4 4 4 2 2

63.0 65.0 71.5 66.0 66.6 77 66.6

52.0 58.5 48.7 42.9 35.1 14.0 16.0

0.87 0.58 0.28 0.26 0.19 0.10 0.14

6 5 4 3-5 5 3 5

4 4 4 24

77 71.5 77 77

27.9 13.7 1.74 47.1

0.11 0.053 0.005 0.039

3 4 3 3

15 33 350

24

77

9.2

0.006

3

2400

24

77

0.00

0.00

0

-

-

m

b

-

8.56 8.44 8.68

-

8.39

"X = k: (benzene)/k; (substance), where k: are rate constants, standardized for different catalyst-reagent ratios and different reaction conditions (A,B,C,D); the latter was achieved by the inclusion of reference samples. c

0 (0

110

J. L. GARNETT AND W. A. SOLLICH-BAUMGARTNER

may be attributed to smaller losses in rcsonance energy. However, the observed trends in adsorption strength together with the difficulties discussed in Section I indicate that chemisorption of this kind is relatively unimportant. Alternative explanations available to the classical theories assume that TABLE I11

Exchange Reactions of Monoha2ogenated Benrenes and Naphthalenes" Reaction Run

Reagents

Quantities (moles x lo2)

_ I

Completion of reaction

Time (hours)

Temp.

("C)

(70)

la

D*O Fluorobenzene

2.5 1.0

46.5

130

100

lb

D*O Chlorobenzeno

2.5 1 .o

46.5

130

66

D,O

1.0 2.5

46.5

130

2-4

Bromobenzene

DZO Iodobenzene

1.o

48.6

130

0.0

2.5

2a

DaO Bromobenzene

7.6 1.5

45

180

8

2b

DaO Iodobenzene

7.6 1.5

45

180

1-2

3a

D*O

7.5 1.5

22

130

100

7.5

22

130

24

3.5

130

75

3.5

130

56

Ic Id

Fluorobenzene 3b

D,O Chlorobenzene

1.6

D*O

7.5

Benzene

3.0

DaO Fluorobenzene

7.5 3.0

5a

D*O 1-Fluoronaphthalene

7.0 1 .o

72

130

100

5b

D*O 1 -Chloronaphthalene

7.0 1 .o

72

130

22

5c

DaO Naphthalene

1.o 1.o

72

130

16

5d

DaO 1-Bromonaphthalene

1.o 1.0

72

130

6

4a 4b

"All reactions performed with 100 mg of prereduced hydrogen-activated catalysts.

?r COMPLEX ADSORPTION IN HYDROGEN EXCHANGE

111

different toxic side reactions accompany the exchange of each aromatic, or that van der Waals adsorption determines the reagent concentration and consequently the exchange rate. The former explanation may be rejected for several reasons : (a) consistency in results of low- and high-temperature reactions

[(iv) and (i), (ii), (iii)]; ( b ) failure to detect toxic products of side reactions; and ( c ) ready randomization reactions of toxic molecules (e.g., naphthalene). An explanation involving van der Waals adsorption is excluded by the absence of a correlation between toxicity and boiling point of the reagent. This is strikingly exemplified in the case of diphenyl and naphthalene, where toxicities differ greatly despite similar boiling points. The observed trends in toxicities of the three characteristic aromatic series 1, 2, 3 may be explained by the factors governing x complex adsorption. It is important t o realize that in toxicity studies the metal orbital factor in T complex adsorption is held constant by confining investigations exclusively t o platinum catalysts. Considering the polycyclic hydrocarbon series, it is realized that individual compounds differ considerably in ionization potential and electron affinity as well as in the symmetries of bonding and antibonding orbitals, whereas steric hindrance effects are negligible in all species. The ionization potential decreases as the number of aromatic rings increases ( 3 4 , i.e., as the aromatic nodal pattern becomes more complicated. At the same time the electron affinity of the aromatic molecule increases (21).Consequently, changes in ionization potential and electron affinity with increasing ring number act in the same direction; i.e., both tend to TABLE IV Relative Adsorption Strength ~~

~~

Compound Benzene Toluene o-Xylene rn-Xylene p -Xylem 1,2,3-trirnethylbenzene 1,3,S-trimethylbsnzene

Relative adsorption strength

_____

1.0 0.8

0.33 0.22 0.16 0.08 0.02

112

J . L. OARNETT AND W. A. SOLLICH-BAUMOARTNER

increase the strength of n complex adsorption. This effect tends to be compensated for by the increased complexity of the nodal pattern of aromatic bonding and antibonding orbitals which decreases the resonance and overlap integrals and consequently the bonding strength through internal cancellation. Thus differences in aromatic adsorption are readily explained by n complex formation; however, because of the two opposing effects in bonding, it is difficult to predict these adsorption trends. The relative importance of the opposing effects may be gauged from the observation (Table I ) that the aromatic toxicity reaches a maximum at naphthalene and then decreases with increasing molecular complexity, indicating that the advantageous effects of the ionization potential and electron affinity are eventually cancelled by the increased nodal complexity of bonding and antibonding orbitals. The results of the alkylbenzene series may also be readily explained in terms of n complex adsorption. I n this series, the molecular orbital symmetry of individual members remains constant while the ionization potential, electron affinity, and steric factors vary. Increased methyl substitution lowers the ionization potential and consequently favors n complex adsorption. However, this is opposed by the accompanying increase in steric hindrance as a result of multiple methyl substitution, and decrease in electron affinity (36).From previous data (Tables I1 and 111) it appears that steric hindrance and the decreased electron affinity supersede the advantageous effects of a decreased ionization potential. The results of Rader and Smith, when interpreted in terms of n complex adsorption, show clearly the effects of steric hindrance, in that relative adsorption strength decreases with increasing size, number, and symmetry of substituents. I n the monohalogenated benzene and naphthalene series (Table IV) steric hindrance due to the substituent is not significantly different for chloro, bromo, and iodo derivatives (van der Waals radii, 1.8A, 1.95A, and 2.15~4,respectively) whereas that due to fluorine ( 1 . 3 5 4 is relatively small in comparison to methyl substitution. The molecular orbital symmetries are identical for individual members of the naphthalene and benzene series, whereas the ionization potential decreases steadily towards the iodo derivative. From these data a simple correlation between toxicity and ionization potential is evident, similar to that reported for the stability of the homogeneous complexes involving silver ions (37); i.e., the exchange rate decreases and the toxicity increases towards the iodo derivative. However, this trend may not be due exclusively to a n electron interaction, but could also be influenced by the decreasing inertness of the halogen substituents which are capable of charge transfer adsorption (26). Fluorine is the only substituent sufficiently

x COMPLEX ADSORPTION IN HYDROGEN EXCHANGE

113

inert as t o render this kind of complex formation negligible. Thus fluorobenzene and fluoronaphthalene possess properties similar to the corresponding methyl derivatives, in that each exhibits a drastic reduction in toxicity, which again demonstrates the importance of steric hindrance in x complex adsorption.

B.

x

COMPLEXMECHANISMS

Since the associative and dissociative x complex substitution mechanisms are not mutually exclusive, both may participate simultaneously in exchange reactions where deuterium oxide is the second reagent. It is therefore of interest to distinguish between the relative importance of these two mechanisms. 1. Exchange between Normal and Deuterated Aromatics Exchange reactions between two aromatics provide a ready test for the dissociative mechanism. This arises from the fact that aromatics cannot exchange by the associative mechanism in the absence of a second substance which provides transient hydrogen atoms by dissociative chemisorption. Consequently, exchange between two aromatics can only proceed by the dissociative T complex substitution mechanism. Evidence for this mechanism is provided by low- and high-temperature exchange (30 and 120") between lOOyodeuterated benzene and normal hour-'; diphenyl (38). Exchange a t 30" occurs at a rate of 2.0 x the exchange rate for the deuterium oxidelbenzene reaction under hour-'. This result shows therefore identical conditions is 7 . 0 x that the dissociative x complex substitution mechanism operates in aromatic exchange. However, one cannot reach a conclusion concerning the relative importance of this mechanism in deuterium oxidelbenzene exchange since the slower reaction rate (29% of total) with diphenyl may be due to the displacement of benzene from the catalyst surface. Water and benzene on the other hand adsorb noncompetitively (16) and consequently do not produce similar rate-determining displacement effects. To ascertain whether the dissociative mechanism is exclusively responsible for deuterium oxidelbenzene exchange, it is necessary t o eliminate reagent displacement effects. This is best achieved by randomization reactions between normal and 100yo deuterated benzene. The mathematical description of these reactions is complicated by nonrandom deuterium incorporation as a result of multiple exchange. Special procedures are necessary to express changes in the mass spectrum of the reaction mixture by a rate constant. Results of these experiments (38) give randomization rates of 5.9 x hour-l, while those of the

114

J. L. GARNET: A N D W. A. SOLLICH-BAUMQARTNER

deuterium oxide/benzene reaction are 8.6 x hour-’. The fact that randomization occurs a t a rate which is only 7 0 % of exchange is readily explained by the catalyst activation effect of a water-benzene reaction mixture (39). It is probable, therefore, that the dissociative TI complex substitution mechanism is the only process whereby aromatic hydrocarbons can undergo significant exchange with deuterium oxide. Extension of these experiments to higher temperatures (120’) gives identical results. Under high-temperature conditions reagent displacement effects are less severe, since benzeneldeuterium oxide and benzene/ diphenyl exchanges occur at approximately equal rates. The importance of the strength of 7r complex adsorption on the reaction rate through the operation of displacement effects is further demonstrated by naphthalene randomization reactions. Naphthalene exchanges very slowly with deuterium oxide. That this is due to the displacement of water by normal naphthalene and not due t o a toxic side reaction, such as polymerization, is shown in randomization experiments with mono or-deuterated naphthalene. Randomization is completed within 24 hours at 120”’whereas no significant deuteration occurs under the same reaction conditions with water. This result furnishes additional proof for the dissociative exchange mechanism. 2. Isotope Effect Once the dissociative mechanism is established, it is possible to apply Gutmann’s theoretical treatment (40) to the elucidation of the ratedetermining step of the exchange reaction. For deuterium-tritium I % , i t may be double labeling procedures, i.e., D 2 0 % loo%, T,O shown that the following normalized equations apply under initial exchange conditions :

(IV) > (V) M (VI). The more precise formulation of the transition complex of 7r complex substitution reactions makes it possible t o write a reaction scheme [Eq. (17)] showing the possible interconnection of a number of hitherto unrelated hydrogenation and exchange mechanisms. Thus the rr-bonded aromatic (a)may react with a metal radical (active site) and undergo exchange (b) via the dissociative rr complex substitution mechanism; alternatively, it may enter the van der Waals layer and be hydrogenated (c) by chemisorbed hydrogen according t o the mechanism proposed by Beeck ( 4 1 ) ,Rideal et al. ( 4 2 , 4 4 , 4 5 ) ,and Farkas and Farkas (14, 15). Finally there is the possibility of the 7r-bonded aromatic reacting with atomic hydrogen to form transition state (d). If the lifetime of (d) is sufficiently long it may react with another hydrogen atom and be hydrogenated (f) by a mechanism similar t o the one proposed by Rooney ( 6 )or even that of Farkas and Farkas ( 1 4 , 1 5 ) ,i.e., by the simultaneous attachment of two hydrogen atoms. Alternatively, exchange may result by the associative rr complex substitution mechanism (e) if the hydrogen atom in transition state (d) becomes detached before another atom can react with the ring. Finally it is possible that species (d) rotates slightly and forms the classical halfhydrogenated state (g)without involving the serious restriction of metal lattice spacings. The function of species (g)in reactions (h) and (i) closely resembles the exchange and hydrogenation mechanisms in the chemisorption layer (41, 42, 44, 45). Thus species (8) may exchange (h) or hydrogenate (i) by the loss or addition of a hydrogen atom.

V. Conclusion It has been shown that the interpretation of catalytic reactions involving group VIII transition metals in terms of 7r complex adsorption possesses considerable advantages over classical theories by providing a link between theoretical parameters and chemical properties of aromatic reagents and catalysts. The concept has led to the formulation of a number of reaction mechanisms. I n heavy water exchange the dissociative 7r complex substitution mechanism appears t o predominate; it could also play a major role when deuterium gas is used as the second reagent. The dissociative mechanism resolves the main difficulties of the classical associative and dissociative theories, in particular the occurrence

120

J. L. OARNETT AND W. A. SOLLICH-BAUMOARTNER

of randomization reactions, different aromatic adsorption strengths, and the faster exchange rates of unsaturated hydrocarbons. ACKNOWLEDGMENTS Thr authors thank the Australian Iristitute of Nuclear Scicnco and Engincering for financial assistance. Acknowledgment is also made to the donors of The Petroleum Research Fund, administered by the Arncrican Chemical Society for support of this research. REFERENCES 1. Garnett, J. L., Nucleonic8 20, 86 (1962). 2. Garnett, J. L., and Sollich, W. A., AustralianJ. Chem. 14, 441 (1961). 3. Garnett, J. L., and Sollich, W. A., AustraZianJ. Chem. 15, 56 (1962). 4 . Garnett, J. L.,and Sollich, A., J . CaCalyeie 2, 350 (1963). 5. Crawford, E., and Kemball, C., Trans. Faraday SOC.58.2452 (1962). 6. Rooney, J. J.,J.Catalpis 2.62 (1963). 7. Bond, G. C., “Catalysis by Metals,” p. 313. Academic Press, New York, 1962. 8. Volter, J., J . CatUl@8 3, 297 (1904). 9. Gault, F. G., Rooney, J. J., and Kemball, C., J . Catalysis 1 , 266 (1962). 10. Barron, Y., Cornet, D., Maire, G., and Gault, F. G., J. Catalysis 2, 162 (1963). 11. Anderson, J. R., and Avery, N. R., J . Cdalysis 2, 642 (1903). 12. Horiuti, J., and Polanyi, M., Nature 132, 819, 931 (1933). 13. Farkas, A., and Farkas, L., Proc. Roy. SOC. A144, 467, 481 (1934). 14. Farkas, A., and Farkas, L., Trana. Furaduy SOC.35, 906 (1939). 15. Farkas, A., and Farkas, L., Trans. Faraday Soc. 36,622 (1940). 16. Taylor, T. I., “Catalysis” (P.H. Emmett, ed.), Vol. V. Reinhold, New York, 1957. 17. Rader. C. P., and Smith, H. A,, J. A m . Chem. SOC. 84, 1443 (1962). 18. Garnett, J. L., Henderson, L.,and Sollich, W. A., “Tritium in the Physical and Biological Sciences,” Vol. 11, p. 47. Intern. Atomic Energy Agency, Vienna, 1962. 19. Lennard-Jones, J. E., Trans. Faraday SOC.28. .333 (1932). 20. Brown, R. D., J. Chem. SOC. p. 2232 (1959). 21. Streitwieser, A., Jr., “Molecular Orbital Theory for Organic Chcmists.” Wiley, New

w.

York, 1961. 22. Melander, L., “Transition State.” Spec. Pu61. Chem. Soc. (London)16. 77 (1962). 23. Mulliken, R. S., J. A m . Chem. Soc. 74, 81 1 (1952). 24. Mulliken, R. S., J . Phys. Chem. 56, 801 (1952). 15. Mulliken, R. S., J . Chem. Phya. 23. 397 (1955). 26. Matsen, F. A.. Makrides. A. C., and Hackrrmann, N., J . Chem. Phys. 22, 11100 (1954). 27. Orgel. L. E.,“ A n Introdurtion to Transition-Metal Chemistry.” Methiten, London. 1960.

28. Chatt. J., and Dunranson, L. A., J . Chem. 9oc. p. 2939 (1953). 29. Mignolet, J. C. P., in “Chernisorption” (W. E. Gamer, rd.). Academic Press, New

York, 1967. 30. Selwood, P. W . , J .Am. Chem. Soc. 79, 3346, 4837. 5391 (1957). 32. Coulson, C. A., Research (London)4, 307 (1951). 37. Nagakura, S . . and Tanaka, J.. Bull. Chem. Soc. Jnpan 32. 731 (19.59).

r COMPLEX ADSORPTION IN HYDROGEN EXCHANGE

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33. Eley, D. D., Advan. Catalysis 1, 185 (1948). 34. Hanner, 2. K., Acta Chem. Scand. 10, 655 (1956). 35. Watanabe, K., Nakayoma, T., and Mottl, J., A Final Report on Ionization Potentials of Molecules by a Photoionization Method. Contrib. No. DA-04-200-ORD.480and 737. Univ. of Hawaii, Honolulu, 1959. 36. Briegleb, G., Angew. Chem. 76, 326 (1964). 37. Andrew, L. T., and Keefer, R. M., J . A m . Chem. SOC.72, 3113, 5034 (1950). 38. Garnett, J. L., and Sollich, W. A,, J . Phys. Chern. 68, 3177 (1964). 39. Garnett, J. L., and Sollich, W. A., Australian J. Chem. in press. 40. Gutmann, J. R., Intern. J . Appl. Radiation Isotopes 7, 186 (1960). 41. Beeck, 0.. Discussions Faradaysoc. 8, 118 (1950). 42. Jenkins, G. I., and Rideal, E. K., J. Chem. SOC.p. 2490 (1955). 43. Lauer, W. M., and Stedman, G., J . A m . Chem. SOC.80,6433 (1958). 44. Baker, M. M., Jenkins, G. I., and Rideal, E. K., Trans. Paraday Soc. 51, 1592 (1955). 45. Baker, M. M., and Rideal, E. K., Trans. Faraday Soc. 51. 1597 (1955).


Stereochemistry and the Mechanism of Hydrogenation of Unsaturated Hydrocarbons SAMUEL SIEGEL Department

0;

Chemistry. University of Arkansas. Fayetteville. Arkansas

.

I Introduction ....................................................... I1. The Development of Some Stereochemical Concepts ......................

Pagr 124

125 A . The Geometry of Adsorption .................................... 125 B . The Structure of Chemisorbe maturated Hydrocarbons . . . . . . . . . . . . . 129 131 C. The Immediate Source of Hydrogen ................................

I11. Variations in Stereochemistry as a Criterion of Mechanism . . . . . . . . . . . . . . . . 132 ....................... 133 A . Branching Reaction Paths . B . Consecutive Reactions . . . . . ....................... 135 C . The Dissociative Mechanism ation . . . . . . . . . . . . . . . . . . 140 I V . Conformational Analysis and the Geometry of the Pertinent Transition States in the Hydrogenation of Cycloalkenes ............................ 144 A . Interactions with the Catalytic Site or Surface ........................ 145 B Geometry ofAdsorbed Alkenes .................................... 146 C . The Geometry of the Transition State for Adsorption . . . . . . . . . . . . . . . . . . 148 D . The Geometry of the Transition State for the Formation of the “HalfHydrogenated State” ........................................ 150 V . The Reaction of Aromatic Hydrocarbons with Hydrogen . . . . . . . . . . . . . . . . . . 151 A . Geometrical Considerations of the Mechanism for Exchange ............ 153 B . The Stereochemistry of Addition of Hydrogen to Aromatic Compounds .... 155 C. The Detection of Cyclohexene Intermediates ......................... 157 VI . Hydrogenation of Multiply Unsaturated Hydrocarbons . . . . . . . . . . . . . . . . . . . 160 A . Acetylenes ...................................................... 160 B. Allenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 C. 1,3.Dienes-Evidence for Allylic Intermediates . . . . . . . . . . . . . . . . . . . 162 D . The Competitive Hydrogenation of 1,3.Dienes and Acetylenes . . . . . . . . . . . 164 V I I . Some General Mechanistic Considerations ............................... 167 A . The Principle of Minimum Structural Change ........................ 167 B. Surface Sites and the Transition Metal Complex Analog . . . . . . . . . . . . . . . . 168 C . Geometrical Details ............................................... 169 D . Formation and Stability of Unsaturated Surface Complexes . . . . . . . . . . . . 171 E . Reactions of n-Ally1Complexes .................................... 172 References ......................................................... 174

.

123

124

SAMUEL SIEOEL

1. introduction The successful mechanism for a reaction is a theory that correlates the many facts which have been discovered and is fruitful for the prediction of new experiments (I).One approach t o mechanism is the study of stereochemistry which seeks information concerning the geometrical relationships between the reactants a t the critical stages in the reaction. Information is gleaned from the examination of the products, if several isomers differing only in configuration may be formed, or from a study of the reactivity of closely related substances whose molecular shapes are varied in a specific manner. Occasionally a stereochemical fact places a considerable restraint upon the allowable mechanistic postulates, but the most effective employment of stereochemistry generally depends upon its detailed correlation with other experimental methods. This review is concerned mainly with the development of the concepts and methods pertinent to the application of stereochemistry t o the study of the mechanism of the surface-catalyzed reactions of hydrogen (or deuterium) with unsaturated hydrocarbons. Although much of the literature concerning these reactions has been carefully reviewed by Taylor (Z), Smith (3),Bond ( 4 ) ,and Bond and Wells ( 5 ) ,and the stereochemistry has been reviewed by Burwell (6, 7), additional attention t o the significance of the stereochemical evidence seemed in order. I n comparison with its role in clarifying the mechanism of organic reactions, stereochemistry has been of minor service in heterogeneous catalysis. However, as the result of improved instrumental methods for the separation and analysis of complex mixtures, such studies have begun t o make more significant contributions. The use of several different experimental techniques t o investigate a given system is likely to be particularly revealing. Two noteworthy examples in which stereochemical considerations had a part are provided by Taylor and Dibeler (8)on the reactions of deuterium with the butenes on nickel wires and by Meyer and Burwell (9, 10) on the deuteration of multiply unsaturated hydrocarbons. Although reference is made t o other kinds of information pertinent t o the mechanistic problems which are considered, the discussion emphasizes the stereochemical contributions t o mechanism. The recent review of the “Mechanism of the Hydrogenation of Unsaturated Hydrocarbons on Transition Metal Catalysts” by Bond and Wells ( 5 )may be consulted for a more detailed analysis of the kinetic and exchange data which are available as well as a briefer, and in some respects different, evaluation of the stereochemistry of these reactions. The literature has not been searched exhaustively, the examples

STEREOCHEMISTRY AND HYDROGENATION MECHANISM

125

having been chosen as illustrative of the application of stereochemistry in several current problems under the general heading of this review.

11. The Development of Some Stereochemical Concepts A.

THE GEOMETRYOF ADSORPTION

Explanations of the stereochemistry of hydrogenation have been dominated by ideas concerning the manner in which a given unsaturated compound may best be fitted onto a planar surface from which hydrogen is abstracted. Cis addition is readily understood in such terms. The formation of meso-dimethylsuccinic acid from dimethylmaleic acid and the racemic mixture from dimethylfumaric acid implies that both hydrogen atoms add t o the same side of the unsaturated molecule (11). Bourguel (12) also noted that disubstituted acetylenes yielded initially cis-ethylenes but that trans isomers were formed if the hydrogenations were protracted. HSC\

/CHJ

F=?

t.

HSC&""

H

CO,H CO,H

cis

meso

trans

vacemic

R-CSC-R

-

R

R

H

H

,c=c, \ cis

Following their studies of the catalytic hydrogenation of benzene and its exchange with deuterium, Farkas and Farkas (13) stated that the above stereochemical facts were consistent with their proposal that the hydrogenation reaction involved the simultaneous addition of two atoms of hydrogen to a double bond, the hydrogen coming from the catalyst. Greenhalgh and Polanyi ( 1 4 ) showed, however, that a stepwise addition of hydrogen, in which the configuration on carbon is retained at each stage of the reaction, can also account for cis addition. Clearly, the particular stereochemical facts given above allow for a variety of mechanistic

SAMUEL SIEGEL

126

interpretations; but as with other experimental approaches, more detailed information places added restraints upon the allowable hypotheses. To account for the formation of principally cis-2-alkylcyclohexanols from 2-alkylcyclohexanones, Vavon ( 1 5 )suggested that steric hindrance between the substituent and the catalyst directed the attack of hydrogen on the carbonyl group from the side away from the alkyl group. However, the concept of hindrance between catalyst and substrate was more clearly developed by Linstead et al. (16) who asserted that it operated at the adsorption stage of the reaction. They established that the hydrogenation of derivatives of cis-as-octahydrophenanthrene (I) yielded mainly cis-syn-cis-perhydrophenanthrenes(11) and similarly diphenic acid (111) or cis-hexahydrodiphenic acid (IV) gave principally cis-syncis-perhydrodiphenic acid (V).*

(I) *

The concept of catalyst hindrance is illustrated by reference to the hydrogenation of cis-as-octahydrophenanthrene(I) (Fig. 1). If the aromatic ring, lying with its face parallel to the surface, anchors the molecule t o the catalyst, two arrangements are possible. In one, ring A is inclined away from the surface, and the hydrogen atom a t C-13 is directed toward it. If hydrogen adds from the underside of the molecule, the hydrogen which becomes attached a t C-12 is on the same side as that a t C-13 and therefore yields the syn arrangement of the cycles. The opposite situation holds if the molecule is adsorbed so that ring A is directed towards the surface; the addition of hydrogen at C- 12 now yields the anti arrangement with regard to C-13. That the first manner of adsorption is preferred over the second is a reasonable assumption; consequently, the *A heavy dot at a ring junction symbolizes that the hydrogen atom is on the side of the molecule which faces the viewer (17).


127

--Catalyst- Preferred adsorption: Ring A cleor of cotolyst.

Hlndronce between catalyst and ring A

FIG.1 . Catalyst hindrance according to Linstead el al. ( 1 6 ) :reproduced with permission of the publishers.

products of szp hydrogenation should and do preponderate. The same argument may be applied to the diphenic acids assuming that they are adsorbed in a conformation which brings the carboxylic acid groups together. The development of theories which allow the estimation of the relative energies of the various conformations of molecules, a classic paper being that of Beckett et al. (18),was applied by Siege1 (19) to a rationalization of the stereochemistry of hydrogenation of substituted cyclohexanones. Stereoselective hydrogenation of a ketone (platinum oxide in acetic acid) was assumed to be a consequence of the adsorption of the ketone onto the catalyst in a conformation which minimized the nonbonded interactions between the surface and the cycle, while the substituents a t the 2 , 3 , and 4 carbon atoms tended t o be equatorial (Fig. 2). The contrasting stereochemical behavior on reduction of cholestanone and coprostanone (20) was explained by reference to these conclusions. Accordingly, the stereospecificity of the reaction does not arise simply from steric hindrance between catalyst and substituent but rather from the steric interactions

R - equoloriol

R - axial

FIG.2. Alternative conformations of a 4-substituted cyclohexanone.

128

SAMUEL SIECEL

between the substituent and the atoms of the cycle combined with the requirement of a precise orientation of the carbonyl group on the catalyst. Earlier Balandin (21)in his “multiplet hypothesis” had suggested that there is a necessary relationship between the geometry of molecules and the distribution of at80miccenters on a surface which catalyzes their transformations. Although this concept has stereochemical consequences, the theory was not elaborated in this direction. More recently, Balandin and Klabunovskii ( 2 2 )aiid Balandin (23)have described the hydrogenation of certain derivatives of tryptycene whose geometry prevents their reduction a t a planar surface. From these results they deduce that the catalytic sites reside on eminences or peaks. Again the deductions stem from considerations of the manner in which one may bring together the catalyst aiid substrate, the geometry of the latter being as i t is in the unassociated condition. The possibility that the stercochemistry of hydrogenation might be established a t stages in the reaction other than adsorption was considered by Siege1 and Dunkel (21). However, from their data on the hydrogenation of a group of dimethylcyclohexenes, they concluded that the geometry of the organic moiety a t the product-controlling transition state was like that of the cycloolefin in its most stable conformation. A similar conclusion was reached by Robinson (25) from studies of the hydrogenation of steroids having a double bond in the B ring. Hadler (26)employed conformational analysis t o explain the difference in the proportion of cholestane to coprostane derivatives resulting from the reduction of A* and A5 steroids. He suggested that the hydrogenation process involved the formation of a quasi-ring structure between the unsaturated carbon atoms and two hydrogens originally dissolved in the metal, a mechanism which is similar to one proposed by Beeck (27)and by Jenkins and Rideal(28). He assumed, in effect, that the saturated struc-

ture of the product was fully developed in the transition state for the addition of hydrogen t o the double bond. [For a discussion of the application of this theory t o the hydrogenation of steroids see Fieser and Fieser (29).]More recently Sauvage et a2. (30)proposed that the proportion of &/trans isomers obtained upon hydrogenating various dialkylcyclohexenes could be understood if one assumed that the cycle was adsorbed in a pseudo-boat conformation. It was necessary to assume further that a small substituent a t the 4 position would tend t o occupy the


129

ando rather than the ex0 configuration, but with increasing size the exo configuration is adopted (Fig. 3).

exo- R,

endo- R,

FIG.3. Alterriative boat conformations of a 4-substituted 1 -methylcyclohexene.

The concept that the stereochemistry of reduction of an unsaturated hydrocarbon is determined a t the adsorption stage of the reaction ia evident in each of the above accounts, and others could be cited. However, the development of techniques which permit the identification of different product-controlling reactions directs one to consider the stereochemical consequences of various postulated reaction sequences and this will be discussed in Section IV.

B. THESTRUCTURE OF CHEMISORBEDUNSATURATED HYDROCARBONS

There is little direct evidence concerning the structure of olefins which are adsorbed upon metallic catalysts so that the concepts of these structures have been developed through inference and analogy. Twigg and Rideal(31)assumed that, on nickel, an associatively adsorbed alkene has a saturated structure which is joined by covalent carbon-metal bonds t o two appropriately spaced atoms on the surface. Using reasonable values for the bond distances, they showed that ethylene covered the surface more completely than did methyl-substituted ethylenes. The uncovered nickel atoms provided space for the adsorption of hydrogen, Accordingly, the fact that the ortho-para hydrogen conversion was inhibited much more effectively by ethylene than substituted ethylenes was readily explained. Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica (32).When introduced t o a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C-H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p ) and a deformation associated with a methylene group (6.9 p). A weak band a t 3.3 p was attributed to an olefinic C-H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of

130

SAMUEL SIEOEL

the chemisorbed ethylene has a saturated structure, although if the pressure of hydrogen is sufficiently low, or the temperature high, unsaturated structures are produced. However, the unsaturation may be associated with either 1,2-diadsorbed ethylene or a n-complexed olefin. H H HI\C-~A-I /

/

Ni

\

Nt

H

/

/c=c Nt

n~

H

/ \

Nt Ni

1,a-Diadsorbed ethane

1,2-Diadsorbed ethylene

-

n Cornplexed ethylene

Various authors have suggested that olefins and other unsaturated hydrocarbons may form n complexes by associating with a single atomic center of the catalyst. These structures are assumed t o be analogous t o the n olefin complexes (33,34)of the transition elements which have been the subject of recent intensivc investigations (35). For example, Fukushima and Gallagher (36)discussed the reaction of deuterium with cholesterol in terms of the Horiuti-Polanyi mechanism for the addition and exchange of olefins but proposed that the adsorbed alkenes were n complexes in analogy to the structure of the platinumolefin complexes described by Chatt (33).Similarly, Burwell et al. (37) recognized that this relationship might apply to the reactions of hydrocarbons and deuterium on chromium oxide gel. n complexes, held t o the surface in a manner similar to the bonding in compounds such as ferrocene, were believed by Rooney et al. (38)to provide an explanation for the patterns of exchange with deuterium exhibited by a number of polymethylcyclopentanes. I n particular, they suggested that such complexes could unite with hydrogen from the gaseous or physically adsorbed phase as well as with hydrogen chemisorbed on the surface. I n support of the concept that acetylenes form n complexes with a single surface atom of the catalyst, McQuillin et al. (39) have cited the parellelism between the effect of substances such as amines and phosphines as inhibitors for the hydrogenation of butynediols to butenediols on a palladium catalyst with the ability of these same substances t o form complexes with metals of the class to which palladium belongs (40). According to Dewar (41) the metal-to-olefin bond in such complexes consists in part of the overlap of the z-electron density of the olefin with a a-type acceptor orbital of the metal atom and in part of the backdonation of electrons from filled metal d,, or other dn-pn hybrid oribitals into the antibonding orbitals on the carbon atoms. In a compound such as the ion of Zeise’s salt, [(C,H,)PtCI,I-, the plane of the double bond is approximately perpendicular to the plane contain-

STEREOCHEMISTRY AND HYDRUGENATIUN MEC‘HANISM

131

iiig the platinum and three chlorine atoms which allows for n b o d i n g with the metal ( 4 2 ) (Fig. 4). Because the presence of the metal can cause a mixing of the u and 71 orbitals of the olefin, the C-H bonds will bend out of the plane containing the carbon atoms and away from the metal (33). Similarly, 71 bonding between a metal atom and more unsaturated structures such as acetylenes and dienes results in stable structures (43).The chemistry of these substances may be expected to be analogous to that of the organometallic compounds which are undoubtedly formed on catalytic surfaces; consequently information gained in the study of organometallic compounds in homogeneous systems may be applied to the study of hetero-

Q-

Type bond

tan -Type bond

FIG.4. Orbitals usocl in tho coiiibiiitltioii OF othyloiio with pliitiiiuiii uiiil the spucicrl tlrrangemerit of atoms iu LC!,H,PtCl,] ( 3 4 ) . Hoprotlucecl in modified forin by porrrtission of the publishers. Coates, “Organo-Metallic Compounds.” Wiley (35).

geneous catalysis. The catalytic activation of hydrogen has been discussed by Halpern ( 4 4 )with this in mind, and the review by Bond and Wells ( 5 ) considers these analogies for the hydrogenation of unsaturated hydrocarbons. The similarity between heterogeneous and homogeneous catalytic hydrogenation has been emphasized recently by the demonstration that platinum-tin complexes (45)catalyze the homogeneous hydrogenation of acetylene and ethylene a t room temperature and certain complexes of rhodium (46) are effective for the hydrogenation of 1-hexene. Earlier, Halpern et al. ( 4 7 ) showed that chlororuthenate (11) complexes catalyze the homogeneous hydrogenation of maleic, fumaric, and acrylic acids although simple olefins could not be reduced.

C. THEIMMEDIATE SOURCE OF HYDMWBN Although the mass of evidence indicates that the two atoms of hydrogen which add to a double bond do so in the cis sense, the direction of approach to the hydrogen acceptor is deduced from indirect argu-

132

SAMUEL S I E G I ~ L

meiits. Assuming, for example, that 1 , 2 - t l i t ~ i e t t i y l ~ y ~ l o ~ is ~ ~ad11t~11e sorbed on a surface in the manlier indicated i n the prcvious section, addition of hydrogen from the dircction of thc gas phase, as iii an Eley-Rideal mechanism (48, 49), or from the surface, as postulated by Farkas and Farkas (13) or Horiuti and Polanyi (50),lcads to the samc result. However, in those instances in which different stercoisoniers may form if the hydrogen adds from the alternative sides of the molecule, the main product is the one corresponding to addition from the least sterically hindered side (51).An interesting example has becn noted by Burwell ( 6 ) ; trans-cyclononene hydrogenates readily uudcr mild conditiotis although the conformation of the cycle completely blocks oiic side of the double bond to the attack of any external reagent. Because either the olefin or hydrogen must be adsorbcd oil the catalyst, the hydrogcn must add from the direction of the surface. The precise state of hydrogen which is chemisorbcd on active metallic catalysts is a continuing subject for investigation (52). l’liskin and Eischens (53) interpreted the infrared spectra of hydrogen or deutrrium adsorbed on alumitia- and silica-supported platinum catalysts as implying that two forms of chemisorbcd hydrogen are present-a weakly and a strongly bound species. They proposed that the strongly bound form is attached to two platinum atoms and the weakly bound hydrogen to one. The analysis of such data leans heavily on analogy with the properties of complex hydridcs of the transition metals ( 5 4 ) ; consequently, the clarification of the structure and properties of the latter should aid the characterization of chemisorbed hydrogen. Other cxperimeiital nicthods provide information about the condition of adsorbed hydrogen but direct structural data is lacking. It seems unlikely that “dissolved” hydrogen is directly involved in the mechanism of hydrogenation; indeed palladium, which dissolves hydrogen readily, becomes inactive for hydrogenation if thc pressure is too high (55).

111. Variations in Stereochemistry as a Criterion of Mechanism Studies on the variatioti in the distribution of stereoisomeric prodrrcts as a function of the accessible independent variablw permit deductions concerning the multiplicity of the reaction paths, A classic examplc is found i n Hughes’ and Ingold’s studies into the mechanism of the solvolysis of alkyl halides in aqueous-alcoholic media, an important variable, being the concentration of base or other nucleophilic reagent (56).The obvious variable in hydrogenatioii studies is the pressure of


133

hydrogen and the resulting changes in the distribution of product aid in defining the product-controlling step.

A. BRANCHING REACTIONPATHS The stereochemistry of hydrogenation of 1,2-dimethylcycl~hexene and 1,2-dimethylcyclopenteneis instructive. Each of these substances would be expected to yield only the cis-1,2-dimethylcycloalkanevia cis addition. Both cis and trans isomers, however, are formed from either of these two cycloalkenes when hydrogenated in the liquid phase (acetic acid) over reduced platinum oxide-one of the more stereoselective catalysts ( 5 7 , 5 8 ) .The ratio of isomers which is produced is a function of the pressure of hydrogen, the proportion of cis increasing with increasing pressure (Fig. 5 ) . This fact implies that the trans isomer is formed via a

40

i I

0.2

0.5

I0

I

so

100

so0

PRESSURE OF HYDROGEN (Atm )

FIG.5. Variation with the pressure of hydrogen of the proportion of cis- and transdiinethylcycloalkanes obtained from 1,2-dimethylcyclohexene 2,3-dimethylcyclo1,2-dimethylcyclopenterie and 2,3-dimethylcyclopentene (8); reduced hexene (A), PtO, in glacial acetic acid at 2.5".

(a),

(a),

reaction path.which has a lower dependence upon the pressure of hydrogen than that path which yields only the cis isomer. The Horiuti and Polanyi mechanism (50)accommodates this result and i t is convenient to discuss the stereochemistry of hydrogenation by reference to their formulation which follows.

134

SAMUEL SIEGEL

Hydrogen is assumed to dissociate upon adsorption on the surface, reaction ( l ) , H H, 4 2* + 2 1

(* represents a surface site)

(1)

and the adsorption of the olefin requires two sites, reaction (2),

The remaining steps consist in the union of the adsorbed olefin and its derivative, the “half-hydrogenated state,” with an adsorbed hydrogen atom, reactions which occur with retention of configuration of the substituted carbon atom, reactions ( 3 ) and (4),

-C-L-I

?

+ Hk-l 1 c-c-IT + 2* ~-

*

b.-,

1 1 H H

(4)

The last reaction is effectively irreversible under the usuti, conditions employed to hydrogenate olefins; however much information pertinent to this discussion has been obtained by studies of the exchange of saturated hydrocarbons with deuterium (7, 59), a reaction which is initiated through the reversal of reaction (4). The reverse of reactions (2) and (3) can yield an olefin isomeric with the initial substrate if the hydrogen atom abstracted in (3) is different from the one originally added. Thus, to explain the formation of transdimethylcycloalkane, the 1,2-dimethylcycloalkene must be transformed to the 2,3 isomer which yields both cis and trans saturated products via cis addition (Fig. 6).

FIQ. 6. Reduction and isomerization of 1,2-dimethylcyclopenteneaccording to the Horiuti-Polanyi mechanism.


135

Although this mechanism is plausible, the first studies failed to detect the isomeric olefin, but the fact that the 2,3 isomers were selectively reduced in the presence of their precursors was noted ( 5 7 , 5 8 ) .Recently, the postulated olefinic intermediates were isolated and it is clear that isomerization is the main if not exclusive path leading to the formation of the trans-dimethylcycloalkanes(60) (Fig. 7). Similarly Smith and Burwell (61)concluded that desorbed ~Il,~-octalin is a common intermediate in the formation of cis- and trans-decalin from the hydrogenation, or deuteration, of 9,lO-octalin although they suggest that isomerization probably proceeds via the formation of a hydrogendeficient species, possibly an allyl-x complex (see Section 111,C). Huntsman et al. (62)hydrogenated optically active ( - )-3,7-dimethyl1-octeneon a palladium catalyst and obtained a saturated product which was in part racemic. When the reaction was interrupted (50% completion) the 2,6-dimethyloctane isolated was only 7 yo racemized, CHS

CH,

I ( - )-CHsCH-CH&HaCH&*-CH I I

(

1

IH1

H

+ )-2,6-dimethyloctane

CH,

CHS

I

I

= CHa + CHsCHCHaCH&HaC=CHCH,

1

[HI

( +_ )-2,B.dimethyloctane

although the residual olefin was entirely the inactive 3,7-dimethyl2-octene. The addition of alkali to the reaction mixture caused a reduction in the rate of hydrogenation as well as the rate of double bond migration; however, only the terminal double bond could be hydrogenated under these conditions. The resulting saturated alkane was not racemized, which supported the inference that racemic hydrocarbon came only from the inactive olefin.

B. CONSECUTIVEREACTIONS A different stereochemical pattern is evident in the hydrogenation over a platinum catalyst of 2-, 3-, or 4-alkyl-substituted methylene. these compounds increasing, the pressure cyclohexanes ( 5 7 , 6 3 , 6 4 )With of hydrogen decreases the proportion of the more unstable saturated isomer in the product (cis-1,2-,trans- 1,3-, or cis-1,4-dialkylcyclohexane), a result which is not consistent with a mechanism involving an isomerization to an olefin which yields a proportion of cis and trans isomers different from that given by the methylenecyclohexane. For such a mechanism implies that the hypothetical olefin would yield a larger portion of the more unstable saturated isomer than is obtained from the initial reactant.

136

SAMUEL SIEQEL

I

I

I

I

I

I

1

2

3

4

5

b

Per cent reduclion

FIG.7. The formation of 2,3-dimethylcyclopentene(0) as well as ria-(.) and / r a m (A) 1,2-dimcthyIcyclopcntane during the hydrogcnation of 1,2-cliniethylcyclopentenr; PtO, (60). 10 86-

4-

3v)

C

e :2 k?

u

I -

-

A

A

FIG.8. Variation, as a function of pressure, of the proportion of saturated stereoisomers formed during the hydrogenation of 4-lerl-butylmethylenecyclohexane and 4-feithutyl-I-methylcyclohexene ( A ) ; PtO, ( 6 4 ) .

(a)


137

A particularly clear example is shown in Fig. 8, the hydrogenation of 4-tert-butylmethylenecyclohexane(64).I n this instance, the most likely initial product of isomerization is the endocyclic isomer, 4-tert-butyl-lmethylcyclohexene, which would yield mainly trans-4-tert-butyl-lmethylcyclohexane. Further, the exocyclic olefin, the original substrate, is reduced selectively in competition with its endocyclic isomer so that the latter cannot be an intermediate in the hydrogenation of the former. The Horiuti-Polanyi mechanism can account for the change in cisltrans ratio because the product-controlling step in the given sequence can be altered by varying the pressure, and different product-controlling steps could lead to different ratios of saturated stereoisomeric products, the difference depending upon the nature of the required geometries of the respective transition states. Consider how the relative rates of reactions (2), (3)) and (4)depend upon the pressure of hydrogen if the concentration of hydrogen on t h e surface

is a function of the hydrogen pressure. Whatever the nature of this function, the rate of reaction (2) is independent of the pressure of hydrogen and reaction (4) will increase faster than (3) because its rate is a function of the concentration of the half-hydrogenated state (which itself is a function of the pressure of hydrogen) as well as the surface concentration of hydrogen. Several limiting situations can be imagined. 1. The Reduction of the “Half-Hydrogenated State” is Product Controlling

If the addition of the second hydrogen atom is the rate-controlling surface reaction, then the preceding steps would tend to be reversed, the degree of reversibility being a function of the relative rates of the several reactions. Two effects are expected: (1) the isomerization of the initial olefin is pronounced and (2) the proportion of saturated products should tend towards the equilibrium distribution. Indeed, such effects are commonly observed when palladium catalysts are employed (5, 65, 66) (Fig. 9). I n the reactions so far studied, the formation of an equilibrium distribution of olefins is more readily attained than is an equilibrium distribution of saturated products. But, if two or more isomeric olefins may be formed from a given substrate, the initial distribution may be far from the equilibrium value, as illustrated in Fig. 9 for the hydrogenation on palladium of 2-methylmethylenecyclohexane. This distribution must in part be a consequence of the manner in which the half-hydrogenated

138

SAMUEL SIEOEL

PER CENT

REDUCTION

FIG.9. The distribution of products as a function of the extent of the hydrogenation of 2-methylmethylenecyclohexaneOIL a palladium catalyst; Pd/C (65).

state, formed in reaction (3), reverts to olefin and in part of the quite different competive rates of hydrogenation of the several olefins which decrease in the order 2-methylmethylenecyclohexane > 2,3-dimethylcyclohexene > 1,2-dimethylcyclohexene. Clearly this example is not a truly limiting case for the mechanistic category under disucssion ; however, one might be attained by the use of other substrates, catalysts ( 5 ) ) or lower pressures of hydrogen. 2 . The Formation of the “Iia2f-Hydrogenated State” i s Product Controlling Two other liillitJingconditions can be defined by the use of a formalized

argument (61)basec upoii a simple steady state analysis of the HoriutiPolanyi mechanism. Let OH, 8,, arid O,, represent the fraction of the accessible surface which is covered by hydrogen, alkene, arid “half-hydrogenated state,” respectively. Asuperscript t (e“,, Oit) ki,etc.) will refer t o the adsorbedspecies, which by further abstraction of hydrogen from the surface would lead to the trans-dialkylcyclohexane,and a superscript c to the related cisforming species. Because the suggested pathways leading to the respective cis and trans


139

isomers separate a t the act of adsorption, the rate of formation of each isomer can be written independently as follows (the constants k,, k- , etc., are defined on page 134) :

d[cisl - k;.p,.e, dt

(5)

and the steady state assumption gives Eqs. ( 6 ) and (7))

dt dt

=

ki[E] - kC_,.pE- k;.&.B,

=

&.&.OH

-

+ kT30;

=

0

kF3.PK - k",p8",.O, = 0

(6) (7)

which together yield Eq. (8)) the rate of formation of the cis isomer,

An equation, identical in form, is obtained for the rate of formation of the trans isomer. The assumption that the rate-limiting surface reaction is the formation of the "half-hydrogenated" state [reaction (3)] provides the condition that kC_31ki 1 and, consequently, leads to the approximate expression (9): d[cis] - kik;[E] B, (9) dt k?, G-8,

k3.BlI,e.g., the pressure of hydrogen is sufficiently low, then the equation reduces to (1 1): [cis] _ - k t % g 2 = k; [trans] k",k3$

Kc Kt

where K c and K *are the equilibrium constants for the adsorption of the alkene t o form, respectively, the cis or trans diadsorbed alkane (or n-complexed alkene). Under these conditions, the ratio of isomeric alkanes is determined by the ratio of the rate constants for the formation of the respective "half-hydrogenated states" from the alkene.

140

SAMUEL SIEOEL

3. The Adsorption of the Alkene is Product Controlling At high pressures of hydrogen, it is probable that k , - 9 , > k - , and accordingly the cis to trans ratio would approach k:lki, the ratio of the rate constants for the two modes of adsorption of the alkene. Under these conditions, the reaction consists of the consecutive addition to the double bond of two atoms of hydrogen. Whether the limiting conditions are actually attained under practical operating procedures is not certain, although the stereochemical evidence suggest that it is approached. Other forms of evidence such as experiments with deuterium are needed to answer such questions.

C. THE DISSOCIATIVE MECHANISMFOR OLEFINISOMERIZATION The preceding analysis of the mechanism of the hydrogenation of olefins, though based upon the Horiuti-Polanyi formulation, is also consistent with other alternatives. Indeed it is only by intercomparisons among various forms of evidence that a truly satisfactory theory for these reactions can be evolved. The explanation of the stereochemistry of the hydrogenation of 1,2-dimethylcycloalkenesis centered on the idea of the existence of two competitive reaction paths, the one which yields the cis-dimethylcycloalkane having a dependency of higher order with respect t o the pressure of hydrogen than the other which provides the trans isomer. Although the alternative reactions of the “halfhydrogenated state” satisfy this requirement, a dissociative mechanism for isomerization as suggested by Farkas and Farkas (13)which accompanies the addition reaction is also consistent with the stereochemistry and has experimental support. 1 . Butenes

Taylor and Dibeler ( 8 ) , among the first to apply mass spectrometric methods to the study of the interaction of deuterium with olefins, used this technique as well as infrared spectrometry to study the exchange, addition, double bond migration, and cis-trans isomerization reactions of 1- and 2-butenes on a nickel wire. Hydrogen or deuterium is required for the isomerization of 1-butene to cis- and trans-2-butene or for the interconversion of the cis and trans isomers. Unless hydrogen (or deuterium) is admitted to the reaction chamber first, the rates are not reproducible. At pressures of about 10 cm for each reactant, the rate of addition of hydrogen to 1-butene and its isomerization were about equal a t 60’ but a t 125” the double bond migration was about 2.5 times faster. Because the activation energy for double bond migration is grcater


141

than for hydrogenation, the reactions were assumed to have different rate-controlling steps. Both reactions show a kinetic isotope effect: a t 60' the rate of double bond migration is four times faster in the presence of hydrogen than in the presence of deuterium; a t 130" the rates differ by a factor of 2.5; therefore, a hydrogen (or deuterium) bond must be broken in the rate-controlling step in both reactions. I n the presence of deuterium the initial rate of double bond migration for 1-butene was approximately the same as the initial rate of exchange. Apparently all the above reactions of 1-butene have the same pressure dependencies, i.e., proportional to the square root of both the initial butene pressure and the initial hydrogen or deuterium pressure. The Horiuti-Polanyi mechanism is in accord with the above data. For example, if the formation of the half-hydrogenated state is rate controlling, the rate of both hydrogenation and double bond migration should exhibit a kinetic hydrogen isotope effect and the rates should be proportional to the one-half power of the pressure of hydrogen. The relative rates of isomerization and hydrogenation depend upon different reactions of the half-hydrogenated state, and therefore the activation energies may differ. It is unnecessary t o assume that the two reactions have different rate-controlling steps. However, the exchanged butenes were not highly deuterated and followed a different distribution pattern than the deuterobutanes; consequently, Taylor and Dibeler (8) suggested that the exchange and addition reactions probably occur on different types of sites and by quite different mechanisms. Accordingly, they postulated that the exchange and double bond migration proceeded via an allylic complex : CH,=CH-CH,CHS

-t

D I

f-*-*

-

CHI-CH-CH-CH3

-

H

CH,D-CH=CH-CH,

+

D H

I

:

I--*

the formation of the complex being aided by ally1 radical resonance and the tendency of hydrogen to form a metal-to-hydrogen bond. They suggested that the same intermediate could yield the other products also. Although Taylor and Dibeler (8)found that the isomerization of cis- to trans-2-butene required either hydrogen or deuterium, the reaction showed no kinetic isotope effect. The isomerization was eight to ten times faster than exchange and three t o five times faster than hydrogenation a t 75'. The lack of an isotope effect could be explained by the associative mechanism if the loss of hydrogen from the half-hydrogenated state were rate controlling, because to obtain the trans from the cis isomer the hydrogen removed in this step must be different from that added in the

*-* I

142

SAMUEL SIECfEL

preceding elementary reaction ( 5 ) . However, the exchange and isomerization reactions should then proceed a t the same rate (Fig. 10).

FIQ.10. Isomerization and exchange of cis-2-buteno according to the associative mechanism.

The dissociative mechanism can explain both facts in that the hydrogen removed in the first step may recombine with a n isomeric form of the allylic intermediate to yield the isomeric olefin. Apparently syn and anti x-allylic complexes (67, 68) retain their configurations unless each may be converted into a common a-bonded complex in which the nonterminal carbon atoms of the allyl group are connected by a single bond and the isomerization of the intermediate can be represented as in Fig. 1 I . However, the recombination of the hydrogen atom with the allylic intermediate must be faster than the rate at which it enters the surface pool of

syn

anti

Fro. 1 1 . The interconversion of ,Pyn and anti s-ally1 complexes via a U-bonded allyl structure.

hydrogen and deuterium atoms. Taylor and Dibeler also suggested that the presence of hydrogen or deuterium might be required to reduce the extent of strong two-point adsorption of the double bond. and thus permit the olefin to dissociate on a bare site. 2. Octalins

Smith and Burwell (61) showed that the reaction of deuterium with A gJO-octalin, which is catalyzed by reduced platinum oxide, yields a mixture of cis- and trans-decalins containing an average of nearly three deuterium atoms per molecule. Under the same conditions ~I'*~-octalin gave somewhat less of the cis isomer but the saturated products contained an average of two deuterium atoms per molecule. From a detailed analysis of the distribution of deuterium in the products they concluded that most of the cis- and all of the trans-decalin produced from


143

AS~'O-octalinwas derived from a common intermediate in which three 10-d. Accordingly, hydrogen atoms are equilibrated, possible A1~9-octalinthey assumed that the reaction with A 9~10-octalinproceeded mainly through 1-monoadsorbed A 9,10-octalinand thence A1'g-octalin-lO-d.

m-q==q) *

t

cis

cis and trans

3. Unsaturated Steroids

Because of their relatively fixed geometry, steroids provide excellent material for the study of the geometrical requirements of reactions involving organic structures. A double bond in the 7,8-(VI) or 8,s-(VII)position of a steroid which has the trans A/B ring configuration isomerizes t o the 8,14 position (VIII) when treated with hydrogen and a palladium catalyst or platinum in the presence of acetic acid (69) (Fig. 12). Once the double bond reaches the 8,14 position it cannot be hydrogenated; however it may be isomerized by treatment with HC1 in chloroform t o yield a A14 isomer which can be reduced catalytically.

PI

m

PII

Fro. 12. Relationship between configuration and the ease of the palladium-catalyzed isomerieation of a double bond in the steroid nucleus.

Because the configurations of the reactants and products are known, the above isomerizations demonstrate the facile 1,3transfer of hydrogen by the removal and addition of hydrogen on the same side of the molecule. Presumably, the molecule is adsorbed on the 01 side (arefers to the side of the ring structure opposite t o the direction of attachment of the angular methyl groups a t (2-10 and C-13; B represents the epimeric configuration) which is the Iess hindered for steroids with these particular configurations; and hydrogen is transferred between the surface and the substrate. To account for the influence of the acetic acid in facilitating the isomerization, Bream et ad. (70)suggest that an allylic carbonium ion-catalyst complex is formed. This suggestion is related t o the

144

SAMUEL SIEGEL

mechanism of Taylor and Dibeler referred to previously and to the hydrogen switch mechanism of l'urkevich and Smith (71). The fact that compounds wit11 an 8,14 double bond (VIII) cannot be hydrogenated implies that the isomerization cannot proceed via a halfhydrogenated species, an essentially saturated structure. To avoid the excessive compression between the angular methyl groups at C-10 and C-13 which is enforced by the required geometry of the transition to the half-hydrogenated state (IX) the isomerization proceeds via an allylic intermediate (X) which permits the carbon atom a t C-8 to retain its sp2 hybridization (Fig. 13).

;b I

= I1

Ix

A

FIG.13. Geometrical relationships which affect the mechanism of the metal-catalyzed double boxid migration in steroids.

Apparently, in the reaction of olefins with hydrogen on catalysts such as palladium and platinum, both the dissociative and the associative mechanisms operate for isomerization and exchange. However, the dissociative mechanism accompanies those factors which tend t o slow the addition or accelerate the removal of hydrogen from either substrate or intermediate. These factors may be any of the independent variables, such as the pressure of hydrogen, the structure of the substrate, or the catalyst ( 5 ) .

IV. Conformational Analysis and the Geometry

'

of t h e Pertinent Transition States in the Hydrogenation of Cycloalkenes The development of techniques for identifying possible product controlling steps offers the possibility of examining the influence of various


145

independent parameters upon these rates. It also permits the application of structure-energy relationships such as conformational analysis (72)to the elucidation of the required geometry of the group of atoms associated with the reaction center a t each critical stage (57).A practical aim is the development of theories which allow the prediction of the ratio of the epimeric stereoisomers formed in the reduction of an olefin which would yield different stereoisomers via cis addition to the alternative faces of the double bond. If the arrangement of groups about the unsaturated center precludes the approach of a reagent from but one face of that center, the simplest concept of adsorption onto a surface adequately predicts the configuration of the principal saturated stereoisomer. However, flexible structures such as the alkyl-substituted methylenecyclohexanes or dialkylcycloalkenes (other than the 1,2 isomers) adjust t o the demands enforced by the topography of the reaction center, and yield ratios of products which are the resultant of the forces acting upon the organic moiety in the pertinent transition state. The nature and relative magnitude of these interaction mechanisms may be deduced by appropriate comparisons among structurally related substrates.

A. INTERACTIONS WITH THE CATALYTICSITEOR SURFACE Probably the most important factor governing the geometry of the surface complexes is the required arrangement of those atoms most directly involved in binding the substrate or its derivative (intermediates and transition states) to the surface site. The bonds to the surface constrain the motions of the remainder of the substrate which will tend t o adopt the most stable conformation(s). Other interactions which affect the ratio of the geometrical isomeric states may be subdivided into ( a ) those acting between the organic moiety and the reaction site on the catalyst and ( b )those among the parts of the organic structure. Interactions with neighboring adsorbed molecules will influence the conformation which the critical complex will adopt, This phenomena is demonstrated in the change in the mode of adsorption of toluene on a liquid mercury surface from a flat to a vertical arrangement as the film pressure is increased (73). In the present context, the attraction of the surface for the substrate, whether chemical or physical, will cause neighboring molecules to crowd one another so that an adsorbed molecule may adopt a conformation which is different from the conformation of lowest energy in the isolated molecule. A guide to the determination of the geometry of the atoms most directly involved in the formation of the transition state for adsorption is

146

SAMUEL SIEOEL

the principle that its structure lies between that of the reactants and the products of the elementary reaction t o which it pertains. The geometry will be %uchthat bond breaking and bond forming processes can ba: as concerted as possible. Assumptions must be made about the geometrical disposition of the surface orbitals which are associated with a single atom or a group of two or perhaps more atoms on the surface. In analogy t o olefin complexes of the transition elements the olefin may form a n complex with a single atom, or a binuclear complex (u) with two adjacent atoms. The latter would be the a$-diadsorbed alkane postulated by Horiuti and Polanyi.

B. GEOMETRYOF ADSORBED ALKENES Some of the evidence upon which is based the structure and geometry of chemisorbed alkenes is presented in Section I1,B. Lacking direct information relatively involved arguments have been advanced to deduce these geometries. Burwell et al. ( 7 4 ) have discussed the geometry of a$-diadsorbed alkanes in connection with studies of the exchange of cycloalkanes with deuterium. In agreement with the prior report of Anderson and Kemball (75) they find that the initial isotopic exchange patterns from cyclopentane and cyclohexane exhibit marked discontinuities following the species corresponding t o the complete exchange of hydrogen on one side of the cycle. The effect disappears for cycloalkanes larger than cycloheptane which shows a slight discontinuity after d,-cycloheptane. From this they concluded that the exchange must proceed via a vic-diadsorbed

1,2- diodsorbed

bicycle[ 2.2.11heptone

I - complexed

bicyclo[2.2. (heptene

FIQ.14. Alternative representations of adsorbed bicyclo[2.2. llhepteno.

STEREOCHEMISTRY A N D HYDROGENATION MECHANISM

147

alkane in which both bonds to the surface are in the same plane, an eclipsed conformation. I n the smaller cycles only the cis vicinal hydrogen atoms can be eclipsed but in cycloheptane and higher homologs both cis and trans hydrogens may do so. Accordingly, the exchange of hydrogen atoms on both sides of the cycle may proceed, during a single sojurn on the catalyst, via the a,P-diadsorbed alkane in the larger cycles, but some other process is required in the six-membered and smaller cycles. The exchange of deuterium with bicyclo[2.2. Ilheptane appears to offer a particularly good test of this hypothesis. Because only the hydrogen atoms on C-2 and C-3 may be eclipsed, the initial exchange should be and is limited t o only two hydrogens ( 7 4 )(Fig. 14). As a n alternative, the adsorbed olefin, an important intermediate in the exchange reaction of alkanes, may be represented as a x complex ( 7 ) , and the patterns of multiple isotopic exchange of the homologous cycloalkanes on transition metals can be related to the ease of formation of the derived cycloalkenes ( 7 ) .The ability of hydrogens on both sides of the cycle to participate in the a$ exchange process coincides with the ready formation of both cis- and trans-cycloalkenes in cycles containing eight or more carbon atoms ( 7 2 ) ;trans-cycloalkenes are unknown in C, and smaller rings and the recently prepared trans-cycloheptene is quite unstable (76). Although the difference in the stability of cis- and transcyclooctene is appreciabIe [the difference in AH" being 9.3 kcal/mole ( 7 7 ) ] , this difference in internal strain is apparently reduced in the x complex (78, 79). Recently, Rooney (80)expressed the view that the a,j3 exchange process involved n olefin complexes and asserts that this explains the pattern of exchange on a palladium film of deuterium with 1,l-dimethylcyclobutane. Its failure to exhibit appreciable multiple isotopic exchange was attributed t o the difficulty of forming a x olefin complex because of the strain in cyclobutene. Apparently, the exchange patterns can be explained qualitatively by reference t o either structure for the adsorbed olefin, the eclipsed 1,2diadsorbed alkane or the olefin x complex. This argument should, of course, refer to the transition state for the formation of chemisorbed olefin from monoadsorbed alkane, the critical step in the a,j3 exchange mechanism; however the revised argument would be much the same. Nevertheless we are provided with two alternative descriptions of the chemisorbed alkene under conditions closely related t o those employed in hydrogenation studies.

SAMUEL SIEUEL

148

c. THE GEOMETRYOF

THE

TRANSITION STATE

FOR

ADSORPTION

Whichever structure represents the chemisorbed olefin, the transition state for adsorption will have a geometry which lies between that of the adsorbed and the free olefin. To maximize the overlap between the orbitals of the substrate and of the surface, the olefin must be oriented so that the plane of the double bond is perpendicular t o a line drawn between its center and the surface site-if a single atom-or parallel t o the surface if the site, whether mono or binuclear, is part of the surface array of atoms. The groups attached to the double-bonded carbons become displaced outwardly from this plane as the olefin moves closer to the surface. Both n complex and 1,2-diadsorbed alkane share qualitatively this geometrical feature but the difference in geometry relative to the olefin is maximized in the latter, an essentially saturated structure. According t o the analysis of the preceding section, the stereochemistry of hydrogenation of disubstituted cycloalkenes or alkyl-substituted methylenecyclohexanes obtained at high pressures of hydrogen should relate to the stereochemistry of adsorption of the olefin. Much of the data can be explained qualitatively by the assumption that. the structure of the olefin is largely retained in the transition state for adsorption. Thus the fact that 2-, 3-,or 4-alkyl-substituted methylenecyclohexanes yield more of and cis-l,4the axial-equatorial dialkylcyclohexane (cis-1,2-,trans-1,3-, dialkylcyclohexanes) is explained by the assumption that the olefin which exists mainly in the chair conformation approaches the surface so that the cycle is inclined away from the reactive site and substituents prefer the equatorial positions on the cycle (57, 64) (Fig, 15). The cis addition of hydrogen then yields the more unstable disubstituted cyclohexane from this preferred intermediate. Similarly, to explain the stereochemistry of hydrogenation of dialkylcyclohexenes (other than 1,2 derivatives) a t high pressures of hydrogen, R

I

Methylenecyclohexotm

I

Complex

II u

i

Complex

FIG.16. Reference structures for the transition state for the adsorptionof 4-substituted methylenecyclohexanes.


149

Siegel and Smith (57) suggested that, in the transition state for adsorption, the cycloalkene adopts a pseudo-chair conformation, the most stable geometry of an isolated cyclohexene (81)(Fig. 16). For example, this

4-terf-Butyl- I-methylcycloherene

2,3-Dimethylcyclopentene

FIG.16. Preferred conformationsof the transition states for the absorption of endocycloolefins.

assumption accounts for the virtual 1:1 ratio of cis and trans saturated products obtained a t 100 atmospheres from 4-tert-butyl-l-methylcyclohexene, because in this conformation the double bond is equally approachable from either face (64). I n contrast, the limiting ratio of saturated isomers obtained from the isomeric methylenecycloalkane corresponds to about 60% of the less stable form because the cycle, which in this compound is virtually restricted t o asingle chair form in which the tert-butyl group is equatorial, provides a greater hindrance from the direction which would yield the trans isomer than the other which would lead to the cis. Similar results are obtained with other alkyl-substituted methylenecyclohexanes a t high pressures of hydrogen (63). From the fact that 2,3-dimethylcyclopenteneyields more trans- than cis-l,2-dimethylcyclopentane(63% trans a t 290 atmospheres), Siegel and Dmuchovsky (58)concluded that, in the transition state for adsorption, the geometry of the organic moiety has departed significantly from that of the cycloolefin. They noted that the repulsive interactions between the vicinal methyl groups increase as the complex progresses along the reaction path leading towards cis-l,2-dimethylcyclopentane, a n effect which runs counter t o the interaction of the 3-methyl group and the catalytic site. The effect associated with the interactions between vicinal groups disappears when the substituents are not adjacent because 1,3-and 2,4-dimethylcyclopenteneyield 88% of cis-l,3-dimethylcyclopentane a t 200 atmospheres. Apparently the geometry of the transition state for adsorption is approximately that of a n-complexed olefin in that its structure seems to be only slightly distorted from that of the isolated alkene. However, this does not necessarily mean that the adsorbed state which is formed in the elementary reaction to -which the stereochemistry refers is a n complex, because the same geometry also represents a stage in the progression of olefin to the eclipsed 1,2-diadsorbed alkane. Hopefully other experi-

160

SAMUEL SIEaEL

mental criteria may be found which will distinguish between these alternatives.

D. THEGEOMETRY OF THE

THE

TRANSITION STATEFOR

FORMATION OF

THE

‘‘HALF-HY DROOENATED STATE”

According to the analysis in Section 111,B,2, the ratios of saturated stereoisomers which are obtained a t low pressures of hydrogen on a platinum catalyst are characteristic of the transition state of the elementary reaction which yields the “half-hydrogenated state.” This transition state has a geometry which lies between the geometry of this intermediate and that of the chemisorbed olefin and is likely to be identical with the critical complex implicated in the exchange reactioh between a saturated hydrocarbon and deuterium in which vicinal hydrogen atoms are exchanged via the a$ process (57). Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the “halfhydrogenated state” to either a n-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum a t low pressures can be understood if the transition state has a virtually saturated structure. For example, the proportion of the axial-equatorial saturated isomers obtained at low pressures from substituted methylenecycloalkanes is greater than the ratio a t high pressures of hydrogen which is consistent with a structure in which the distinction between the alternative points of attachment of the surface site to the cycle is large. I n effect the catalyst acts as a large substituent which assumes that position on a cycle in which the repulsive interactions are minimal (Fig. 17). To account for the formation of mainly trans-dialkylcyclohexanefrom R

R&cH3 .I ,’*

endo

-NR,

5

,

II

,,

, I

8

exo- R,

4 -Substituted I - methylcyclohexene

: A:

i-

4 -Substituted rnethylenecyclohexone

FIO.17. Preferred conformations of the transition state which yields tho “half-hydrogenated state” from a 4-substituted methylcyclohexene and a methylenecyclohexane.

STEREOCHEMISTRY AND HYDROQENATION MECHANISM

151

4-tert-butyl-l-methylcyclohexene, Sauvage et al. (30)assumed that substituted cyclohexenes were adsorbed in a boat conformation, a very large group such as the tert-butyl group being forced to take a position exo to the boat. Their arguments sought to explain the results obtained a t about one atmosphere of hydrogen which approximate better the limiting cisltrans ratios obtained at low pressures than those at high. And indeed their suggestion can be modified (58)so as to refer t o the formation of the half-hydrogenated state which the previous analysis suggests is the product-determining reaction under these conditions. To develop the geometry of the above transition state, one assumes first that the eclipsed 1,2-diadsorbed alkane adopts a boat conformation. Clearly, a large group a t C-4 should prefer to be exo (trans to the C-1 methyl group) ; however the driving force which causes the cycle to adopt the boat instead of a chair conformation, as in cis-hydrindane (82),also would cause a small substituent to prefer to be endo (cisto the C-1 methyl group), and indeed the proportion of cis isomers obtained from d-alkyl1-methylcycloalkenes increases in the order tert-butyl < isopropyl < methyl, the per cent being 36 [c.f. Siege1 and Dmuchovsky ( 6 4 ) ] ,47, and 57, respectively, a t about one atmosphere of hydrogen (30). The driving force for the adoption of the above conformation in this instance has been assumed to be the spreading surface pressure arising from the saturation of the surface by various adsorbed species (73). One may gather from the preceding discussion that the application of conformational analysis to the determination of the geometry of these transition states is in an early stage of development. However further studies coupled with refinements in the theory of interaction between nonbonded groups can be expected t o lead t o a clearer picture of these important structures.

V. The Reaction of Aromatic Hydrocarbons with Hydrogen Studies of the kinetics of the addition and exchange of deuterium with benzene resulted in the first detailed mechanistic proposals of Polanyi and his associates (50,83)and of Farkas and Farkas (13,84,85). Farkas and Farkas suggested that the critical step in hydrogenation involved the simultaneous addition of two hydrogen atoms t o an adsorbed benzene molecule, whereas exchange with deuterium required the prior dissociation, on the surface, of benzene t o form a phenyl radical and a hydrogen atom. The phenyl radical then combined with a deuterium atom, which had been produced by the dissociation of a deuterium molecule, and the monodeuterobenzene was desorbed.

152

SAMUEL SIEOEL

Hydrogenation (Farkas and Farkas) : Ha C,H, C,H, ads

+ surface ;r 2H ads + surface $ C,H, ads

fwt + 2H ads slow -+ C,H, ads +C,H,, + surface

Ezchange (Farkas and Farkas) :

+ surface C,H, ads + D ads C6H6

--f

C,H, ads

--f

C,H,D

+ H ads

+ surface

Horiuti and Polanyi (50) argued differently and concluded hat the exchange reaction had an associative mechanism as does the addition of hydrogen to benzene. However, they assumed that in the latter the two atoms of hydrogen added consecutively.

Hydrogenation (Horiuti and Polanyi) : H

H,

+ 2* 'r 2 cI

CeH,

+*

$

(* represents surface atom)

CEH, (ads) H

I

C,H, (ads)

fast

+ H1 "r C,H, -+ C,H, (ads) +C,H,, I nlow *

*

Exchange (Horiuti and Polanyi):

Later Greenhalgh and Polanyi (14)formulated kinetic expressions for this mechanism. They showed that the dissociative mechanism for exchange according t o Farkas and Farkas would yield the same mathematical function as the associative mechanism if the combination of deuterium atoms with phenyl radicals was the slow step. However, they continued to favor the associative mechanism. One or the other mechanism for exchange is preferred by different authors, the choice being based upon facts or theories other than the kinetics of the reaction (86-88).However, there seems to be general agreement that the hydrogenation of benzene requires the addition of two hydrogen atoms to attain the composition of the critical complex (14,84,86, 88,89),this being in accord with the most reliable kinetic data showing the first-order dependence of the rate upon the hydrogen pressure.

STEREOCHEMISTRY AND HY DROOENATION MECHANISM

153

A. GEOMETRICAL CONSIDERATIONS OF THE MECHANISM FOR EXCHANGE 1, The Associative Mechanism

Stereochemical arguments can be used to deny that the associative mechanism for exchange can take place in the manner proposed by Horiuti and Polanyi. If an adsorbed deuterium atom unites with an adsorbed molecule of benzene, the deuterium atom will occupy a position which bears a different geometrical relationship t o the surface (reactive site) than does the hydrogen atom originally present on the same carbon atom of the cycle (A or B). The reversal of the reaction will not lead t o exchange unless the intermediate can turn over, a process requiring the breaking of the bond to the surface.

i

(B)

(A)

Recently Harper and Kemball (90) have elaborated the associative mechanism in a manner which avoids the above geometrical limitations. Much of the reasoning upon which they base their ideas is recorded in papers by Gault et al. (91) and by Rooney (80). They postulate that r-bonded benzene or molecular species such as B can combine either with a hydrogen atom from the surface or with a hydrogen molecule from the gas or physically adsorbed phase:

-(p;

(

*

*

*

*

* The removal of hydrogen must proceed along the same paths; consequently exchange will occur via this associative mechanism only if these alternative reactions occur a t approximately the same rate. One difficulty is that reaction (13) would seem to require a much larger activation

154

SAMUEL SIEGEL

energy than reaction (12) (c.f. refs. 59 and 92) and there is as yet no compelling evidence that n-adsorbed hydrocarbon species combine with molecular hydrogen as postulated. Because the latter mechanism has geometrical implications it should be subject t o test by appropriately designed stereochemical studies. 2. The Dissociative Mechanism

Although the bond dissociation energy of a hydrogen-carbon bond in benzene is about the same as that of methane (93),the rate of exchange of deuterium with benzene on metallic surfaces is much faster than with methane. This result has been thought to support some kind of associative mechanism (87), but perhaps, in the transition state for dissociative adsorption, the r-electron cloud of the aromatic ring can interact with vacant d orbitals of the metallic center to which it is becoming attached in a manner similar t o the interaction which stabilizes phenyl complexes of transition metals according to Chatt and Shaw (94). Garnett and Sollich (95) suggest that the transition from wbonded benzene t o o-bonded phenyl reaches the critical stage when the benzene ring is inclined a t 45' to the final bonding direction. At this point the hydrogen has weakened its hold on carbon and has begun t o be attracted to the metal. I n support of the dissociative mechanism they cite the deactivation of the platinum-catalyzed exchange with deuterium oxide of the ring hydrogen atoms in alkyl benzenes which are flanked by two methyl groups or are ortho t o one tert-butyl group, and attribute it t o a steric effect.

Horizontally n-bonded

Transition state, inclined at 45"

Edge on o-bonded

Crawford and Kemball (87) had earlier demonstrated that, on films of nickel, deuterium exchanges rapidly with both side chain and ring hydrogens of alkylbenzenes. The hydrogen atoms in these molecules can be classified into groups according to their ease of exchange. The most reactive included those on carbon atoms a to the ring and in ring positions not ortho to any substituent. The next most reactive hydrogens are those ortho to a single alkyl group such as methyl or isopropyl while the least reactive were hydrogens ortho to two methyl groups as in nz-xylene. They assumed that the exchange of the hydrogen atom on the a carbon atom occurs through the formation of species in which the benzene ring is


155

rr-bonded to the surface. On the basis of experience with the exchange

M

M

reactions of saturated hydrocarbons on nickel (59) they questioned that multiple exphange might proceed via species which have lost two CY hydrogen atoms as in :

However the analogy may be inappropriate (see Section VI1,D).

B. THE STEREOCHEMISTRY OF ADDITIONOF HYDROGEN TO AROMATIC COMPOUNDS The mechanism for the union of hydrogen with benzene also has been illuminated by stereochemical studies. Linstead and his students demonstrated that the hydrogenation of substituted benzenes occurred with considerable stereospecificity (16).Thus the hydrogenation of diphenic acid yielded mainly compounds with either cis (one ring reduced) or cis-syn-cis (both rings reduced) configurations.

Q-pQ-pQp / \

HO,C

CO,H

HO,C

CO$

cis

HO,C

COsH

cis-syn-cisperhydrodiphenic acid

They concluded that when a benzene ring is hydrogenated during a single period of adsorption the stereoisomeric products are formed in a proportion which is determined by steric interactions between the substituents on the cycle and the catalyst. The benzene ring was assumed to be adsorbed with its face parallel to the surface from which hydrogen was abstracted. Further, the formation of mainly syn isomers suggested that the diphenic acid, or the intermediate with one ring reduced, was adsorbed in a conformation which brings the carboxylic acid groups together. Undoubtedly, the most important of these concepts is that steric effects determine the manner in which a molecule is adsorbed upon a surface and this is revealed in the configuration of the products. These ideas, based upon a classic example of stereochemical research,

156

SAMUEL SIEGEL

served a generation of organic chemists and were not seriously questioned until convenient methods for the analysis of mixtures of stereoisomers became available. The demonstration that the ethyl esters of both A'- and A2-tetrahydrophthalic acid as well as ethyl phthalate yielded only cis-hexahydrophthalate when hydrogenated under the same conditions (reduced platinum oxide in acetic acid a t one atmosphere of hydrogen) showed that the proportion of stereoisomers obtained from hydrogenating an aromatic compound did not of itself distinguish between the postulates that ( a )the benzene ring is hydrogenated during a single period of residence on the surface or that (a) intermediates, such as the tetrahydro derivatives, were desorbed from the surface before the final step of saturation was accomplished (96).

Although the principal stereoisomer formed a t ambient temperatures in the hydrogenation of disubstituted benzenes has the cis configuration, trans isomers are also produced, the amount being a function of the structure of the substrate, the pressure of hydrogen, the temperature, and the catalyst (97-100).Mixtures are formed although the products are virtually unaffected under these conditions; consequently the trans isomers result from a kinetically controlled process. Detailed studies of the stereochemistry of hydrogenation of the several xylenes and their tetrahydro derivatives suggested that the cycloalkenes which would result from the cis addition of four atoms of hydrogen to the xylene molecule were released from the surface before final readsorption and reduction (24, 97). Indeed the ratio of the cis and trans saturated stereoisomers which are produced from a particular xylene can be estimated from a knowledge of the ratios obtained from each of the possible derived cycloalkenes under the same conditions and the assumption that the cycloalkenes are formed in a proportion corresponding to the random addition (cis) of hydrogen to the aromatic ring. Accordingly, o-xylene should yield the following proportion of cycloalkenes: 1,2-dimethylcyclohexene, 16.6y0; 2,3-dimethylcyclohexene, 33.3% ; cis-3,4-dimethylcyclohexene, 33.3%; and ~is-4,6-dimethylcyclohexene, 16.6%. Separately, 1,2- and 2,3-dimethylcyclohexene yield ratios of the dimethylcyclohexanes which are a function of the pressure of hydrogen (Fig. 5) while the cis-3,4-and cis-4,5-dimethylcyclohexenes should form only cis-1,2-dimethylcyclohexane.Indeed Mahmoud and Greenlee (101) have shown that the geometric isomers of 3,4,5-trimethylcyclohexene

167


retain their stereochemical integrity upon reduction under comparable conditions. Accordingly the percentage of the cis isomer a t 1, 4, and 100 atmospheres is estimated to be 90, 87, and 88 while the observed values are 93, 88, and 91, respectively (Fig. 18). Similar results are found with other xylenes. .#"

20

-

10

-

tn 6 -

p \

t

4 -

2 -

I 0.2

I

I

I

0.5

I

5

I

,,.I 10

1

I

I

I

I

50

I00

J 500

PRESSURE OF HYDROGEN (Atm )

FIG. 18. Comparison of &/trans ratios obtained from o-xylene and its tetrahydro derivatives 1.2- and 2,3-dimethylcyclohexene as a function of hydrogen pressure; PtO, (97).

Alternatively, if only a single cycloalkene is released from the surface, then a t least 40--50%of the aromatic precursor must be reduced via this alkene.

C. THEDETECTION OF CYCLOHEXENE INTERMEDIATES The postulate that olefins are released from the surface during the hydrogenation of aromatic hydrocarbons has gained considerable support. Madden and Kemball (89) observed cyclohexene during the early stages of the vapor phase hydrogenation (flow system) of benzene over nickel films a t 0" t o 50". The ratio of cyclohexene to cyclohexane diminished with time, and little or none of the alkene was detected if the films were annealed a t 50" in a stream of hydrogen. Later, Weitkamp (102) isolated the octaliiis formed in the hydrogenation of methylnaphthalene, or naphthalene, over a platinum sup-

158

SAMUEL SIECEL

ported on alumina catalyst at about 100-150". Unfortunately this work has not appeared in the usual channels of publication. More recently Hartog and Zwietering (103)used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate; for example, a t 25' o-xylene yields 0.04, 1.4, and 3.4 mole yo of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole yoof cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethylcyclohexene (0.002 mole yo)from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from m-xylene in reductions catalyzed by reduced platinum oxide. I n a detailed study of the reduction of the xylenes in the liquid phase on a 5% rhodium on carbon catalyst, Siegel and Ku (105)showed that both 1,2- and 2,3-dimethylcyclohexenes are formed from ortho-xylene (Fig. 19). The initial (extrapolated) ratio, 1,2-/2,3-,lies between 0.5 and 1 but rises as the reaction proceeds. If the initial distribution of cycloalkenes was random as previously postulated (97)the ratio should be 0.5.

:::L v,

I

10

20

30

40

5 0

I

,

,

bO

70

00

Per cent reductLon

FIG. 19. Formation of 1,2- and 2,3-dimethylcyclohexene in the hydrogenation of o-xylene in the liquid phase (acetic acid) on a 6% Rh/C catalyst (105).


159

The rapid change in the ratio as the reaction proceeds reflects the relative reactivity of the 2,3 isomer in competition with the 1,2 isomer (57, 60). I n comparison, the ratio of 2,4-/1,3-dimethylcyclohexene obtained from m-xylene is close to unity (1.2) and the ratio changes little during the course of the reaction. An initial random distribution should yield equal amounts of these isomeric cycloalkenes and the relative constancy of the ratio is consistent with the fact that, in competition with one another, 1,3- and 2,4-dimethylcyclohexene are reduced a t comparable rates. The failure to observe the other postulated dimethylcyclohexenes is to be expected, because none would have substituents attached t o the double bond in the cycle. Consequently, because of their expected greater reactivity in competition, their maximum concentration should be no more than a few per cent of the most reactive of the cycloalkenes actually observed in these experiments. Although there is no question that olefins are released from the surface during the hydrogenation of an aromatic hydrocarbon, the significance of this fact is a subject of controversy. The stereochemical argument requires that a large fraction (greater than 50%) of the aromatic hydrocarbon is reduced via the desorbed cyclohexene ; whereas Hartog and Zwietering (103) conclude from their kinetic analysis that the fraction is small. They show that the manner in which the concentration of cyclohexene changes during the course of hydrogenating benzene can be described by a two-parameter equation derived from a kinetic scheme of the form

x

--

k

x, +Yo

k

2

It Y

where X is the aromatic compound, Y the intermediate tetrahydro derivative, and Z the fully hydrogenated product, and the subscript a means adsorbed. Later, Hartog et al. (106)conclude that, in the hydrogenation of benzene on ruthenium black, only 1yoof the benzene that is hydrogenated leaves the surface as cyclohexene, the remainder being hydrogenated during one soiourn on the surface. If the above interpretation of the kinetics is correct, there must be a mechanism whereby a xylene molecule may be converted to a transdimethylcyclohexane without leaving the surface. Indeed, this is proposed by Harper and Kemball in their formulation of the associative

SAMUEL SIEGIEL

160

mechanism for hydrogenation and exchange (90). However, the kinetics may be interpreted in other ways and a definitive answer must await further study. Noteworthy in this connection is van Bekkum's report (107)that the hydrogenation of 2-tert-butylbenzoic acid over a rhodium catalyst yields cis-2-tert-butylcyclohexanecarboxylicacid and 2-carboxy-3-tert-butylcyclohexene (which attains a concentration of 22 mole yo)and only in the final stages, after most of the aromatic compound has disappeared, is the trans isomer formed, obviously via the hydrogenation of the tetrahydro derivative. t -Bu

t-Bu

cis

t -Bu

I

VI. Hydrogenation of Multiply Unsaturated Hydrocarbons Meyer and Burwell's (9, 10) recent studies of the hydrogenation of multiply unsaturated hydrocarbons illustrate the value of employing a variety of techniques, including stereochemical, t o an examination of the mechanism of a reaction. These studies were designed t o better identify the structure bf the surface intermediates involved in the reactions of hydrocarbons with hydrogen and to illuminate the pathways of their further transformations. Following gas chromatographic separations, nuclear magnetic resonance spectroscopy served to locate the deuterium in the products and mass spectrographic analysis gave the distribution of the deuterated species. The reaction between deuterium and 1-butyne, 2-butyne, 1,2-butadiene, and 1,3-butadiene, respectively, was conducted in a flow system at near ambient temperatures. The catalyst (0.03 wt yo palladium on alumina) was prepared by impregnating hard alumina pellets with palladium chloride so that the metal was probably confined t o an outer shell of each particle.

A. ACETYLENES The reactions are highly selective in the sense that little if any of the product butene reacts further as long tts any of the more saturated substrate remains. 2-Butyne yields cis-2-butene-2,3-d2almost exclusively;

STEREOCHEMISTRY AND HYDROU ENATION MECHANISM

161

multiply exchanged cis-butene, the other butenes, and butane sum to only 1yo of the product. This is in accord with the classical mechanism,

**

CH,C=CCH,

HSC,

*'

,CHI

c=c *'

the formation of the minor products arising via alternative paths of decomposition of the half-hydrogenated intermediates. The 1,2 addition of deuterium to 1-butyne is accompanied by an additional exchange reaction involving the original acetylenic hydrogen. Exchanged 1-butyne appears in the vapor phase but the amount of 1-butene-(h,d), which is formed is much greater than can be expected from the simple addition of deuterium t o this exchanged acetylene. Consequently, 1-butene-(h,d), must be formed largely from an adsorbed species derived from the original substrate. A suggested path involves the sequence : HCrCCH,CH,

-

H;C=C/CHaCH'

*

*'

CH,CH, I

111 C I

which is an alternative to the more direct reaction for exchange: CH,CH, I

H-CECCH,CH,

H

tl

+ i

C I

The first sequence includes one reaction (the last) which requires a greater amount of molecular reorganization than any of the other steps which are needed to explain the major course of the reaction. A perhaps more satisfactory sequence is the following:

*

162

SAMUEL SIEOEL

The last step is analogous to the reaction which accounts, in part, for multiple exchange in methane (59),e.g.,

B. ALLENES The proportion of products obtained from 1,2-butadiene are 0.53 cis-2-butene, 0.07 trans-2-butene, and 0.40 1-butene. The predominant deuterated species are again those which result from simple 1,2 addition to one or the other double bond. However 1-butene is more extensively exchanged than cis-2-butene. The fact that much more cis than trans-2-butene is formed is considered to be the consequence of a steric effect. If adsorption of 1,2-butadiene is to occur a t carbon atoms 1 and 2, the allene must approach the surface so that the plane of the CH,=group is parallel to the surface. Of the two possible directions of approach, one ( b ) which would yield the H\

1'') 1

c, =c =c,

,CHs

"t t (a)

trans isomer is hindered by the methyl group; consequently the other.(a) which generates cis-2-butene is favored. Application of the concepts developed in Section I V would yield a qualitatively similar argument, attention being focused on the geometry of the transition states for either adsorption or the formation of the half-hydrogenated states.

c. 1,Q-DIENES-EVIDENCE

FOR

ALLYLICINTERMEDIATES

The addition of deuterium to 1,3-butadieneyields mainly I-butene and trans-2-butene. The isotopic distribution in these products is nearly identical and 70% of the initial product corresponds to simple 1 , 2 or 1,4 addition. Meyer and Burwell suggest that 1,3-butadiene is adsorbed on the surface in the trans conformation. Addition of deuterium to a terminal carbon atom produces an allylic species which is a common intermediate for the formation of both major products, 1-butene and frans2-butene.


CH,D H \ /

CH2=CH-CH=CH2

J‘

163

/c=c\

13cHaD

2

CHP H \ / ,CZC.\

H

CH,

H

CH,

CH,= CH-CHD-CH,D

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-~yclononadiene and 1,2-~yclodecadiene led Moore (108)to conclude that ally1 complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d , the trans isomer. Examination of

(a)

@)

molecular models of the dienes showed that adsorption along c and d is hindered seriously by the methylene chain of the cycle, whereas the alternative approaches, a and b, are unhindered. Thus the classic view would predict the formation of cis-cycloalkenes. The formation of substantial amounts of the trans isomers, which are the less stable geometrical isomers in cycles with fewer than eleven carbon atoms, implies that an intermediate is produced which is capable of yielding both cis- and trans-cycloalkenes a t comparable rates. As Moore suggests, the addition o f a hydrogen atom to C-1 (A) gives a half-hydrogenated state (B) identical to that which would be formed from the corresponding acetylene and would yield the cis-cycloalkene by the replacement, with retention of configuration, of the bond t o the catalyst by hydrogen. However, if the first atom of hydrogen is added to C-2, an allylic species is formed and models indicate that configuration (C) should be preferred in the nine- and ten-membered cycles. Hydrogen

164

SAMUEL SIEGEL

/

\

should be equally accessible to either C-1 or C-3 of this structure. Addition to C-1 would give a cis-cycloalkene, to C-3 the trans. The same allylic species can be formed from the related 1,3-diene and indeed cis,cis-1,3-~yclodecadieneyields a t least 38% trans-cyclodecene although the saturation of one double bond which did not involve its neighbor should obviously yield the more stable cis isomer. The result implies that the allylic intermediate, which would be formed initially in the cis,& conformation, has time t o rearrange on the surface t o its more stable cis,trans configuration before it reacts further with hydrogen. The isomerizes first to the less possibility that the cis,cis-l,3-~yclodecadiene stable cis,trans-diene was discounted. The rearrangement of the allylic intermediate could be accomplished via a a-bonded complex, as indicated an page 142 of Section II1,C.

D. THE COMPETITIVE HYDROGENATION OF 1,Q-DIENES ACETYLENES

AND

According to Meyer and Burwell (lo),a further indication that the initial chemisorption of 1,3-butadiene involves more than the formation of 3,4-diadsorbed 1-butene is the fact that 1,3-butadiene competes for


165

hydrogen on equal terms with 2-butyne, although the enthalpy of hydrogenation of the dieiie is approximately 10 kcal less than that of the acetylene. Generally there is a rough correlation between the enthalpy of hydrogenation, the strengths of binding to metallic ions (and presumably the catalyst), and the relative rates of hydrogenation of unsaturated hydrocarbons. The expected relationship fails, presumably because 1,3-butadiene is adsorbed in a manner which is quite different from 2-butyne, perhaps as a 7 complex as suggested in another connection by Rooney et al. (.38). However, a different view focuses attention upon the nature of the half-hydrogenated species. For simplicity the mechanism of hydrogenation of 2-butyne is represented as a sequence of three reactions (14-16) and the chemisorbed acetylene is written as a 7r complex.

+

H,

2*

H 2!

*

*I

* If the last reaction is effectively irreversible, the rate of disappearance of 2-butyne (Y)is -d[Y]/dt

=

k3k&/k- zk-,[Y] [HI

=

k3KlK2[Y] [HI

(17)

where [HI represents the surface concentration of hydrogen atoms and K , and K , the equilibrium constants for reactions (14) and (15). An analogous sequence can be written for 1,3-butadiene (E);again the chemisorhed diene is represented as a 7 complex.

166

SAMUEL STEOEL

+

1

H

k, k--t

+

1-butene, cis- 2-butene,

or trans-2 butene

(20)

-

Both reactions (19) and (20) are more complicated than shown in that two different n complexes may be formed, the syn and anti, although apparently mainly the latter is formed in this instance (1 0 ).Each complex can yield l-butene and either cis-2-butene from the syn or trans2-butene from the anti complex. However to emphasize the more immediate question, reactions (19) and (20) are presented in this simplified fashion, With the last step controlling, the rate of hydrogenation of 1,3butadiene is

-d[E]/dt = k7k6k6/k-5k-6[EJ (21) The relation between the constants of the above mechanism and the apparent rate constants measured when 2-butyne or 1,3-butadiene are examined separately depends upon whether the most stable species on the surface is the r-complexed butyne (or 1,3-butadiene)or the respective half-hydrogenated states. If the former situation prevails, the apparent rate constants are k,K, for 2-butyne and k,K, for 1,3-butadiene; if the latter, the respective constants are k, and k,. If the two compounds are allowed to react in competition with one another, the relative rates will be d [ Y l / d [ E I = k&iK,[YI/k,K&6[KI

(22)

Therefore, the measurement of the relative reactivities in separate and in competitive experiments will permit the evaluation of either K J K , or K I K , / K , K , depending upon whether the principal surface species are the 7-complexed multiply unsaturated hydrocarbons or the respective half-hydrogenated states. If the former situation exists, the evaluated ratios might be expected to correlate with the association constants of the hydrocarbons with silver ion (78),but nGt if the main surface species are the half-hydrogenated states. Apparently, it is the latter condition which prevails. Hussey et al. (109) have recently reported measurements of the kinetics of hydrogenation in the liquid phase of a large number of cycloalkenes, both individually and competitively. The data permit the kind ofanalysis


167

outlined above and it is clear that such information is useful for the identification of some of the factors which influence the rates of the surface-catalyzed reactions. Wauquier and Jungers (110) have employed a similar treatment to abstract from kinetic data the relative adsorption constants of a number of aromatic compounds on a nickel catalyst. Rader and Smith (111)have extended the measurements to all the possible methyl-substituted benzenes on a platinum catalyst and Smith and Campbell (112) have studied the same series on rhodium.

VII. Some General Mechanistic Considerations A. THE PRINCIPLE OF MINIMUM STRUCTURAL CHANGE The preceding discussion has shown that the major course of the reduction of multiply unsaturated compounds can be understood in terms of a relatively small number of elementary reactions. Other reactions have been postulated for various reasons and it is obviously desirable to find criteria for judging the probable importance of the many conceivable changes. Perhaps the most important criterion is an experimental one which is coupled with the principle of minimum structural change. Thus the demonstration that 2-butyne yields, almost exclusively, cis-2-butene-2,3-d2implies that the structure (A), a logical

(A)

intermediate, reacts with hydrogen on the surface to yield desorbed cis-2-butene. By analogy, the dissociately adsorbed olefin (B), which Sauvage et al. (30) advanced to explain why the hydrogenation of 1,2dimethylcyclohexene gives trans as well as cis-l,2-dimethylcyclohexane, must proceed to 2,3-dimethylcyclohexene rather than be transformed on the surface to the saturated products (97). Later Siege1 ct al. (60)showed

I:&(

- qCHs

cis- and trunsl,2-dimethylcyc~ohexane

H

*

(B)

CH,

168

SAMUEL SEIGEL

that the isomerization of 1,2- to 2,3-dimethylcyclohexene (which is able to form both cis and trans products via cis addition) and the reduction of the latter were fast enough to account for the production of most if not all of the trans isomer.

B. SURFACE SITESAND

THE TRANSITION METAL COMPLEXANALOG

The practice of considering the catalyst as a featureless surface or a planar array of atomic centers deprives theory of a n adequate concern for the geometry of the transition from reactants to products. Balandin (23) recognized the importance of the concept of a transition state t o the development of a mechanistic theory of catalysis, and in his hands the “multiplet theory” proved fruitful. However the directional properties of binding orbitals, a subject of more recent development, apparently has not been incorporated into his theory. A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113),Arlman ( I r a ) ,and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers a t which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. There is a growing recognition that the reactions which are catalyzed by metals may best be analyzed in terms of a localized domain of the surface with little dependence upon the theory of the gross properties of metals. The surface presents geometrically different sites depending upon their location on the various faces, edges, or dislocations of the crystal (116).The reactions which may occur a t one of these sites will be a function of the number of liganda which may be associated with the atomic center; and the energy of such associations will depend upon the nature of the metal and its oxidation state, the nature and number of neighboring atoms of the lattice, and the sweep of unobstructed space associated with the optimum bonding directions. The last factor in particular might be explored by stereochemical studies. I n discussing the mechanism of the para hydrogen conversion or the hydrogen deuterium exchange reaction, Eley (117)suggested that i t could occur a t a single metal atom. Two orbitals are needed t o bind the activated complex and these can be provided easily by the d orbitals of


169

an atom a t the surface. The number and spacial 'distributions of the available orbitals depends on the crystal structure and the lattice planes

which are exposed. However, hydrogen is the smallest possible ligand and a metallic site which might coordinate with several hydrogen atoms could accept fewer large groups. This is one source of selectivity for reactions which have quite different steric requirements. For example, if the dissociative mechanism for the exchange of benzene given in Section V,A,2 is correct, this reaction should have a smaller steric requirement than the addition of hydrogen to benzene; in the latter, the molecule must present, R face to the reaction site, in the former, an edge.

C. GEOMETRICAL DETAILS 1. The Exchange of Saturated Hydrocarbons

A noncommital representation for the exchange reaction of methane is H CH, CH,+2*F?.l

H

CH,

+!"!I

€I

I

C

H

+!'!I+!

which coupled with the reaction D D,+2*F?.2

I

provides the minimum number of simple reactions required to account 0

FIG.20. Geometrical relationships for the simple exchange of methane with deuterium involving adjacent surfwe atoms each of which haa available two octahedral bonding directions suitable for the reaction.

170

SAMUEL SIEGEL

for its exchange on a variety of metallic surfaces. The first step in the reaction can proceed a t a single atomic center. However, the exchange with deuterium probably requires the cooperation of a second site which is written in accordance with the model employed by Sherman et al. (118) (Fig. 20). The loss of a second and possibly a third atom of hydrogen from methane (accounting for multiple exchange during a single period of residence on the surface) could be represented as in Fig. 21. The particular r

l*

Fro. 21. Geometrical relationships for the multiple exchange of methane.

orientation of the groups in the transition state is chosen so that, as the C-H bond is loosened, the overlap between a p orbital on carbon with an appropriate d orbital of the metal can be maximized. 2 . The Associative Mechanism for Exchange and the Addition of Hydrogen

The reaction of an olefin with hydrogen may be analogous to the The olefin may form a T complex related homogeneous reaction (44,46). with a single center and combine with hydrogen in the sequence shown in Fig. 22A. Besides the reverse of the preceding sequence, an alternative

A Additlon and exchange at o single center.

p

-$

c

H

-

B. Exchange involvinp two centers.

FIQ.22. Geometrical relationships involved in the associative mechanism for addition and exchange of an olefin.


171

mode for the loss of hydrogen from an alkyl group is indicated in Fig. 22B, a reaction which corresponds to the multiple a$ exchange process which is so important on metals (see page 146). The direct desorption of an olefin from the 1,2-diadsorbed state is likely to be slower than the desorption of olefin from a rr complex, because of the much greater electronic reorganization which the first process implies. Consequently, the proposed sequence would allow a rapid reversible adsorption of the olefin to form a n complex, but once the half-hydrogenated state is reached it could undergo multiple exchange through the 1,2-diadsorbed state without returning an appreciable amount of exchanged olefin to the gas phase. Such a relationship may account for the phenomena of selectivity in the competitive hydrogenation of olefins and the smeared pattern of deuterium in the product while relatively little exchanged olefin reappears in the gas phase (8). Indeed the distinction between the mechanism for the reaction of hydrocarbons with hydrogen on metals compared to oxides may be that the metals have open the above avenue for a$ exchange while the reaction on oxides is restricted t o a single atomic center (37).

D. FORMATION AND STABILITY OF UNSATURATED SURFACE COMPLEXES Structural theory may be applied to a consideration of the relative stability of the several complexes on a particular metal, or the variation in energy which follows a change in the metallic atom or its neighbors (ligands).For example, the relative stability of CH,

and

I

CH,

/I

(and presumably the transition states leading from one t o the other) will depend upon the ability of the metallic center to form d - p x bonds with the

(a)

(b)

(c I

Fro. 23. Diagrammatic representation of overlap between dzlorbital of metal atom and p orbital(8)of (a)methylene; ( 6 ) carbon monoxide; and (c) vinyl group.

172

SAMUEL SIEGIEL

methylene group (Fig. 23), and similar considerations apply to the stability and formation (Fig. 24) of vinyl or aryl complexes (94) and

FIG.24. Geometrical relationships for the dissociative adsorption of ethylene at a single atomic center.

explain the rates of dissociative exchange of benzene. Such an effect may account for the comparable rates of exchange of methyl and ring hydrogens with deuterium over freshly deposited films of nickel and other metals (59).The bond dissociation energy of the benzyl-hydrogen bond is much less than an aryl-hydrogen bond so that an interaction mechanism which enables the aryl hydrogens to overcome this disadvantage must enter into its transition state for exchange. The fact that sintering the nickel diminishes the rate of exchange of aryl hydrogen more than i t does the exchange of benzylic hydrogens may be explained as a change in the fraction of sites a t which p - d n bonding is most favorable. Indeed the fact that the introduction of a small amount of carbon monoxide has an effect similar t o sintering the film (119)supports this notion because a likely mode of interaction between CO and a metal involves the same kind of metal orbitals (120,121).

E. REACTIONS OK T-ALLYLCOMPLEXES A final example concerns the question of n-ally1 and related complexes with hydrogen. Again the evidence cited in the preceding section suggests that the principal reaction of r-ally1 complexes with hydrogen is to yield olefins, desorbed from the surface, although the possibility that a r-bonded olefin is formed first is a geometrically feasible process (Fig. 25). To account for the exchange and isomerization of a number of polymethylcyclopentanes, Rooney et al. (38) postulated that intermediates corresponding to the n-ally1 structures written above were not only able to abstract hydrogen from the surface as in the classical mechanism, but also could accept an atom from molecular hydrogen according t o an Eley-Rideal mechanism (Fig. 26). Studies on the exchange of 1,1,3,3-tetramethylcyolohexanewere claimed to support strongly the above mechanism (80);however, the

STER,EOCHEMISTRY A N D HYDROGENATION MECHANISM

-H

173

j?+:=

H

L

FIG.25. Geometrical relationshipsfor the formation and reaction of a n-ally1complex at a single atomic ccnter.

FIG.26. The alternative paths for the combination of a n-ally1structure with hydrogen according to the mechanism of Rooney, Gault, and Kemball.

Fro. 27. Geometrical relationships for a possible dissociative mechanism of exchange involving an allylic intermediate.

same exchange pattern could result from other reactions of a r-ally1 complex such as that indicated in Fig. 27. The latter mechanism recognizes that allylic hydrogen atoms, because of their lower bond dissociation energy, are more easily removed than nonallylic and can give rise t o multiple exchange via an cu,a-diadsorbed species. I n this sense, the exchange of benzylic and allylic hydrogens with deuterium is analogous, the reaction being faster than with nonallylic. Consequently, one hydrogen atom in the trimethylene chain is set apart from the other five. The same type of intermediate may also account for the racemization of (+)3-methylhexane during its exchange with deuterium ( 7 4 ) .

174

SAMUEL SIPGEL

One may inquire whether the evidence that n-ally1 complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances: in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. With its emphasis on the concepts of molecular structure and the relationship of structure to reactivity with all its implications, stereochemistry provides a unifying theme for the study of mechanism. This review will have served a useful purpose if it conveys this sense to the reader. ACKNOWLEDGMENTS The author’s contribution t o research on the subject of this review has been supported by grants from the National Science Foundation and the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made to the donors of this fund. Valued support was also given through research grants by the Monsanto Company, St. Louis, Missouri. The review was begun during the tenure of an International Award in Petroleum Chemistry administered by the American Chemical Society; and through the kind offices of Professor Charles Kemball, the author enjoyed the stimulating environment for the study of surface chemistry provided by the Department of Chemistry of the Queens University of Belfast. Discussions with Dr. George Blyholder concerning bonding in surface complexes were also helpful. Figures 1, 8, 9, and 18 are reproduced with the permission of the American Chemical Society; Figure 4, with the permission of Professor J. Chatt and the Chemical Society, London; and Figure 14, with the permission of the North-Holland Publishing Company, Amsterdam.

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175

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Chemical Identification of Surface Groups H. P. BOEHM Institute of Znoqanic Chemistry, University of Heidelbery Heidelberg, Uermany Page I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 11. Surface Groups on Carbon . A, Surface Compounds on . . . . . . . . . 182 B. Surface Groups on Graphite . . . . . . . . . . . . . . . . . . . 217 C. Surface Groups on Diamond . . . . D. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 111. Surface Groups on Silica . . . . A. Identification of Surface Groups on Amorphous Silica. . . . . . . . . . . . . . . . . . 226 B. Identification of Surface Groups on Crystalline Silica ..... . . . . . . . . . 246 C. Summary ... ............................ . . . . . . . . . 247 IV. Surface Groups o V. Surface Groups o VI. Surface Groups o VII. Conclusion ......................................................... 264 References . . ........................................ 264

1. Introduction The surface of crystalline solids may be considered as an extreme case of lattice defect. On a perfectly clean surface, there is an abrupt termination of the regular array of atoms. The coordination of the atoms must be different from that within the structure. This is true for the covalent bond type as well as for purely ionic lattices. Consequently, the atoms in the surface will have unsaturated “bonds” capable of reacting with other elements or compounds. The strengths of the bonds thus formed with other elements foreign to the bulk structure can vary to a very great degree. We speak of a true surface compound only if the bonding is similar in character to the bonding within the structure and if its strength is of the same order of magnitude as is found in chemical compounds. There is no distinct threshold between physical adsorption, i.e., reversible adsorption with small activation energy of desorption, chemisorption with a significant activation energy of desorption, and formation of surface compounds with a high activation energy for 179

180

H . P. ROEHRl

decomposition. Usually, it is not possible to remove the heteroatoms or surface groups as such from the surface; other stable compounds are formed, sometimes containing surface atoms of the bulk structure. The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards “bond formation” between cations and anions has been shown by precise determinations of the electron density distribution ( I ) ] . Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp3 hybridization of silicon in silica. Here, too, transitions from “Crystalline” t o “amorphous” do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. Each particle of a predominantly covalently bonded amorphous substance can be considered as a macromolecule. The surface groups are equivalent to the “end groups” of macromolecular chemistry. The subject of this article is the discussion of the functional groups on various surfaces, their chemical identification, and possible chemical reactions with these groups. Surface groups consisting of atoms foreign t o the structure can be formed on a great variety of substances. It is not intended t o discuss all possibilities; this would surpass the scope of an article limited in volume. Furthermore, research in this field has but begun; surface compounds have been studied only on a selected group of substances. Most of the investigated substances, however, are very important from an industrial viewpoint. Therefore, in this article the chemistry of surface compounds will be described for a few characteristic and well-known examples. Borderline cases, such as the chemisorption of carbon monoxide on metals, will not be considered. The properties of a surface are influenced by the surface groups to a very great extent, Knowledge of their existence and of their chemistry is important for many technological processes. Apart from heterogenous catalysis, surface chemistry is important in lubrication, in re-enforcement of rubber and other elastomers, in flotation, in the behavior of pigments in laquers, printing inks, and textile additives, and in many other applications. The existence of surface groups was first observed with finely divided

CHEMICAL IDENTIFICATION OF SURFACE GROUPS

181

substances of high surface area. Only when the surface area exceeds 50-100 mZ/gm does the amount of heteroatoms bound to the surface become analytically significant. Research on surface groups started with active carbon and with silica. The chemistry of an important group of naturally occurring materials is characterized by surface reactions: many clay minerals possess what can be considered “surface” a t its extreme. All clay minerals capable of intracrystalline swelling with separation of the silicate layers are-to overstate it-surface with a silicate layer on each side. Many principles and techniques of surface chemistry were first found with clay minerals. Nevertheless, the clay minerals will not be considered in this article, except for some comparison and analogies with surface compounds. Only a few examples, chosen also for reasons of the author’s familiarity with them, will be discussed: surface compounds on carbon, on silica, on titaiiia, and, less extensively, on alumina and silica-alumina.

1

II. Surface Groups on Carbon

There are three forms of carbon known: diamond, graphite, and black microcrystalline carbon. The diamond structure consists of a regular network of tetrahedrally bonded carbon atoms with cubic symmetry. Diamond is the parent of all aliphatic saturated compounds with sp3 hybridization of carbon. The graphite structure, on the other hand, is the prototype of all aromatic compounds. The carbon atoms form hexagon layers, each carbon atom bonded by c bonds to three neighbors (sp2hybridization). The fourth electron of each carbon atom is present as a T electron. There is some overlap of the T orbitals of neighboring atoms (2). In consequence, electricity and heat are conducted along the layers. The distance of 1.420 A between neighboring carbon atoms is intermediate between the values expected for single and double bonds and is what is expected for the bond order 1.5. The carbon layers are stacked with a separation of 3.354 A (at 15”).This distance is typical for van der Waals bonds. In well-crystallized graphite, consecutive layers are shifted + or - a13 with respect to each other; the stacking sequence is ABAB. In the direction of the c axis, perpendicular to the layers, electrical and heat conductivity are very small. The anisotropy of the electrical conductivity is p,,/p, = 104(3).Each layer of the graphite structure can be considered as a macromolecule of condensed aromatic rings: Microcrystalline carbon was formerly called “amorphous” carbon. It is known in many varieties: active carbons, carbon blacks, carbon

182

H. P. BOEHM

brushes for electrical machines, cokes, etc. Its structure derives from graphite as was shown by Hofmann and Wilm ( 4 )using X-ray diffraction. It consists of graphitelike layers of limited size stacked parallel to each other without further ordering. The carbon-carbon distance within the layers is the same as in graphite. The interlayer spacing, however, is larger than in graphite; it is about 3.6 A. The various forms of microcrystalline carbon differ in the size of the crystallites (from a few tens t o a few hundred angstroms) and in their mutual orientation. Sometimes, there is a considerable content of disorganized, tetrahedrally bonded carbon (5-7), often cross-linking different layers. Also, a considerable amount of foreign elements is usually found by chemical analysis. The heteroatoms can be bound at the edges of the crystallites and form surface compounds, or they can be incorporated within the carbon layers forming “heterocyclic” ring systems. This is analogous to substitution defects in crystals. I n the surface of the crystals or crystallites of either form of carbon, the rcgular array of carbon bonds is disrupted, forming “free” valeiices which arc very reactive. Usually, most of these valences do not remain free for any length of time, but form compounds with any suitable element being present. The surface compounds of carbon were investigated first with microcrystalline carbon. Therefore, a detailed discussion will be in the order : microcrystalline carbon, graphite, diamond.

A. SURFACE COMPOUNDS ON MICROCRYSTALLINE CARBON Most important and best known among the surface compounds of carbon are those with oxygen and with sulfur. Other elements, e.g., chlorine and hydrogen, can also serve as “end groups.” 1. Surface Oxides

Surface oxides of carbon were first observed more than 100 years ago. The first observation was published by A. Smith (8) in 1863. He discovered that oxygen was chemisorbed by charcoal and could be recovered on heating only as carbon dioxide. In 1887, Baker (9) found that oxygen was bound by freshly outgassed charcoal even at - 13”;the gases evolved at 450“ consisted mainly of carbon monoxide. The combustion of charcoal was studied in detail by Rhead and Wheeler (10)in 1913. Reaction occurred at temperatures as low as 300”. More oxygen was consumed than was contained in the evolved carbon oxides, CO and CO,. On subsequent heating to higher temperatures, up to lOOO”, the fixed oxygen came off the surface (as CO and CO,). The authors concluded that the first step of oxidation was the formation of

CHEMICAL IDENTIFICATION OF SURFACE CROUPS

183

carbon oxides with a low oxygen content, e.g., C,,,O. They did not realize, however, that this oxygen was bound on the carbon surface. The structure of carbon was not yet known a t the time, even though Aschan (21)had visualized already in I909 the correct structure made up from polycondensed aromatic nuclei. Another investigation worth mentioning was made by Lowry and Hulett (12), who confirmed that oxygen was irreversibly adsorbed a t 25" by previously outgassed charcoal. Beginning in 1922, Bartell and Miller (13) and Miller ( 1 4 , 15) published a series of observations on the adsorption of acid by sugar charcoals which had been activated at 800-1000" by admittance of a limited supply of air. From neutral salt solutions, the anions were adsorbed and exchanged for hydroxide ions. Burstein and Frumkin (16) and Frumkin ( 1 7 ) observed in 1929 that acid was adsorbed by carbon outgassed i n a high vacuum at 1000" only if oxygen was present. The reaction was formulated as C,O

+ N20 -+

C,

+2

0

+ 20H-

The oxygen was adsorbed irreversibly; it was not liberated in molecular form on addition of alkali (17). A t the same time, it was found that carbon could acquire acid properties as well on oxidation. If it is heat-treated and cooled in a high vacuum, i t shows basic behavior on admittance of molecular oxygen a t room temperature. It turns acidic, however, if it is exposed to oxygen a t moderate temperatures, e . g . ,400". This was first observed by Ogawa (18) in 1926. Kruyt and de Kadt (19)found 3 years Iater by measuring the electrokinetic potentials that the Same sample of outgassed carbon could acquire basic or acidic properties depending on the temperature at which it reacted with oxygen. This finding was later confirmed by Kolthoff (20)arid by King (21). An electrochemical explanation of the basic reaction was suggested by Frumkin ( l 7 ) , while Shilov et al. (22, 23) claimed surface oxides of definite structure to be the only cause of either acidic or basic reaction. Shilov formulated the acidic surface oxides as carboxylic acid anhydrides bound to the edges of the carbon layers. Thus, two kinds of surface oxides became known. Basic surface oxides are formed always when a carbon surface is freed from all surface compounds by heating in a vacuum or in a n inert atmosphere and comes into contact with oxygen only after cooling t o low temperatures. As is now known ( 2 4 ) , the irreversible uptake of oxygen starts a t ca. - 40"; there is only reversible, physical adsorption a t lower temperatures. Acidic surface oxides are formed when carbon is treated with oxygen a t temperatures near its ignition point. King ( 2 1 ) found the maximum

184

H. P. BOEHM

amount of acidic groups when the carbon had been oxidized a t 420". Later on, i t was observed that acidic groups are formed also on reaction with oxidizing solutions a t room temperature. Both types of surface oxides are found on technical products. Rubber grade carbon blacks are produced in different processes. Channel blacks are made by cooling a flame on iron plates, the so-called channels. The resulting carbon blacks are acidic in character because an excess of air is present (25). I n the production of furnace blacks, the fuel, mostly oil or natural gas, is burned with a limited supply of air. Thermal blacks are obtained by thermal cracking of the gas, which sometimes is diluted with hydrogen. In consequence, both types show weakly basic reaction in aqueous suspension. Starting the next period of development, chemical investigations of the oxygen-containing groups with the aim of their identification were published by Villars (26, 27) in 1947 and by Hofmann and Ohlerich (28) in 1950. The work of the latter group was impired by the results obtained on graphite oxide (also called graphitic acid). This compound must not be mistaken for surface oxides on carbon or graphite. Graphite oxide is a lamellar compound which is formed by the action of potassium chlorate or permanganate on graphite suspended in concentrated sulfuric or nitric acid. I n this compound the carbon layers are still intact; however, oxygen or oxygen-containing groups are bonded to the carbon atoms which assume, a t least partially, sp3 hybridization. I n consequence, the layers become puckered. Graphite oxide is capable of intracrystalline swelling (29) and is similar t o the clay mineral montmorillonite in this respect. By chemical methods, hydroxyl and carboxyl groups were detected (30, 31). The carboxyl groups are situated a t the edges of the carbon layers. A short summary with literature references is contained in an article by Boehm et al. (32). It was assumed from the beginning that functional groups known from classical organic chemistry would be present in the surface oxides. a. Acidic Surface Oxides. The studies on acidic surface oxides will be discussed first, because they are rather well-known. Pretreatment of carbon. In all experiments it has to be ascertained that no adsorbed impurities are present on the carbon surface. Extractable matter should be removed by extraction with organic solvents, e.g., xylene. This is especially important for carbon blacks (25). Oxidized carbon may contain small amounts of oxalic acid. King ( 3 3 , 3 4 ) found 0.002 meq/gm of oxalic acid in oxygen-treated sugar charcoal. More severe is the contamination of the surface with adsorbed gases, mainly carbon dioxide and water. Activated carbon with narrow pores may contain considerable amounts of carbon dioxide (28).The best

CHEMICAL 1DENTIFICATION OF SURFACE GROUPS

185

purification is outgassing in a high vacuum a t elevated temperatures. In my laboratory, a temperature of 100"is considered sufficient. Outgassing a t 300" practiced by Hofmann and Ohlerich (28) destroys the most sensitive groups on the surface (35).The decomposition, starting near 240", is easily recognized by a sudden evolution of gas, blowing fine carbon particles through the apparatus. A similar gas evolution observed near 100"is due to adsorbed water. Of course, the outgassed samples must be stored in tightly stoppered bottles. I n order to obtain a clean surface, the carbon is heat-treated either in a vacuum or under argon or nitrogen. It is not known at which temperature precisely all surface oxides are removed. Even after outgassing a t lOOO", there is a small oxygen content in many samples. At least a part of this oxygen is very probably incorporated within the carbon layers. The acidic groups are completely removed after outgassing at 900" according t o Puri and co-workers (36). It is advisable to heat the samples as high as possible. However, most carbons start recrystallizing a t temperatures exceeding 1200"; definite crystallite growth can be observed (37). Near 2400 to 2700", some carbons will graphitize; perfectly or nearly perfectly crystallized graphite is formed (37).Therefore, heating to 1200" seems to be the optimum temperature for heat treatment without significant structural changes. Oxygen content. The oxygen content of oxidized carbon is primarily a function of the specific surface area. Values as high as 13% were observed with an activated sugar charcoal that had been heat-treated at 1200" (Table I). 11.6% oxygen were found in "Carbolac No. 1," a carbon black used as a pigment (38).The color blacks are oxidized in the course of production (25).Commercially produced rubber grade channel blacks contain around 3% of oxygen (25),the greater part of which seems to be bound a t the surface. Wetting properties. A clean carbon surface is hydrophobic. Surface oxides provide sites of adsorption for water and other polar compounds. The more surface oxides there are, the more distinct is the hydrophilic behavior of the carbon. This was confirmed by Healey et at. (39) for graphitized carbon black, and by Kraus ( 4 0 ) and A. V. Kiselev and his group ( 4 1 ) for carbon black. Beebe and Dell (42) measured the sulfur dioxide adsorption on channel black and found an increase after oxidation a t 600". Further evidence for selective adsorption of polar compounds was provided by Gasser and Kipling (43). Neutralization behavior. A neutralization isotherm of sodium hydroxide on oxidized active carbon was determined by Hofmann and Ohlerich (28). The maximum neutralization was achieved with 0.01 N solutions

186

H. P. BOEHM

TABLE I Neutralization Behavior of Acidic Surface Oxides on Microcrystalline Carbon” Neutralization in meq/100 gm by Productb

SC 1, H.T. SC 3, H.T. SC 1, act. SC 2, act. SC 3, act. SC 1, act., H.T. SC 4, act., H.T.

Eponit CK 3, sample 1 CK 3, sample 2

CK 3, H.T. Philblack 0 Spheron 6 Spheron C

%or

5.4 9.2 7.3 13.6 15.9 10.7 13.2

5.5

_ _ _ _ I _ _ _ _

_____

NaHCO,

Na,CO,

NaOH

NaOEt

16 42 21 47 GO 36 47 15 31 76 14 57 59 63

32 82 43 94 106 72

69 124

85 165 89 215 214 137

ion

71 163 167

102

118

139 62 89 237 39 164 196

128

188

34 60 152 27 1(J9

106

315 70 233 295 255

“All carbon samples had been oxidized by heating in moist oxygen to 420-450” [after Uiehl ( 4 7 ) ] . *Explanation of the abbreviations: SC 1, SC 2, etc.: sugar charcoal, preparation No. 1, No. 2, etc. Eponit: activated charcoal, “Eponit,” manufactured by Degussa, Frankfurt a.M., Gcrmsny. CK 3: carbon black, “CK 3,” manufacturecl by Uogussa, Frankfurt a.M., Germany. Philblack 0:carbon black, “Philblack 0,”manufactured by Phillips Petroleum Co., Akron, Ohio. Spheron 6: carbon black, “Spheron 6,” manufactured by Cahot Corp., Boston, Mass. Spheron C: carbon black, “Sphcron C,” manufactured by Cabot Corp., Boston, Mass. H.T.: heat-treated under nitrogen a t 1200”. act.: activated by partial oxidation with CO, a t 950”. The burn-off was about 40-500/o. ‘Determined from elcnientary analysis by clifferrnce from 100%.

of sodium hydroxide and no further increase was observed with higher concentrations. Direct poteiitiometric titration with alkali gave rather flat curves without distinct inflection points (26, 4 4 ) . Villars (26)concluded that no chemical groups of distinct acidities were present. However, very often the potential becomes constant only several hours after the addition of alkali. Therefore, it was attempted in my laboratory (45-47) t o differentiate the acid groups by neutralization with bases of different basicities. The samples were agitated for at least 16 hours with 0.05 K solutions of four bases: NaHCO,, Na,CO,, NaOH, and Na ethoxide. The


187

acidities of the conjugate acids are: H,CO,, pK, = 6.37, pK, = 10.25; H,O, p K = 15.74*; C,H,OH, p K = 20.58.* Other authors (48) think t h a t still longer periods, e.g., 3 or 10 days, are needed to reach tru e equilibrium with alkali hydroxides. \Vith carbonate a n d bicarbonate, a few hours are sufficient. With a great number of oxidized carbons, simple ratios of the four neutralization values were observed (Table I), Twice as many acid groups were neutralized with Na,CO, as with NaHCO,; with NaOH a n d with Na ethoxide three and four times the NaHCO, neutralization was achieved. Sometimes, there was a little higher NaOH consumption. Invariably, more sodium ethoxide was bound by the carbon blacks than was expected from the previous experience. We think this due t o the presence of disorgaixized, aliphatic carboll in carbon black ( 6 , T), which on oxidation may produce groups capable of reacting with sodium ethoxide, e.g., alcohols or carbonyl groups. P The figures presented in Table I were confirmed by other reactions giving identical results, as will be shown later. It seems very improbable tha t the ratio of 1:2:3:4of the neutralization values occurred by chance. The simple ratio implies th at four different groups of characteristic acidities occur side by side in equivalent amounts. Very likely, they are part of a bigger complex. This stoichiometric ratio was observed only when the samples had been completely oxidized, Complete oxidation was achieved by heating smaller samples in an oxygen stream t o 420-450" (after 5 hours the burn-off was about 25-50%). It was difficult t o achieve by heating in air; only after prolonged oxidation, 36 hours at 420°, was a neutralization ratio near 1 :2:3:4obtained. Another important point is the slow cooling of the samples under oxygen. The oxidation temperature is well above the decomposition temperature of some of the acid groups. B y rapid quenching, samples were obtained which had a neutralization ratio of 1 :2:2:3.Na,CO, and NaOH neutralizations were equal. Incomplete oxidation seems t o be the main reason for the discrepancies in the results found in different laboratories. Surface oxides on commercial carbon blacks, with the exception of color blacks, never show this ratio because they are never completely oxidized. There is evidence t h a t the NaHCO, neutralization is caused by carboxyl groups ( 3 5 , 4 5 , 4 6 ) Sometimes . carboxyl groups were determined by neutralization with sodium or calcium acetate. This method is inaccurate because it does not allow for the presence of basic surface sites which bind some of the acetic acid liberated. It is shown in Table I1 th a t *Calculaterl from tho ion products (IO-" ant1 4.5.10-?", rcspwtivcly) m u l t i p l i ~ ~ hyl t h r number of moles per liter.

188

H. P. BOEHM

TABLE I1 Neutralization of Acidic Groups by Acetates and of B a s k Surjace Oxides by AcitE“

Product NaHCO, (oxidized at neutralization

NaOAc

420-450”)*

SC 1 , H.T. SC 1, act. SC 3, H.T.

Acetic acid liberated from

16 21 42

Ncutralization Sum of liberated of HCI AcOH and HCl Ca(OAc), neutralization

-

0

11

27

5

15

-

9

17 20 36

‘All results are given in meq/lOO gm. *For abbreviations see Table I.

the NaHCO, neutralization was equivalent t o the sum of acetic acid set free from acetates and of the basic surface oxides as determined independently by hydrochloric acid neutralization. With NaHCO, solutions, the amount of Naf ions remaining in the solution is determined by adding an excess of standard HCl and backtitrating after boiling off the CO,. The same procedure is used with Na,CO,. With sodium acetate, the liberated acetic acid is usually titrated directly. Preferably, the acetate solution should be exchanged to NaOH by an anion exchanger, allowing a determination of the decrease in the Na+ ion concentration. Similar results were obtained when carbon was oxidized in liquid medium. Carbon in aqueous suspension is attacked by many oxidizing agents, e.g., permanganate (49-52), chromate (52-54), hypochlorite (52, 55), persulfate (52, 56, 57), and bromate ions (52, 56, 57); chlorine (49),dilute nitric acid (52,58), and concentrated nitric acid (28). The neutralization behavior against the four bases used in Table I was studied with a few samples oxidized in liquid medium (45, 46).The same ratio was observed as with the oxygen-treated carbons, except that twice the amount of groups reacting with sodium bicarbonate was found (Table 111). In this case, too, complete oxidation was essential for finding the simple ratio of the neutralization values of 2:3:4:5. Completion of the oxidation was recognized by the appearance of brown colloidal solutions when the samples were washed with alkali. The colloids were removed by washing first with alkali, then with dilute hydrochloric acid and water, before any further experiments were done. Half of the more strongly acidic groups present on the “liquid oxidized” carbons were destroyed 011 vaciiiim outgassing a t ctt. 200”. Att

189


TABLE 111

Neultalization of Acidic Croups on Corbon Oxidized in Liquid M e d i u m Neutralization in meq/100 gm by Carbon"

Oxidant

___

Preparation NaHCO,

RC 12, act., H.T. KMnO, KMnO, KMnO, Eponit, H.T. KMnO, NaOCl (NH4)xSxOn (NH, )&O* (NH,),S,O, CK 3, H.T. "H,)*S,O*

1st 2nd 3rd -

let 2nd

3rd

-

30 39 61 I9 107 145 89 87 21

Na,CO,

NaOH

NaOEt

45 02 89 116 163 203 89 132 30

63 87 125

81 95 147 220 262 339 171 236 57

160 214 269 119 173 36

"For abbreviations see Table I

the same time, an equivalent amount of carbon dioxide was split off (35). The neutralization ratio became 1 :2:3:4 again. On further heating, the group reacting with sodium hydroxide but not with carbonate started to decompose a t 240" with evolution of CO. It was completely destroyed before the other groups were attacked, too. CO, was formed on destruction of the group neutralized by carbonate and stronger bases. Puri and collaborators (36, 59) found that the amount of CO, given off on heating to 1200" was always equivalent to the Ba(OH), or NaOH neutralization. Evolution of CO, was complete between 750 and 900". Samples oxidized in liquid medium evolved more CO, in relation to CO on heating than did samples treated with oxygen (36,55). Puri and Bansal (59) suggested that the neutralization of alkali was caused by carbon dioxide chemisorbed on the carbon surface ('TO, complex"). If .carboxyl groups were responsible, 1 mole of CO, should be formed for each equivalent of alkali consumed. The author of this article thinks, as will be shown below, that very likely carboxyl groups of different environment are responsible for bicarbonate and carbonate neutralization as well as CO, evolution. The carbon blacks used by Puri and Bansal(59) seemed to be different from the carbons used by Boehm et al. (35).Titration curves published by Puri and Bansal(59)show distinct breaks at p H 10-1 1. Other authors (26, 44) did not find this inflection. Possibly, 110 phenolic groups were present in Puri's samples. As will be shown below, the group neutralized by NaOH, but not by Na,CO,, was identified as phenolic hydroxyl group,

190

H . P. BOEHM

No phenols are formed if the carbons are quenched rapidly after heating in oxygen. It should be pointed out here that the use of Ba(OH), in the place of other alkalies has two disadvantages. First, adsorbed CO, will give a precipitate of BaCO,, while soluble carbonates are formed with NaOH or KOH which can be backtitrated as well as the hydroxides. More severe is the fact that, in general, two ways of neutralization are possible, as shown here. -COOH + Ba(OH),

-COOH

I

-I

-COOH

-COOH

-coo-coo-

-COO+ 2Ba(OH), ----+

Ba2'

+ 2 H,O

Baa OH-

-COO- BaZ+OH-

+ 2H,O

Equivalent neutralization is normal if the acid sites (carboxyl groups in the example) are close together as is the case with most ion-exchange resins. If, however, there is some distance between the acid sites, equimolar neutralization will occur. The charges will be compensated much better in this way. As Pauling (60) has pointed out, positive ions will seek the places of highest negative potential and vice versa. Experimental proof of this tendency which is not only valid for the interior of crystals, but for surface as well, was first presented by A. Weiss ( 6 1 ) using kaoliiiite and by Boehm and Schiieider (62) using finely divided silica. Studebaker (63)studied the potentiometric titration of surface oxides in nonaqueous medium. Using sodium aminoethoxide in ethylene diamine, he found indications for the appearance of two breaks in the titratioii curves. The first break was attributed to carboxyl groups or groups of similar acidity, the second one to phenols. Only two-thirds of the acidity that was determined by NaOH neutralization could be titrated in iioiiaqueous medium. Sitr of the acidic surfaw oxides. The question whether the acidic surface oxides are bound to the periphery of the carbon layers or to the basal planes of the crystellites could be resolved by oxidation of a graphitized carbon black ( 3 6 ) .The particles of carbon black arc, at first approximation, spherical. The graphite-like crystallites show such preferential orientation that their c axis are aligned in a radial direction (64, 65). A schematic representation of this secondary structure is given in Fig. 1. On recrystallization between 2000 and 3000", maiiy small

CHEMICAL IDENTIFICATION O F SURFACE GROUPS

191

crystallites merge forming a pyramidal-shaped graphite crystal. The particles assume the shape of polyhedra consisting of a few graphite crystals (65, 66).The apices of the pyramids are turned to the center of the polyhedron (Fig. 2). Therefore, the surface of graphitized carbon black consists of graphite basal planes only and is very homogeneous in consequence. When graphitized carbon black was oxidized with oxygen a t 420", no acid groups were formed a t all within the usual limits of detection ( 4 6 ) .

FIG.1 . Schematic representation of the structure of the particles of carbon black and graphitized carbon black [after Roehm (66)j.The short lines indicate the orientation of the layer planes.

The conclusion from this observation is that the acidic groups are bound only to the peripheral carbon atoms of each layer, as had been expected. Hennig (67) proved in an elegant way that almost no oxygen is bound a t the basal planes of graphite. Large single crystals of graphite had been heated to 800" and cooled in pure oxygen of low pressure. Afterwards, the surface oxides were decomposed in a vacuum a t 900"and the amount of carbon oxides was determined. The same quantity of gas was evolved when the crystals had been cleaved five times before oxidation. The number of atoms in the cleavage faces was multiplied by this cleaving, while the number of edge atoms remained constant. It is remarkable that the oxygen content exceeded the number of peripheral carbon atoms significantly. Hennig thought it likely that part of the oxygen was bound in between the carbon layers forming ether links between adjacent layers which are puckered a t the periphery. Using tritiated water as a tracer, Montet (68)showed in a very similar manner that water vapor is adsorbed predominantly by the prism faces of graphite crystals. The water film could he removed completely only by outgassing at 800-900". Methylation. The reaction with diazomethane has often been used for differentiating the acidic groups (28, 35, 38, 45, 46, 69). Diazomethane reacts, in general, with carboxylic acids, forming methyl esters which are easily hydrolyzed by dilute hydrochloric acid. With phenols, ethers are formed which are stable to hydrolysis. Alcohols are methylated only if catalysts are present, e.g., BF,, ZnC1, (TO),or H,O (72). As Garten et al.

192

H. P. BOEHM

(44)pointed out, diazomethane reacts with lactones of the fluorescein type transforming them into the quiiioid form (72):

A similar grouping could be present at the edge of a carbon layer (73):

dilute acid

\

Diazomethane is capable of addition reactions with double bonds. Pyrazoline rings are formed. Studebaker et al. (38)saw in this reaction an indication of quinone groups:

OH

0

However, from the nitrogen content a lower percentage of quinoiie oxygen was calculated than from the hydrogen uptake during reduction with sodium borohydride. Other double bonds can react with diazomethane as well (74):

mo moMefwM ChN, &O/MeOH

CH

~

\

HC’

I

H,C-N

‘N

I1

Therefore, i t is not feasible to estimate the quinone content from the nitrogen uptake. The results of methoxy determinations after the reaction with diazomethane are presented in Table IV. The same carbon samples were used as in Table I. When dry ether was taken as solvent, acidic groups

1!U


TABLE IV Comparieon of Neutralization Values and Methoxy Determinations after Methylation with Diazornethane [after Diehl (-17)r

Productb

-OCH, in meq/100 gxn Neutralization in meq/100 gm by __-_ Resistant to Na,CO, NaOH NaOEt Total Hydrolyzable hydrolysis

SC 1

50

81

-

SC 1, H.T. SC 3, H.T.

32 82

69 124

85 165

SC 1, art.

43

71

89

SC 2, act. SC 3, act.

94 106

103 167

215 214

7-2 34 60

102 62 89

137 106

SC 1, act., H.T.

Eponit CK 3, sample 1

-

82 109 72 124 122 72 92 211 168 215 140 107 89

50 79 34 81 81 42 62 141 106 156 99 46 56

32 30 3x 43 41 30 30 70 62 59 41 61 33

"The carbons were oxidized with oxygen at 420-450". CH,N, was used in anhydrous etheric solution. Figures in bold face were obtained using ether saturated with water as solvent. *Forabbreviations see Table I.

equivalent to the sodium hydroxide neutralization were methylated. Using ether saturated with water, higher methoxy values, equivalent to the sodium ethoxide neutralization, were obtained. A part of the methoxy groups was readily hydrolyzed by refluxing dilute hydrochloric acid. Presumably, methyl esters of carboxyl groups were saponified. Their number was equivalent to the carbonate neutralization. The methoxy groups resistant to saponification seem to be phenol ethers. I n accordance with this conclusion, the NaOH neutralization decreased after hydrolysis to the Na,CO, value (Table V). This behavior would fit the assumption by Garten et al. (44)that fluorescein- or phenolphthalein-type lactones were present. A s is well known, the lactone ring in phenolphthalein is opened by sodium carbonate, giving rise to the red coloration. It does not react with bicarbonate. The results of Tables IV and V could be interpreted by assuming the presence of free carboxyl groups and lactones of this type. However, there is another, equivalent explanation (35,46). Lactols, i.e., hydroxylactones of the type

194

I€. P. AORHM

R OH

react in a similar manner. Such lactols are tautomers of formylacid carboxylic acids. For instance, 4-formylphenanthrene-5-carboxylic is formed by the action of ozone on pyrene dissolved in acetic acid (75)or by oxidation of pyrene with hydrogen peroxide in the presence of osmium tetroxide (76):

This compound is known to exist normally in the lactol form (77, 78). TABLE V Neutralization Behavior ajter Hydrolysis of Methyl Esters [ajter Diehl (47)y

Productb

sc 1 SC 1, H.T. SC 3, H.T.

SC 1, act. SC 1, act., H.T.

CK 3, sample 1

Neutralization in meq/100 gm by -_____ NaHCO, Na,CO, NaOH

27 (27) 16 (16) 40 (42) 21 (21) 36 (35) 32 (31)

"The carbons were oxidized with oxygen at 420-450". Esters were hydrolyzed by refluxing with 1 N HCI. The results for the starting materials are given in parentheses. *For ahhreviat,ions RRA Tahln T


195

Likewise, aromatic ketocarboxylic acids with the carbonyl and the carboxyl groups attached in the 1,2 position react as lactols ( 7 8 ) :

Diazomethane gives with these compounds the easily hydrolyzed carboxyl methyl esters (78):

K:roMe Am')

With methanol and hydrochloric or sulfuric acid the pseudoesters of the lactol form are obtained (77, 7 8 ) :

Very likely, the pseudo-esters are susceptible to saponification as well. Both esters can be distinguished by their reactions with carbonyl reagents. A phenylhydrazone is formed only by the compound with a free carbonyl group. A definite decision between the models proposed by Garten et al. ( 4 4 ) and by Boehm et al. (46)is not yet possible. An attempt was made by de Bruin and van der Plas (79) to identify lactones in carbon blacks using ammonia dissolved in methanol or hydrogen bromide in glacial acetic acid. I n the first case, either an amino acid or an amide of an hydroxycarboxyl acid should arise. Ammonium salts could be formed, too. Therefore, the increase in nitrogen content is not very specific. With hydrogen bromide, formation of the corresponding bromocarboxyl acid is expected. Uptake of bromine was observed. However, no increase was found in the carboxyl content as determined by the sodium bicarbonate and the calcium acetate methods. A definite proof of either presence or absence of lactones could not be given. Further evidence must be obtained by reaction with carbonyl reagents and by reduction experiments. Instead of diazomethane, dimethyl sulfate can be used for the methylation as well (35).If the reaction is allowed to proceed in the presence of

196

€ P. I. BOEHM

an excess of alkali, only phenols are methylated. The same methoxy contents were found as after saponification of the diazomethanetreated samples. The carboxyl groups were methylated, too, if dilute sodium hydroxide was added very slowly, so that the medium was kept only slightly alkaline. A pH-stat might be useful in this operation. The resulting methoxy content was then equivalent to the NaOH neutralization. Methylation by refluxing with methanol in the presence of dry hydrogen chloride is thought (31) to attack carboxyl groups only, I n Table VI it is shown that after repeated reaction the methoxy of content TABLE VI Formation of Methyl Ester8 by Rejluxing Oxidized Carbon with Methanol in the Preeence of Hydrogen Chloride [after Diehl (47)r ~

~~

Productb

NaHCO, neutralization

Difference Na,CO,-NaHCO, neutralization

SC 3, H.T. sc 1, act. SC 2, act. SC 1 , act., H.T. CK 3, sample 1 Eponit, H.T. (oxidizd with NaOC1) Eponit, H.T. (oxidized with KMnO,, incomplete oxidation)

42 21 47 47 31 107 14

40 22 47 53 29 56 16

Methoxy content 44 23 48 67 35 60

16

‘All results in meq/lOO gm. *Forabbreviations see Table I. If not otherwise stated, the samples were oxidized with 0, at 420-460’.

was only equivalent to the NaHCO, neutralization or, rather, t o the difference between Na,CO, and NaHCO, neutralization. This was confirmed by the neutralization behavior of the esterified sample “SC 3, HT. 020x.”:NaHCO, and Na,CO, neutralization t e r e equal. Therefore, the less acidic carboxyl group must have been esterified. The neutralization of sodium ethoxide decreased from 166 to 82 meq/100 gm, indicating that the group capable of reaction with ethoxide, too, had been affected by the methylation. The same degree of methylation as by refluxing with methanol was achieved by blowing a stream of methanol vapor and nitrogen over the carbon heated to 120”.


197

These results made the presence of various functional groups very likely. Definite proof by other, independent methods was needed. IdentiJication of carboxyl groups. It was assumed that the stronger acidic groups, giving methyl esters with methanol, were carboxyl groups. In agreement with this assumption, CO, is evolved on thermal decomposition (35, 36). Puri and co-workers (80, 81) observed that water and alcohols were strongly adsorbed by the groups which give CO, when the samples are outgassed a t 1200". No adsorption occurred on the groups disposed as CO. About 1 mole of water was very firmly held per mole of CO, forming groups. The heat of immersion in water was proportional to the amount of oxygen present as CO, complex and not to the total oxygen content (82). Clearly, these effects are due to groups capable of forming strong hydrogen bonds with water and alcohols. Hydroxyl and carboxyl groups are such sites. The groups capable of reaction with NaHCO, reacted with solutions of potassium iodide and iodate liberating an equivalent amount of iodine according to 51-

+ 10,- + BH+ + 31, + 3H,O

(46).The pH of such a solution is stabilized close to 7 . Rivin ( 8 3 , 8 4 )found a close agreement between NaHCO, neutralization by carbon blacks and their adsorption capacity for diphenylguanidine from benzene solution. The carbonate neutralization, again, was twice the bicarbonate value (84). The carbon dioxide evolved on vacuum pyrolysis was roughly equivalent to the carboxyl content. Acyl chlorides of the stronger acidic groups are formed by the reaction with thionyl chloride (35, 45). The amount of easily hydrolyzed chloride was equivalent to the NaHCO, neutralization or t o the difference between Na,CO, and NaHCO, neutralization. Both values were equal in most examples. About twice this amount of chlorine was bound a t the same time in a form that was resistant to hydrolysis. The nature of the chlorine bonding is not yet known; there was no sulfur in the samples. Unambiguous identification of carboxyl groups was achieved by two reactions of the acyl chlorides (35). I n a Friedel-Crafts reaction, the chloride was treated with dimethylaniline in the presence of aluminum chloride. Nitrobenzene was used as solvent. After exhaustive extraction, the nitrogen content was equimolar to the former chloride content. No NaHCO, was neutralized by the reaction product; the other neutralization values decreased correspondingly. Further confirmation was gained by the successful Schmidt rearrangement. This was done in the variation described by Schroeter (85).

108

H. P . BOEHM

The acyl chloride was treated with a solution of sodium azide in ethanol. The nitrogen content of the extracted substance was equimolar to the initial chloride content, which was hydrolyzed easily. The reaction stopped at the urethane stage: R-COCI

NaN,

--+ R-CON,

- N. __f

t EtOH

R-NH-COOEt

No free amino groups could be detected. The ethoxy conteiit corresponded to two ethoxy groups, one apparently being present as ethyl ester of the carboxyl group neutralized by Na,CO, or stronger bases only. This information was obtained from neutralization experiments. Information concerning the position of the carboxyl groups relative to each other was obtained from the neutralization behavior after reaction with thionyl chloride (35,4 7 ) . More base ought t o be consumed by such products because additional alkali is used for the neutralization of the hydrochloric acid liberated on hydrolysis. As is shown in Table VII, the additional alkali consumption was equivalent t o the quantity of chloride ions found in the solution. However with all the samples which had been activated with carbon dioxide, less sodium ethoxide was consumed than had been expected. The deficit was equivalent to half the NaHCO, neutralization value. This strange behavior can be explained only by TABLE V I I Reaction of Acidic Surface Oxides with Thionyl Chloride [after Diehl (47)y ~

Sample*

SC 1, H.T. SC 1, act. sc 2, act. SC 3, act. SC 3, act., reduced with LiAlH, Eponit, H.T., oxidized with (NH,),S,O,

NaHCO, neutralization

Difference Na,CO,NaHCO, neutralization

C1hydrolized

Change in neutralization by NaHCO, Na2C0, NEOH NaOEt

16 21 47 60 60

15 22 47 46 35

16

+16

23 45 48 38

+20 +45 +47 +35

$19 +17 +47 $45 +35

+18 +21 $49 +41 +35

59

30

31

+30

+32

+30

0

-7.5 -29 -34 -31

+ti

~~~

“All results in meq/100 gm. ‘For abbreviations soe Table I. With the exception of Eponit, all samples were oxidized with 0, at 420-450’.


I99

assuming that with the “activated” samples always two of the more acidic carboxyl groups were in close vicinity, allowing the formation of a carboxylic acid anhydride. Acids like phthalic acid will give anhydrides, rather than chlorides, on reaction with SOCl,. Chlorides of carboxylic acids will consume 1 mole of sodium ethoxide: R-COCl

+ NaOEt + R-COOEt + NaCl

Anhydrides of carboxylic acids will consume only half a mole of sodium ethoxide per original carboxyl group: It-CO

\ R-CO

/

0

+ NaOEt -+ R-COONa + R-COOEt

Therefore, no change in the sodium ethoxide consumption should be expected with samples transformed to the chlorides. A decrease points to the formation of acid anhydrides. The conclusion that the groups neutralized by NaHCO, were in the close vicinity was confirmed by the reaction with ammonia. Here, too, a differentiation between “activated” and “nonactivated” oxidized carbons was observed. The amount of ammonia retained after outgassing at 100” was equivalent to the NaHCO, neutralization with carbons originally not “activated.” With carbons that had been activated, the ammonia retention was equivalent to only half of this value (35, 46). There is normally a considerable difference in acidity of neighboring carboxyl groups due to the formation of one hydrogen bond between the groups. Stable ammonium salts are formed only by stronger acids. An imide, e.g., phthalimide, is formed when the ammonium salts are heated to higher temperatures. The formation of imides could be excluded since ammonia could be expelled with magnesium hydroxide from the samples (outgassed at 100”).

After treatment with ammonia gas and outgassing a t 20” an NH, uptake equivalent to the Na,CO, neutralization value was observed repeatedly (35). There is a strong fixation of ammonia at higher temperatures. Hofmann and Ohlerich (28)found that the fixed ammonia was resistant to hydrolysis and that it formed no salts with hydrochloric acid as would be expected of amino groups. The formation of amides was not excluded. Studebaker ( 8 6 ) investigated the reaction of carbon blacks with ammonia. He observed also that the nitrogen became completely unreactive after heating, towards acids as well as towards alkali.

200

H. P. BOEHM

Van Slyke determinations for primary amino groups were unsuccessful. There remains the question why “activated” carbons differ from carbons heat-treated at 1200”with respect to the relative position of the carboxyl groups. Perhaps this difference is based on the structure of the edges of the carbon layers. Hennig (87, 88) found, by observations with single crystals of graphite, that after oxidation with dry oxygen the “armchair” configuration of the periphery resulted:

I n the presence of small amounts of water, however, the “zigzag” structure was observed:

Since siigar charcoal-and nearly any other carbon-always contains some hydrogen, the “zigzag” structure should be expected after oxidaTABLE VIII Identijcution of Phenolic Hydroxyl Groups on Oxidized Curbona [after Diehl (37)l’’ Reaction with Product6

SC 1, H.T. SC 3. H.T. SC 1, art. SC 2. art. SC 3, art. SC 1 , act., H.T. SC 4, act., H.T. CK 3, sample 1

Difference NaOH - Na,CO, neutralization

Acid-resistant -OCH, groups

38 42 29 69

38 43 50

61

62

62

30 39 32

41

31 37 13

_ I _ _ _ -

DNFB

p-Nitrobenzoyl chloride

37 42 30

TO

33

40

29

“All results in meq/100 gm. ’Vor abbreviations see Table I. All samples were oxidized with oxygen at. 420-450”


20 1

tion. It is not known, unfortunately, which structure of the edge prevails after activation with carbon dioltide. If it were the “armchair” form, it might be preserved, a t least partially, during the subsequent covering with surface oxides. However, this seems rather unlikely in view of the substantial weight loss during oxidation. Identijication of phenolic hydroxyl groups. The groups which are neutralized by sodium hydroxide, but not by the carbonate, and which give acid-resistant methoxy groups, seem to be phenols. This was confirmed (35,46) by the reaction with typical phenol reagents, dinitrofluorobenzene, and p-nitrobenzoylchloride (Table VIII). Dinitrofluorobenzene (DNFB) reacts with phenols and, which is not of interest here, with amino groups. Hydrogen fluoride is eliminated. DNFB does not react with carboxylic acids. Alcohols, if they react at all, form dinitrophenyl ethers very slowly. Very weakly dissociated phenolic hydroxyl groups, e.g., in salicylic acid (pK = 13.4), are inert towards DNFB. A procedure given by Zahn and Wiirz (89)was followed. 0.5 gm of the carbon was agitated with 25 ml of a 1 M solution of DNFB in dimethylformamide (DMF) for 20 hours. In order t o neutralize the H F set free, 10 ml of a 1% aqueous solution of NaHCO, were added. The reaction products were washed and extracted successively with DMF, dilute HCl, methanol, and ether. Nitrogen determinations, Kjeldahl as well as Dumas determinations, gave identical results. Treatment with piiitrobenzoylchloride followed standard procedures. In all examples, excellent agreement was observed for the extent of the DNFB reaction or for the p-nitrobenzoyl contents with the difference between NaOH and Na,CO, neutralization and with the content of hydrolysis-resistant methoxy groups. Of course, other hydroxyl groups of similar acidities, e.g., enols, would react in the same way as phenols. However, the presence of phenols at the periphery of an aromatic system is much more likely. With acetyl chloride, the carboxyl groups were acetylated as well as the phenols (45).The resulting acetyl contents were equivalent to the NaOH neutra!ization. Determinations of active hydrogen by the Zerewitinoff method were undertaken by Studebaker (63).With many carbon blacks, there was a very good correlation with the difference between the first and second breaks in the titration curves attributed to phenolic hydroxyl groups (see page 190). This seems peculiar, since the stronger acidity titrated at the first break is very likely due to a free carboxyl group and, furthermore, lactones seem to be present also. There would be an active hydrogen either in a free carboxyl group or in a fluorescein-type lactone. No active

H. P. BOEHM

202

hydrogen would be contained either in a carboxylic acid anhydride or in a normal lactone. The presence of normal lactoiies in oxidized carbons was postulated by Garteii et al. ( 4 4 ) . However, Zerewitinoff determinations proved to be impracticable with porous carbons, such as sugar charcoal, because of extremely slow and incomplete methane evolution. An attempt was made by Diehl (47) to obtain information concerning the relative position of the phenolic groups with respect to each other. Comparison of the ion exchange with univalent and divalent ions can provide information on the relative distance between ion exchange sites. As was pointed out on page 190, equivalent exchange of hydrogen ions for barium ions is t o be expected only if there is no great distance from one acid group to the next one. Otherwise equimolar ion exchange will occur. A barium ion will be bound by each acid group, the remaining charge being neutralized by an anion, e.g., an hydroxide ion. Actually, a higher neutralization value resulted with barium hydroxide than with sodium hydroxide as is shown in Table IX. This indicates widely spaced acid groups. The Ba(OH), neutralization value was not twice as large as the NaOH value, however. Therefore, some of the acid groups were sufficiently close together to allow equivalent ion exchange. By use of the same oxidized carbon after diazomethane treatment and saponification, i.e., with the phenolic groups converted to methyl ethers, i t became clear that the equivalent exchange was caused by the phenolic groups. Twice as much barium hydroxide was neutralized now as sodium hydroxide. No experiments have been made yet with “activated” carbons. Evidence for the presence of hydroquinones, which are phenols as TABLE IX Equimolur and Equivalent Ion Exchange with Barium Hydroxide [after Diehl ( 4 7 ) ]

Neutralization of

SC 3, H.T., 0, oxidized meq/100 gm

NaHCO,

SC 3, H.T., 0, oxidized, CH,N, treated, saponified meq/100 gm

NalCO, NaOH Ba(OH),

42 82 124 199

40 81 163

Ba(OH),’

206

164

83

“Calculated for equirnolar exchange with carboxyl groups (NaHCO, and Na,CO, neutralization) and equivolent reaction with phenol groups.


203

well, was presented by Hallum and Drushel (90). They used polarographic methods in the investigation of carbon black surfaces. The results will be discussed in detail in the following section. Identi$cation of carbonyl groups. It has often been claimed that the carbon layers of graphite terminate in carbonyl groups:

No proof for such structures was ever presented. Although the existence of carbonyl groups has been proven by other research workers, their amount was never sufficient to cover the whole surface in this way. Studebaker et al. (38)attributed the nitrogen uptake on reaction with diazomethane to the formation of pyrazoline rings by addition to quinones. This assumption was substantiated by the ability of the samples t o bind hydrogen from sodium borohydride or on catalytic hydrogenation. With acidic carbon blacks, twice as much hydrogen was bound catalytically (using Adams catalyst) than by reaction with sodium borohydride (63).With basic furnace blacks, the quantities were equal. The results with NaBH, indicate that 18% of the oxygen on the carbon black surface reacts like quinone. Garten et al. ( 4 4 ) thought the existence of fluorescein-type lactones very likely (see page 192). There is a carbonyl group formed on change to the carboxyl form of the lactone. In the similar proposal of a lactol (page 194) an aldehyde would be formed. Garten and associates substantiated their evidence by infrared spectra of the carbon black “Carbolac No. 1 ” (color black). It is difficult to obtain useful infrared spectra with carbon due to the scattering by the particles and to a high background absorption. “Carbolac” showed an absorption at 1600 em-’ which was attributed either to aromatic C =C bonds or to carbonyl groups chelated to phenolic hydroxyl groups. Another absorption at 1760 cm-l arises from the presence of a carbonyl group which may be that of a lactone. This band disappears when the sodium salt is formed. Instead, a weaker band appears at 1720 cm-l which may be caused by an aldehyde or a ketone. Using the same type of color black, Hallum and Drushel (90) observed that the band near 1585 cm-I decreased in intensity after methylation with diazomethane, while a distinct band appeared at 1750-1700 em-’, the wave number usually found with normal carbonyl groups. At the same time, a weak absorption band arose at 1240 cm-I which can be attributed to aromatic methoxy groups. The authors concluded that originally an aromatic hydroxyl group had been hydrogen-bonded to a

204

H. P. BOEHM

carbonyl group. Identification of hydroxyl, carbonyl, and carboxyl groups by their infrared absorption was also reported by A. V. Kiselev and collaborators (91).They, too, noticed interaction between hydroxyl and carbonyl groups. Valuable information was obtained also by Hallum and Drushel ( 9 0 , 9 2 )from polarographic analysis of carbon black slurries in dimethylformamide using tetra-n-butylammonium iodide as supporting electrolyte, Distinct waves were found in the polarograms. From the half-wave potentials (between - 0.6 and - 0.8 volts) the existence of quinones was inferred. The reduction was a two-electron process. Treatment with lithium aluminum hydride or with methyl magnesium iodide caused the wave to disappear completely. Hydroquinones were detected in a similar manner by anodic polarography. This wave disappeared upon treatment with hydrogen peroxide or diazomethane. The authors proposed a model based on the following types of surface groups:

A possible mechanism for the chemical interaction of carbon blacks with elastomers on the basis of this model is discussed in their paper. Similar waves in the cathodic polarogram were observed by Donnet and Henrich (58) using oxidized carbon black. The wave disappeared after treatment with isobutyronitrile. It was assumed that isobutyronitrile gives an addition reaction with quinones. No reaction with this reagent was observed after reduction with hydrogen iodide, after treatment with aniline, or after treatment with diazomethane. The latter finding confirms the assumption by Studebaker et al. (38) that diazomethane is added to the quinones in the carbon black surface. Further evidence for the presence of quinone functions was presented by Garten and Weiss (93). Attempts were made in my laboratory to determine carbonyl groups with various carbonyl reagehts, e.g., hydroxylamine, semicarbazide or dinitrophenylhydrazine. With hydroxylamine, oximes were formed to an extent that was equivalent to the difference between NaOEt and NaOH consumption. Errors due to binding of hydroxylammonium ions, which would show up in nitrogen determinations as well, were prevented either by methylation of the acidic groups with diazomethane or by ion exchange with dilute sodium hydroxide; oximes are stable towards dilute alkali. However, only half the quantity of carbonyl groups reacted with semicarhazide nr with dinit,rnphenylhydrrteiiie.


205

This behavior indicates that two carboiiyl groups are always close neighbors, since it is typical for 1,2- or 1,3-diketones. De Bruin and van der Plas (79)also used hydroxylamine in an attempt t o identify carbonyl groups. I t is not stated whether this reagent was used as free base or as a salt. The considerable nitrogen uptake was very nearly the same as on reaction with amnioiiia in methanol. Perhaps ammonium satts had been formed in both reactions. The same authors found a pronounced reduction with TiCI, which exceeded by far the extent of all other reactions. The TiCI, reaction gave easily reproducible results. Reduction with LiAlH, was used by Rivin ( 8 4 ) in the determination of carbonyl groups. An increase in active hydrogen after reduction was ascribed to formation of Iiydroquinones from quinones. The difference t o the total reductioii was thought to be due to lactones. Oxygen had to be excluded scrupulously in these experiments i n order to preclude basecatalyzed oxidation of the carbon. At lower temperatures, the reaction with LiAlH, was used for the determination of active hydrogen. There was good correlation with the results of sodium hydroxide neutralization. The total number of acidic and nonacidic groups thus determined agreed well with the quantity of carbon oxides evolved on vacuum pyrolysis. The neutralization values were influenced by reduction with strong reducing agents, lithium aluminum hydride, sodium borohydride, and amalgamated zinc plus hydrochloric acid (35,46).For the most part, the consumption of Na,CO, and of NaOEt decreased in equivalent amounts. This is further confirmation of the assumption that lactones of the fluorescein type or of the lactol type are present. The reaction with sodium ethoxide was shown to be no true neutralization, that is, exchange of Hffor Naf, a t all, but an addition reaction with the formation of the sodium salt of a semi-acetal or ketal:

3s

+ 2NaOEt

or

-

COONa

3 d H ONa \

'OH

$'

OEt

-7

+ 2NaOEt

OH

+ EtOH

-7

-

-PrnNa

H. P. BOEHM

206

The usual consecutive reactions, Cannizzaro reaction or aldol condensation, would be prevented by the spatial fixation of the carbonyl groups on the carbon surface. Ethoxyl groups were detected in the reaction product obtained with NaOEt solution ( 9 4 ) .Since semi-acctals and their salts are extremely sensitive to hydrolysis, the ethanol used as solvent for the NaOEt reagent was removed by thorough washing with dilute NaOH. The samples were dried without removal of adhering NaOH and ethoxyl determinations were run. The results agreed always with the difference between NaOEt and NaOH consumption. Identification of f r e e radicals. The existence of free radicals in carbon has been shown by electron spin resonance (e,s.r.) techniques by a large number of scientists, notably by Bennett, Ingram, and I‘apley (95), Etienne and Uebersfeld (96n),Uebersfeld et al. (96b),and Mrozowski arid Andrew (97).It is not possible to summarize here all publications dealing with e.8.r. measurements on carbon. Attempts a t chemical identification by combination with free radicals were made. The reaction with “benzidine blue” was used by Garten and Sutherland (98).The reagent is obtained by controlled oxidation of benzidine. It is very unstable, however, and other methods proved to be better suited. Szwarc (99)found a great affinity for methyl radicals in carbon black. Donnet and co-workers (58, 100, 101) determined the concentration of free radicals on carbon black surfaces by the fixation of the radicals of isobutyroni trile, 3,5-dichlorobenzoyl peroxide, and lauroyl peroxide. The number of radicals bound by the surface coincided satisfactorily with the number of unpaired electrons determined by e.s.r. The reaction with the isobutyronitrile radical proved to be most TABLE X Free Radicals in Oxidized Carbon Blacka Quinonc groups” (mmoles/100 gm)

Free radicals“ (meq1100 gm)

Original

6 20 62

VI VIII

64 84

19 24 51 65

Sample

.- .-

20

F

3

78

““Philblack 0” oxidized with 40% nitric acid. Results calculated from values given by Donnet el al. ( 1 0 1 ) . lFrom fixation of isobutyronitrile. rFrom difference in fixation of isobutyronitrile radical and isobutyronitrile.

CHEMICAL IDENTIFICATION OF S U R F A C E G R O U P S

207

convenient. The radical is obtained by heating a solution of azoisobutyronitrile in benzene to the boiling point: h b 2 --t N,

hle,C-N=N-C

I

I

+ 2 Me--*

CN

C"

Me

I

CN

The nitrogen uptake is easily determined by the Kjeldahl method. However, quinones are powerful acceptors for free radicals, too. Therefore, it is necessary to determine the quinone content by other reactions, for instance with isobutyronitrile (see below). Besides, this check is necessary because isobutyronitrile is always present in solutions of its radical. The difference in the nitrogen content after reaction with isobutyronitrile and with isohutyronitrile radical is attributed by Doniiet to the free radicals in the carbon surface. Table X shows that after reaction with isobutyronitrile radical very nearly twice the amount of nitrogen was found than after reaction with isobutyronitrile. There seems to be a ratio of one free radical per quinone oxygen. Donnet et al. (101) concluded that either paraquinones or aroxylic radicals were present, which would react according to the following schemes. 0 Quinone: I1

6 0

after reaction with isobutyronitrile Me,CH-CN OH

(R-€I) :

OH

after reaction with isobutyronitrile radicals Me,C*-CN present) :

oR

Aroxyl:

I

OH

I

OR

(R*, R-H

is

208

H. P. BOEHM

after reaction with isobutyronitrile: OH I

after reaction with isobutyronitrile radical: OH

OR

Since the number of free radicals calculated from the difference in nitrogen uptake (1.13. 1OZ0/gm)agreed well with the number of unpaired electrons as determined by e.s.r. (0.80. 1OZ0/gm),the aroxylic structure seemed very likely. The reaction of oxidized carbon black with styrene can be explained on this basis (102). The reaction with free radicals plays an important role in the interaction of carbon black with rubber (103) and with styrene (58, 102). However, some workers doing research on e.s.r. are convinced that the unpaired electrons are not localized on the carbon surface. This point is not yet decided, as was pointed out by Singer (104).The concentration of unpaired electrons is diminished by formation of surface oxides as was shown by Jackson, Harker, and Wynne-Jones (105). I n contrast to these results, Antonowicz (206) found that spin centers originated on formation of surface compounds with oxygen, sulfur, chlorine, etc. Very likely, the type of starting material is decisive for its behavior on surface compound formation. 6 . Basic Surface Oxides. Although the basic surface oxides have been known even longer than the acidic surface oxides, their structure is not yet elucidated to an entirely satisfactory degree. As was mentioned in the introduction, Frumkin and his group [Kuchinsky et al. (107) and Frumkin et al. ( I O S ) ] proposed an electrochemical theory, according to which the adsorption of electrolytes by carbon would be determined by the electrical potential a t the carbonsolution interface, and by the capacity of the double layer. At higher concentrations some physical adsorption of acid might occur in addition. Shilov (23) attributed the adsorption of acids to genuine surface oxides of basic nature. According to Steenberg (109)) adsorption of inorganic acids involved primary adsorption of protons, by physical forces, and secondary adsorption of anions. The adsorbed anions were


200

exchangeable. A part of the adsorbed acid could be displaced from the surface by water-immiscible unpolar liquids, e.g., toluene. This was confirmed by Strashessko et al. (110). Steenberg claimed physical adsorption t o be the main force in the binding of acid. Toluene is more strongly adsorbed than the acids. The opposing theories were reviewed in detail by Garten and Weiss (73,111).To summarize the facts: acid is adsorbed only in the presence of oxygen (16),the adsorption is dependent on oxygen pressure, a t least at low pressures (22,lie), and a part of the adsorbed acid can be removed by solvents such as toluene (109, 111). According to Burstein and Frumkin (113),hydrogen peroxide is liberated when acid is adsorbed in the presence of oxygen: C,O,

+ 2H+ + 2X- -+

C,2+Xp-

+ H,O,

The quantity of hydrogen peroxide found was not equivalent to the acid sorption. Catalytic decomposition might be responsible for this. The formation of peroxide-like substances when moist air and acid reacted on carbon was deducted by Lamb and Elder ( l l a ) ,Kolthoff (ZO), and King (33) from the positive potassium iodide-starch test and, in my laboratory, from the reaction with titanyl ions. The only concept able to explain all the observed phenomena was brought forward by Carten and Weiss (111). They suggested that the oxygen was bound in chromene-like structures. On oxidation in the presence of acid, carbonium ions would be formed:

A transfer of the positive charges would be possible:

Benzpyran and benzpyrylium salts are model substances. Carbonium and oxonium bases are usually very weak bases, and complete hydrolysis of the salts should be expected on exhaustive washing with water. However, complete removal of the acid is apparently not possible (83, 115). As Garten and Weiss (73) pointed out, free radicals in the surface might as well be responsible for the apparent neutralization of acid, giving the same carbonium ion :

210

H. P. BOEHM

The presence of cationic surface sites on carbon blacks which had been treated with oxygen in the presence of acid was confirmed by Rivin (83) by various hydride-transfer reactions. Isopropanol was oxidized to acetone in refluxing 50% sulfuric acid: B+X- + Me,CH-OH

HISO,

---+

B-H

+ HX + Me,C=O

(B stands for the carbon black surface). Unoxidized carbon black reduced triphenylmethyl perchlorate to triphenylmethane and was then able to oxidize isopropanol : B-H

+ Ph,C+CIO,- + B+ClO,- + Ph,CH

Quantitative measurements were made using the reaction with refluxing formic acid. A determination was made of the evolved CO,: B+X-

+ HCOOH

4

B+HCOO-

B+HCOO- -+ B-H

+ HX

+ CO,

The amount of liberated carbon dioxide was equimolar to the HC1 adsorption. Rivin confirmed also that hydrogen peroxide is formed by reaction of carbon black with formic acid in the presence of oxygen. Physically adsorbed hydrochloric acid was removed by washing with dioxane. The remaining chloride ions on the surface were replaced by hydroxide ions on treatment with sodium hydroxide. The reaction was formulated as production of a carbinol: B+X-

+ OH- -+B-OH + X-

When the carbon blacks were treated with solutions of iodine in carbon tetrachloride, some iodine was chemisorbed. The samples thus treated adsorbed less acid in the presence of oxygen. Small amounts of iodide ions were desorbed in this process. None of these reactions could establish whether the positive charge is located on oxygen or on a carbonium ion, although the latter possibility seems more likely. Either way, the positive charge is due to the presence of bonded oxygen. One is justified in speaking of basic surface oxides. The evidence in favor of the chromene structure is rather circumstantial, although many phenomena are explained. A more direct proof should be desirable. It was found by Burstein and Frumkin (116) that a carbon that had been exposed to hydrogen at 1000"lost its ability to neutralize acid a t low concentrations, but was able to neutralize alkali instead. Experiments in

21 1

CHRMTC'AL TDRNTTFTCATTON O F SURFACE GROUPS

TABLE XI Persistence of Basic Surfuce Oxides upon Oxidation of Carbon [after Diehl ( I 1 7 ) ]

HCI consumption in meq/100 gm Product Before After oxidation with 0, at 420" SC 1, act., H.T. CK 3 CK 3, H.T. 1100" CK 3, graphitized at 3000" Graphite oxide soot, graphitized at 3000"

39 8

5 17 8

19 8 6 18 11

my laboratory (117)showed that with most carbon samples the ability to bind 0.05 N HC1 was not impaired by oxidation (Table XI). No connection between acid and base neutralization capacity became apparent. The graphitized carbon black adsorbed acid to an astonishing degree, while no acid oxides were formed on its surface. This is remarkable because the surface of graphitized carbon black is made up almost exclusively of basal planes (see page 191). Perhaps, the basic surface oxides are bound a t the basal planes; further studies in this direction are warranted. No change of the base adsorption capacity was observed on treatment with carbon dioxide a t 600 and 800"; an activated sugar charcoal that had been heat-treated a t 1200" was used in this experiment. I n conclusion, i t must be pointed out that the identified acidic and basic groups on oxidized carbon usually do not account for all of the oxygen present on the surface (35). 2. Sulfides

Carbon is able to bind sulfur to its surface. The reaction of sugar charcoal or wood charcoal with elementary sulfur a t temperatures of 400-1000" was studied in detail by Wibaut (118-122). In this reaction some carbon disulfide was formed as well as hydrogen sulfide, if hydrogen was present in the samples. The solid reaction products contained considerable amounts of sulfur, up to 20% by weight. The maximum sulfur uptake was observed a t 600". The sulfur was not completely volatized even by heating in a vacuum to 1000" (122). The sulfur came off in elementary form and as carbon disulfide. Neither could the sulfur be removed from the samples by extraction. It was disposed of by powerful chemical attack, e.g., by oxidation or by reduction with hydrogen a t 700". The formation of hydrogen sulfide started a t 450".

212

H. P. BOEHM

The presence of high-molecular weight p-sulfur with chain structure seemed improbable since the sulfur was not extractable with boiling toluene. The p-sulfur is known to convert to the soluble ring structure (8,) rather rapidly a t 115". Wibaut (119) thought the formation of a carbonsulfur complex similar to the surface oxide formed with oxygen very likely. He was not able, however, to analyze definite surface groups. Hofmann and Nobbe (123) established that the sulfur content was dependent on the specific surface area. Enoksson and Wetterholm (124) confirmed by X-ray diffraction that no crystalline sulfur was present in exhaustively extracted charcoal with 13% sulfur content. Juza and Blanke (125)investigated the reaction of carbon and sulfur between 100 and 1000" at various pressures. They thought it unlikely that there was genuine chemical bonding. The phenomenon of sulfur fixation was ascribed to capillary condensation, adsorption, chemisorption, and solution in the carbon structure. Hofmann and Ohlerich (28) found that the same quantity of sulfur was taken up a t 600" by activated sugar charcoal before and after covering with surface oxides. About equivalent amounts of oxygen and sulfur were bound by the same charcoal. A similar observation had been made by Baraniecky, Riley, and Streeter (126) who charred cellulose in the presence of hydrogen sulfide. The sulfur content corresponded t o the oxygen content of samples treated similarly in the presence of air. Dogadkin, Skorodumova, and Kovaleva (127)studied the reaction of carbon black with sulfur a t low temperatures. A solution in toluene was used at 145" in the presence of an accelerator. The sulfur sorption was negatively influenced by surface oxides. The oxygen-containing groups were not affected by the reaction, since no change in the water vapor adsorption was detected. No hydrogen sulfide was evolved under the reaction conditions. Carbon reacts also with other sulfur-containing compounds with resulting fixation of sulfur. Reaction of various forms of carbon with hydrogen sulfide was reported by Baraniecky, Riley, and Streeter (126) and by Polansky, Knapp, and Kinney (128). The reaction of carbon blacks with hydrogen sulfide and with sulfur was studied extensively by Studebaker (86).At 150°, the increase in sulfur content was proportional to the quinone content as measured by catalytic hydrogenation using Adams catalyst (see page 203). The same author (25)has shown that sulfur is bound to carbon blacks by the action of carbon disulfide a t 150". I n the presence of hydrogen sulfide, less carbon disulfide will react with activated carbon (129). The interaction of carbon disulfide and sulfur with coconut charcoal was studied by Sykes and White (130) a t low pressures in the temperature


213

range 627-927". The results were discussed according to a scheme which postulates the chemisorption of sulfur and carbon disulfide as distinct but interconvertible entities. Reaction of carbon with sulfur dioxide was observed by Fischer and Prauschke (131).In my laboratory it was recently found that sulfur is bound by heat-treated carbon blacks and by graphitized carbon black on treatment with hydrogen sulfide, carbon disulfide, or sulfur dioxide at low temperatures, even a t room temperature. The sulfur content cannot be eliminated by outgassing at 100" or by prolonged extraction with various solvents. Very little is known of the constitution of the surface sulfides of carbon. Sykes and White (130) assumed that the same type of surface sulfide results from the action of sulfur or carbon disulfide. A speculative structure was postulated : s s

A t least a part of the sulfur is chemically bonded to the surface and is not present in elementary form. This was proven by the catalysis in the oxidation of azide ions by iodine: 2N,-

+ I, + 3N, + 21-

As Feigl (132) pointed out, this reaction does not proceed by itself. It is catalyzed by sulfide ions, insoluble inorganic sulfides, or organic sulfides. Elementary sulfur has no influence, nor does normal carbon. On addition of sulfurized carbon to solutions of sodium azide, potassium iodide and iodine, decoloration and the appearance of nitrogen bubbles was observed (69). This indicated that sulfides were present. Nevertheless, some long-chain sulfur might be present in the narrow pores of activated carbons which took up much sulfur at elevated temperatures. It is conceivable that rearrangement to soluble eight-membered rings might be sterically hindered in pores narrower than the diameter of S, molecules. At the temperature of sulfuration, e.g., 600", there is quite a high proportion of S, in the vapor, which could easily penetrate into the pores and polymerize there. Observations by Wibaut and van der Kam (122),that on evacuation at 600" some elemental sulfur came off the carbon, are in accordance with this assumption. All attempts of identifying the sulfides in the surface have been unsuccessful so far. Sulfhydryl groups (-SH) seem t o be present in minor quantities in some samples, as was shown by their ability to

214

H. P. BOEHM

adsorb Hg2+ ions from mercuric chloride solutions. The -SH groups account for only a small part of the total sulfur content. The importance of the knowledge of the reaction of sulfur and sulfurous compounds with carbon for understanding the mechanism of rubber reinforcement was pointed out by Studebaker (25). 3. Other Surface Groups

The interrupted bonds on carbon surfaces can bind other elements as well as oxygen and sulfur. Not much research has been conducted in this direction, however. The more important of the remaining surface compounds contain hydrogen or chlorine. Hydrogen is present in nearly all carbons. With many types of carbon, notably such that were prepared by pyrolysis of hydrocarbons or other hydrogen-containing compounds, considerable hydrogen contents were found by combustion analysis. Hydrogen contents ranging from 0.1 to 0.9% in carbon blacks were determined by Studebaker (25). The hydrogen content calculated on a molecular basis exceeds the oxygen content even with highly oxidized carbon blacks, the so-called color blacks. Four to six atom percent of hydrogen were found in most blacks. Very sensitive and precise hydrogen determinations by Smith et al. (233a)showed that in graphon (graphitized "Spheron 6" carbon black) less hydrogen was present after treatment with hydrogen a t 1000° than after vacuum outgassing a t 450". The hydrogen content of the hydrogentreated sample was 26 meq/100 gm. Furthermore, the authors concluded that the hydrogen content was distributed throughout the particles. However, it is not feasible t o determine the location of the hydrogen within the carbon black particles by stepwise combustion as was done by Smith et al. (133a)and by Snow et al. (133b).When carbon black is oxidized, the particles do not burn evenly from the outside in, but, instead, the interior is attacked first, leaving a hollow shell. This was shown by Hofmann et aE. ( 6 4 ) ,Clauss ct al. (133c),Donnet et al. ( 1 3 3 4 , and Heckman ( 1 3 3 e ) using the electron microscope. Moreover, some particles are attscked rapidly while others remain untouched. This is presumably caused by t,races of impurities which act as catalysts. Similar effects occur with graphitized carbon blacks (133f). A similar decrease in hydrogen content was observed with a sugar charcoal hydrogen-treated a t 1000" (133a).The oxygen bonded to the carbon surface was completely removed by hydrogen at 1000". No evidence of surface hydride formation was found in these experiments. However, graphon is not very well suited for such experiments because of the homogeneity of its surface which consists almost entirely of basal planes.


215

Barrer (134a,b)demonstrated that hydrogen was taken up a t 790" by microcrystalline carbon as well as by graphite. Hydrogen sorption on graphite at high temperatures was studied by Redmond and Walker (134c).Samples previously outgassed in a high vacuum a t 2000" were heated to predetermined temperatures in pure hydrogen. A maximum of adsorption occurred in the adsorption isobars around 1100" with pressures exceeding 10 mm Hg. This indicates that the adsorption is an activated process. The hydrogen could not be removed by pumping after cooling of the samples. It was estimated for a nuclear graphite that more surface was covered by the hydrogen than was available for nitrogen adsorption in surface area measurements by the Brunauer, Emmett, and Teller (BET) method; very probably, at high temperatures internal pores became accessible to hydrogen. Thomas (135) also observed activated adsorption of hydrogen a t temperatures above 450". At low temperatures, e.g., - 183") some nonactivated chemisorption was found. The high-temperature adsorption increased after removal of surface oxides by carbon monoxide treatment a t 500". For each surface oxygen atom lost, two additional hydrogen atoms were bound. Rapid chemisorption of hydrogen a t room temperature on graphite wear dust prepared in a vacuum was noticed by Savage (136)and Savage and Brown (137). The formation of several volatile carbon hydrides in the hydrogengraphite reaction between 360 and 800" was reported by Breisacher and Marx (138). The formation of ethane, ethylene, propylene, and even butane suggests that the edge of the carbon layers became hydrogenated in the first step of this reaction. The results were discussed on the basis of a mechanism proposal by Zielke and Gorin (139). The difficulty in detecting surface hydride formation is that hydrogen is nearly always contained in the bulk structure of the carbons. Hydrogen is not easily determined with sufficient accuracy if only small concentrations are present. Precise analytic methods should be used in the measurement of the hydrogen uptake of carbons that were prepared with rigorous exclusion of hydrogen-containing contaminants. Carbon formed by the disproportionation of carbon monoxide

2co

* c + co,

would be well-suited, since i t has a sufficiently large surface area. The hydride surface is quite unreactive. Substitution by halogens, chlorine in the first place, is possible a t elevated temperatures. The reaction of carbon blacks with chlorine was first described in a patent by Kloepfer (140). As is shown in Fig. 2, the maximum chlorine uptake occurred between 400 and 500".It was nearly equivalent t o the hydrogen

21 6

H. P. RORHM

Fro. 2. Reaction of carbon black “CK 3” with chlorine [after Hoehni el ul. (6!1)]

content of the original samples (69). Bromine was bound to a lesser extent by the carbon blacks under the same conditions. After heat,ing the carbon blacks to 1100” the hydrogen content and the chlorine uptake decreased considerably. Chlorine was bound a t 450” even by graphitized carbon black “CK 3” with a surface area of 76 m2/gm to the extent of 30 meq/100 gm. This chlorine is presumably bound 011 the surface sites where the basal planes of adjacent graphite crystals meet (see page 191). The chlorine uptake of the original carbon black was 730 meq/100 gm. About 30 meq/100 gm chlorine were also bound by a natural graphite with 8 m2/gm surface area. Nearly all the prism faces of the crystals are accessible with graphite flakes but not with graphitized carbon black. The chlorine bound to the carbon black surface can be used for further reactions. On fusion with sodium hydroxide, i t was completely removed. A large part had been replaced by CN groups after fusion with sodium cyanide or treatment with copper (I) cyanide (69).Reaction wasobserved also with ammonia. However, no amino groups could be detected on the surface by the iisiinl methods.


B. SURFACE GROUPSON

217

GRAPHITE

Because of the similarity in structure between microcrystalline carbon and graphite, it would be expected that the same type of surface compounds are formed on both modifications of carbon. There are only very few detailed investigations of the surface oxides on graphite, mainly because of the small available surface areas and the ensuing difficulties in determining extremely small quantities of functional groups. Graphitized carbon black, e.g., graphon, was used in some studies. Because of its peculiar structure, only a very small percentage of its surface area of 80 m2/gm is able t o form surface compounds. The first indications of surface oxide formation were obtained in the course of combustion studies. Bonnetain et al. (141)and Bonnetain (142) studied the kinetics of the graphite-oxygen reaction and concluded that oxygen was intermediately bonded to the periphery of the carbon layers. Graphite wear dust prepared by grinding of graphite in a vacuum or under argon is very reactive. Irreversible adsorption of oxygen, carbon monoxide, and carbon dioxide was observed by Savage and Brown (137). The crystal structure of graphite is seriously disturbed in the wear dust. A turbostratic ordering of the layers was found (143, 144). I n a finely ground graphite, Mrozowski and Andrew (97)observed an electron spin resonance which was in part irreversibly destroyed on admission of air. The authors assumed broken bonds in the carbon layers to be the cause. The reaction of graphite wear dust with carbon dioxide and with oxygen a t low pressures was studied by Vastola and Walker (145). Surface oxides were formed already a t 200". Carbon dioxide was the only gaseous reaction product. One molecule of carbon dioxide was formed for three molecules of oxygen fixed on the surface. On thermal decomposition of the surface complex, carbon dioxide was formed in limited amounts. Its evolution ceased virtually above 700", the main part being disposed of already a t 500". The evolution of carbon monoxide had its maximum at higher temperatures. The reaction of graphitized carbon black with oxygen at very low pressures was studied by Laine et al. (146, 147). Graphon with various levels of burn-off was used. The number of active sites on graphon was shown by Graham (148)to correspond to only 1.25% of the surface area. This active surface area increased rapidly on partial oxidation, while the total surface area was affected but little. The formation of a stable surface complex was inferred from the difference in consumed oxygen and evolved carbon oxides. At 625", one molecule of CO, was formed for each 0, molecule bound on the surface. The remainder of consumed oxygen appeared as CO. The ratio of CO to CO, was temperature-dependent with a good reproducibility. The oxidized

H. P. BOEHM

218

graphon behaved similarly t o oxidized wear dust on thermal treatment. The formation of surface oxides on the prism faces of graphite single crystals was shown by Hennig (67).The experiments have been described on page 191. Adsorption studies with tritiated water by Montet (68) confirmed this result. The most detailed investigation of surface oxides 011 graphite wear dust was accomplished by Kiselev and collaborators (24, 149-152). Natural graphite was ground under argon using a vibration mill. A pyrophoric wear dust with 380 m2/gmsurface area was obtained. Adsorption of oxygen and water vapor was studied without exposing the samples t o other gases. Gravimetric techniques were used. More oxygen was irreversibly adsorbed a t room temperature than by active carbons outgassed a t high temperatures, No or very little reaction with oxygen occurred a t -196", however (24, 149). On outgassing a t elevated temperatures, the oxidized graphite evolved molecular oxygen as well as CO and CO,. Gas evolution started a t 100". The formation of oxygen ceased at 250". The surface oxides were hydrated irreversibly on treatment of oxidized graphite with water vapor. An amount of 0.4 pmole/mz of water was bound by a surface containing 4.1 pmole/m2 of oxygen. A chemical analysis of the functional groups gave the results represented in Table XII. Carboxyl groups were determined by Na,CO, neutralization, tertiary hydroxyl groups from the difference between NaOH and Na,CO, neutralization. Hydroperoxides were determined iodometrically in aqueous isopropanol. The carbonyl content was estimated from the

TABLE XI1 Functional Group8 on Oxidized Cold-Milled Graphite [after Kiselev el al. (15Z)l

Content (meqlm')

-COOH

\

0.48

-C-OH

0.10

/ \ -C-OOH / \

0.04

/

c=o

0.07


219

reaction with p-bromphenylmagnesium bromide. The groups in Table XI1 account for only 29% of the oxygen of the graphite. From the difference in the oxygen balance it was concluded that neutral or basic hydroxyl groups, radicals, and interstitial oxygen bridges might have been present also. The authors suggested tentatively that the primary oxidation products had radical character, e.g., 0

\

-c-0-0.; /

-c

//\

0.

The oxygen disposed of in molecular form on heating might be adsorbed by radicals in the surface or by defects in the basal planes possessing a high rr electron density. The differential heats of adsorption of oxygen on freshly ground graphite started with 100 kcal/mole, and decreased t o 50 kcal/mole at a surface coverage of 2 pmole/m2 (152).The high values seem to indicate the formation of carbonyl groups. The heat of adsorption of water on oxidized graphite was 40 kcal/mole a t low coverages. It decreased t o the value of condensation of water a t a coverage of 0.5 pmole/m2. Bobka (144) studied the adsorption of diphenylguanidine on graphite wear dust. As was shown by Rivin (83),diphenylguanidine adsorption is equivalent to NaHCO, neutralization in the determination of carboxyl groups. With increasing grinding time, surface area and diphenylguanidine adsorption increased parallel t o each other.

C. SURFACE GROUPSON DIAMOND Diamond is the prototype of all aliphatic compounds. One would expect on its surface free valences which are capable of surface compound formation. The surface compounds on diamond should differ somewhat in character as compared to the surface compounds on “aromatic” graphite or microcrystalline carbon. Apart from singly bonded carbon atoms on the edges and corners of diamond crystals

carbon atoms anchored with two or three valence bonds to the bulk structure ought to be present in the crystal faces (100) and ( 1 11):

220

H. P. BOEHM

(100)

(111)

The electronic structure of the free valeiices was recently discussed by Kouteckjl ( 1 5 3 ~ ) . Farnsworth and co-workers (153b) and Marsh and Farnsworth (153c) demonstrated recently by use of low-energy electron diffraction that the structure of “clean” diamond, silicon, or germanium surfaces is different from the ideal bulk structure. Fractional order diffractioii maxima were observed, indicating a regular displacement of surface atoms from their ideal positions. The displacement of carbon atoms on diamond was . changes relieved after treatment with hydrogen or oxygen ( 1 5 3 ~ )Such were most pronounced on the (111) crystal face. The results indicate mutual interaction of free bonds in the surface. It was shown (153b)that on (100) faces of silicon or germanium alternating rows of atoms are displaced in opposite directions. After oxidation, chemisorbed oxygen atoms as well as those of Si or Ge have the ideal array. Diamond occurring in the “blue ground’’ of volcanic pipes as well as freshly pulverized diamond show hydrophobic behavior. This is used in its isolation by flotation. Diamond found in sediments is hydrophilic, however. According to Plaksin and Alekseev (154),hydrophobic diamond turns slowly hydrophilic on storing with exposure to air. Hofmann (155) reported that fine particle size diamond forms stable suspensions in dilute ammonia after treatment with calcium hypochlorite. It seems rather obvious that formation of surface oxides is responsible for the hydrophilic properties. The adsorption of oxygen on diamond was studied by Barrer (156). Essentially no chemisorption was observed a t -78”. From 0 to 144” oxygen was chemisorbed, but no carbon oxides were liberated. Some carbon dioxide was formed as well from 244 t o 370” by interaction of oxygen and diamond surface not covered with surface oxides. Surface oxide formation was observed at low pressures. The coefficient of friction of diamond increases considerably after heating in a high vacuum. The measurements by Bowden and Hanwell (157) showed a decrease in the friction on access of oxygen, even at very low pressures. A chemical investigation of the surface oxides on diamond was undertaken by Boehm et al. (35). Using a fine particle size diamond powder with a specific surface area of 17 m*/gm, the oxidation was studied by use of a vacuum microbalance. Formation of surface oxides started a t a measurable rate with pure oxygen at 260”. A weight loss due t o formation of carbon oxides became apparent above 360”.

22 1


The neutralization behavior was different from that of oxidized microcrystalline carbon. No relationship was observed between the neutralization values with different bases as is found in Tables I and I11 for black microcrystalline carbon. Samples outgassed a t .8OOo adsorbed no alkali. The results of several reactions are summarized in Table XIII. There TABLE XI11 Surface Reactions of Diamond [after Boehm et al. (35)]

Treatment of diamond

Reaction

Extent of reaction (meq/100 gin) ._

Oxidized with 0, at 400" Oxidized with 0, at 400" Oxidized with NaOCl Oxidized with 0, at 400"and outgassed at 25"

looo 200" 300' 400" 500" 960" Oxidized with 0, at 400" Outgassed and treated with H, at 800" Outgassed at 800"

-~

~-

~

NaHCO, neutralization 2.5 Thionyl chloride 2.6 Acetyl chloride 3.7 Active H, (with CH, MgI or D,O) 65 Potassium 23 Potassium 20 21 Potassium Potassium 22 18 Potassium Potassium I1 Potassium 5 Potassium Chlorino at 100' Chlorine at 320" Chlorine at 410" Chlorine at 500'

4. I

20 20 21 17

was good agreement between the neutralization of NaHCO, and the formation of acyl chlorides with thionyl chloride. The reactions are very likely caused by carboxyl groups a t the edges and corners of the diamond crystals. It was first thought that tertiary hydroxyl groups were present on the (111) faces of diamond. However, the existence of substantial quantities of tertiary hydroxyl groups can be excluded since determinations of active hydrogen by the Zerewitinoff method or by deuterium exchange with D,O agreed approximately with the carboxyl content. When potassium was distilled in a high vacuum onto the samples, about 20 meq/100 gm of potassium were irreversibly adsorbed after removal of surplus potassium by vacuum distillation. Presumably, ketyls of surface carbonyl groups were formed. The retention of potassium decreased after outgassing a t 400 and 500". Considerably less

H. P. BOEHM

222

potassium was bound by a diamond sample that had been treated with hydrogen at 800". It was assumed that free radicals might be formed on destruction of the surface compounds at high temperatures. The signal obtained from paramagnetic spin resonance measurements with samples outgassed at 800" corresponds to only 1.2 meq/100 gm of free radicals. The signal was influenced in its size and shape by subsequent surface oxide formation, but it was not destroyed. The absence of free radicals in the surface of heat-treated diamond is not surprising if one considers the results obtained by Lander and Morrison (158a, 158b) with atomically clean surfaces of silicon or germanium. They concluded from low-energy electron diffraction data that the diamond-type lattice is distorted in these surfaces and that two or more neighboring atoms are shifted towards each other with apparent mutual saturation of the "dangling" bonds. In certain crystal faces, interatomic distances were found to be shorter than a single covalent bond (158u, 158b). Hydrogen is chemisorbed by diamond a t temperatures from 400" upwards as was shown by Barrer (1346). Apparently, surface hydrides are formed as is indicated by the decrease in the capacity for potassium chemisorption (Table XIII). A significant decrease was also measured for the heat of immersion in water after hydrogen treatment a t 800" [ ( 3 5 ) , Table XIV]. Methane is liberated when hydride-covered diamond is heated in a vacuum ( 1 5 3 ~ ) . Surface chlorides, too, were formed on diamond. The samples were outgassed in a high vacuum a t 800" and immediately afterwards treated with chlorine a t temperatures from 100 to 500'. With reaction temperatures up to 400", 20 meq/100 gm were bound on the diamond surface, a quantity which is equivalent to the amount of potassium retained after treatment with this alkali metal. (Table XIII). The chlorine on TABLE XIV Heat of Immersion of Diamond Powder i n Water" Hent of immersion Sample

___ Diamond, oxidized with 0, at 400" Same sample, after vacuum outgassing at 800" Same sample, after hydrogen treatment at 800'

"Ca. 17 m*/gm specific surface area.

cnllgm

erg/cmP

0 .7 0.5

170 120

0.35

85


223

the surface was resistant to hydrolysis, even with hot sodium hydroxide solution. Surface su[fide formation was attempted by Wibaut and van der Kam (222).The results were negative. However, i t seems doubtful whether a sufficiently finely divided diamond powder was used. Otherwise, the analytic methods used by the authors would have been too crude for the detection of the extremely small sulfur concentrations. One would expect that similar surface compounds as on diamond might be formed 011 silicon, silicon carbide, or germanium surfaces. Formation of surface oxides on silicon, occurring already a t very low pressures, changes the diffraction pattern that is obtained with lowenergy electrons (153b, 258a).Lander and Morrison ( l 5 8 a )described the existence of definite surface phosphides as well as surface chloride and iodide on silicon. A surface h.ydride was apparently not, formed.

D. SUMMARY A large variety of surface groups have been prepared and identified on microcrystalline carbon. Best known among them are the acidic surface oxides. Various functional groups known from organic chemistry have been detected. The existence of carboxyl groups, phenolic hydroxyl groups, and carbonyl groups is certain; the existence of lactones similar to fluorescein or to lactols of ketocarboxylic acids is very likely as well as the occurrence of free radicals associated with peripheral carbon or oxygen atoms. Acidic groups are bound only t o the edges of the carbon layers. Frequently, a difference was observed between the oxygen content of oxidized carbons and the oxygen accounted for by the identified functional groups. Therefore, it seems likely that still other ways of bonding oxygen to the carbon surface are possible. The existence of ether-type oxygen bridges between carbon layers of one stack cannot be excluded. One might imagine hydroaromatic buckling at the periphery of the aromatic carbon layers. A simple stoichiometric relationship of the amounts of the various groups has been observed with thoroughly oxidized carbons. This implies that a definite structure containing these groups is formed by the oxidation mechanism. A discussion of models, although tentatively tried (35,46),is very difficult because insufficient evidence has been obtained as yet on the relative position of the various groups. There are some indications that some of the groups, especially carboxyl and carbonyl groups, do occur pairwise; one might conceive that carboxyl groups are left when a carbon ring is destroyed in the process of oxidation:

224

H . 1’. ROEHM

However, with different pretreatment of the carbon, a different spatial arrangement of the groups seems to result during oxidation. These relationships deserve further studies. The effect of oxidation catalysts or inhibitors on the formation of the functional groups has not been studied yet. A definite influence is to be expected since the oxidation mechanism is certainly changed by additives such as water vapor or chlorine (87,88). A part of the oxygen not accounted for is bound in basic surface oxides which are present on any carbon. The structure of these groups has not been satisfactorily explained so far. Their chemistry seems important in oxygen transfer reactions. Very little is knowii of the nature of the surface groups which form when carbon is treated with sulfur or sulfur-containing compounds. Most authors were content to determine the increase in sulfur content. Rubber chemists might be interested in the nature of the bonding of sulfur t o carbon black. The large quantities of sulfur bound under certain conditions imply that long sulfur chains are attached to the surface. It seems likely that rearrangement of the sulfur chains with formation of S, rings is hindered by steric reasons only, since sulfur contents of 20 wt yoor more have been observed with porous activated carbons only. The existence of surface hydride groups of the types known in classic organic chemistry is very probable in most carbons. Direct chemical evidence is very difficult to obtain due to the relative inertness of the carbon-hydrogen bond. However, the fact that hydrogen is stroiigly chemisorbed on carbone and released at high temperatures only in the form of hydrocarbons is sufficient proof of the existence of true carbonhydrogen bonds. Investigation of the surface groups by physical methods, especially spectroscopy, is very difficult with microcrystalline carboii due t o its high absorptivity and its poor crystallinity with random orientatioii of the crystallites. Such methods might be more useful in future investigations with well-crystallized graphite. With graphite, most chemical reactions 011 the surface are difficult t o observe because of the extremely small quantities due t o the low specific surface area. Quite interesting results have been recently obtained with large crystals of diamond or silicon by low-energy electron diffraction (1536,c, 158). I n general, one would expect that surface compounds on graphite are identical in structure with those on microcryRtalline carbon. The only apparent


225

difference-apart from the occurrence of “disordered,” i.e., nonaromatic carbon in microcrystalline carbon-would be the size of the layers and the correspondingly large reservoir of n electrons in graphite. Donation of electrons or acceptance without change of the planar layer structure seems more likely with graphite. With diamond, surface oxides, hydrides, chlorides, etc. are formed as well. Rather little hydrogen was detected in the surface oxides. Therefore, coverage with tertiary hydroxyl groups must be ruled out. Considerable distortion of the diamond structure near the surface was observed by low-energy electron diffraction. Linkage of neighboring carbon atoms by etherlike bridges seems more probable, therefore. It was shown that, under normal conditions, surface oxides are always present on diamond. A pure carbon surface can be obtained and preserved for some time only in an ultrahigh vacuum. The best approximation to an undistorted surface structure is obtained by hydrogenation. It should be interesting t o investigate the friction and wearing behavior of diamond in hydrogen or halogen atmospheres. Altogether, foreign atoms chemisorbed on any type of carbon are held very strongly by covalent bonds. Removal without simultaneous removal of surface carbon atoms is almost impossible. Even exchange reactions are difficult t o achieve.

111. Surface Groups on Silica Only with silica was the nature of the surface groups studied as extensively as with carbon. Silica, like carbon, has several polymorphs. Apart from the amorphous state, it is known to exist in numerous crystalline modifications. The most important forms are quartz, tridymite, and cristobalite. Each of these can occur in a low-temperature form and in a high-temperature form of somewhat higher symmetry. Tridymite is only stable if small amounts of alkali ions are present in the lattice (159). Ar. Weiss and Al. Weiss (160) discovered an unstable fibrous modification with the SiS, structure. Recently, a few highpressure modifications have been synthesized: keatite (161),coesite (162), and stishovite (163).The high-pressure forms have been found in nature in impact craters of meteorites, e.g., in the Arizona crater or in the Ries near Nordlingen (Bavaria). Stishovite is very interesting because it has the rutile structure with octahedral coordination of silicon. I n all other forms of silica, each silicon atom is surrounded tetrahedrally by four oxygen atoms. The bonding is intermediate in type between purely covalent and ionic (164). There is some dn-pn bonding between silicon and oxygen.

22G

H. P. BOEHM

The bond angle is near 140” (165).A review on the structural peculiarities of silica and silicates was recently given by No11 (166). Amorphous silica is similar t o the crystalline modifications in the close ordering of the atoms. However, the three-dimensional array of the SiOa tetrahedra is not as regular. An extensive review on the knowledge of the structure and chemistry of silica and silicic acid up to 1955 was given by Iler (167).Unfortunately, there is no recent edition of this book covering research in the last decade. Most work on the surface of silica has been done with amorphous silica of colloidal dimensions. This is due to its large surface area and to its technical importance. We shall therefore discuss first the identification of surface groups on amorphous silica. A. IDENTIFICATION OF SURFACE GROUPS ON AMORPHOUS SILICA The surface chemistry of amorphous silica is somewhat less complicated than that of carbon. Generally, only two kinds of “end groups” are possible on the surface: silanol groups and siloxane groups:

Hofmann and collaborators (168)were probably the first t o postulate that the free valences of silicon atoms in the surface of silicates must be saturated with silanol groups. Carman (169) visualized the structure of a particle of colloidal silica as a network of interlinked SiO, tetrahedra with hydroxy groups attached t o the surface, due to the tendency of silicon to complete tetrahedral coordination. Each particle of silica can be considered as a macromolecule of polysilicic acid. 1. Identijication of SilanoE Groups

a . Determination of Water Content. On heating, the silanol groups will release water with formation of siloxane groups. Repeatedly, the weight loss a t 1000 to 1100” has been used for the quantitative determination of silanol groups. It seems doubtful for various reasons, however, whether this simple method is applicable. In very narrow pores, water may be held even after drying at 100-120”. Furthermore, many types of silica contain hydroxyl groups within the bulk of the structure. This is especially true of silica which has been prepared by condensation of low-molecular silicic acids. It can easily be imagined that a few silanol groups will remain in the network not having


227

found a partner for condensation. The presence of hydroxyl groups within the bulk structure has been shown using infrared spectroscopy for fused silica ( 170, 171) and even for crystalline quartz ( 17 2 ) . Of course, the larger the specific surface area is, the more of the remaining silanol groups will be located in the surface and the less severe will be the error due to this cause. Stober (173)found that, even after thorough outgassing at lOO", one molecule of extremely tightly adsorbed water is retained for each two silanol groups in the surface. This surprising result was confirmed by us (174) using chemical reactions as well as deuterium exchange (see Section 111,A, 1, e ) . Stober arrived a t his conclusion from the fact that the sites of reversible water vapor adsorption (in the first layer) were exactly half the number expected from the quantity of water expelled on heating. An unprobably dense packing of the silanol groups would result if the water came from silanol groups only. Further confirmation was provided by Darlow and Ross (175) who studied the desorption of water from saturated surfaces at 100-190". The rate of desorption was proportional to the square root of the amount adsorbed, The authors concluded from this result that there was one water molecule adsorbed for every two silanol groups. Consequently, one molecule of water is evolved on heating for each silanol group. Stober (173) found also that some hydrogen is given off during outgassing above ca. 500". He assumed that this hydrogen originated from silanol groups also. Hydrogen evolution from heated quartz was reported also by Zhdanov (176).Krasil'nikov, Kiselev, and Sysoev (177) found that silica gel acquired oxidizing power after dehydration in a vacuum. The oxidizing equivalent was three orders of magnitude smaller than the quantity of silanol groups. Also, evolution of hydrogen in addition to water was observed (178,179)when silica gel was outgassed a t 1000". There has been some speculation on the packing density of surface silanol groups. Iler (180) estimated the number of silicon atoms in the surface as 7.85/100 A=. He assumed that each silicon atom carries one OH group. However, it is more likely that only half of the silicon atoms have free valences protruding from the particle surface. Otherwise, each particle would be coated by a (H,Si,O s)n layer which had no bonding to the particle itself. Thus, a value of 3.93 OH groups per 100 A2 seems more likely. De Boer and Vleeskens (181)calculated the packing density of surface silanol groups from the crystal structures of cristobalite and tridymite. The results varied between 4.55 and 4.851100 A%.Similar estimates by Schneider (182) are presented in Table XV. The silanol groups on

228

H. P. BOEHM

amorphous silica must be arranged in a similar way to those on the crystal faces of the crystallized forms. By correcting for the lower density of amorphous silica (a? = 2-20), somewhat lower values are obtained. The average is ca. 5 OH/lOO Aa. TABLE XV Pack?& Denaity of st&molGroup8 on the Surface of Cryslalline and Amorphous Silica [afterSchneider (ISZ)] Packing density in OH/100 An SiO, structure Quartz (d = 2.665) Cristobalite (d = 2.32)

Crystal face Crystallized SiO,

Amorphous SiO,"

(001) (101) (100)

9.6 6.0

(iO1)

5.6 4.5 4.6 4.8

8.5 5.2 7.6 5.3 4.4 4.5 4.7

(111)

Tridymite (d = 2.26)

(001) (100)

7.9

"Calculated on the basis of the lower density of amorphous silica (d = 2.20).

A determination of the surface hydroxyl groups was made by No11 et al. (183).The total water content was determined by heating to 1100". The content of molecular water was titrated by the Karl Fischer method. Silanol groups react with this reagent only very slowly. Good agreement was observed with silanol contents determined independently by other methods. With silica gel, 5.2 silanol groups were found per 100 As. b. Neutralization Reactions. The silanol groups on the surface of silica react weakly acidic. Carman (169) found that silica particles aquire a negative charge in alkaline media. H+ ions are replaced by Na+ ions which, unlike protons, cannot form an undissociated compound by entering the electron shell of the 0-ions. The isoelectric point of silica is near a pH of 2 (184, 185). Silanols are more acidic than comparable carbinol compounds; they show a more pronounced tendency for hydrogen bonding and association, as was shown by West and Baney (186). Since the surface silanol groups react weakly acidic, neutralization with strong bases can be used for their direct determination. However, care must be taken that no dissolution of silica takes place. Greenberg (187) found that the adsorption of calcium hydroxide waR roughly

CHEMICAL IDENTIFICATION OF SURFACE QROUPS

229

proportional to the surface area. He noticed that less than the equivalent amount of sodium hydroxide was bound under the same conditions. Repetition of this work by Boehm and Schneider (188)showed that good agreement with other reactions was found if one assumed that calcium hydroxide was adsorbed not equivalent but equimolar to the silanol groups. The mechanism which was proposed by Weiss (61) has been explained on page 190. With this assumption, Greenberg’s results with Ca(OH), and NaOH adsorption agree better with each other. The Ca(OH), adsorption was determined by conductometry. Superposed on the neutralization reaction is a slower formation of insoluble calcium silicates which is analogous to the dissolution of silica by sodium hydroxide. Sears (189) and Heston et al. (190) used the adsorption of sodium hydroxide for the determination of the surface area of colloidal silica. An empirical factor was used for the conversion of alkali consumption into surface area. This is permissible provided the packing density of surface silanols is constant. The determination was performed in concentrated sodium chloride solution in order to keep down the dissolution of silica. Using the same technique, it was found in my laboratory that all surface silanol groups as determined by other methods are neutralized a t pH 9.0. At higher pH, siloxane boiids in the surface were opened. A maximum in the sorption of Naf ions occurred usually a t p H 10.5-10.6 which corresponded to a packing density of ca. 5 OH/lOO A,. On further addition of alkali, silicate ions H,SiO,- went into solution. Bolt (191),however, found in determinations of the charge density of “Ludox” silica sols a value of 1.8 negative charges per 100 Aa a t p H 10. Schneider (182) attempted to measure the neutralization of sodium ethoxide by Aerosil. About twice as many sodium ions were bound as from sodium hydroxide a t pH 9. However, the reproducibility was poor and still higher values were observed after prolonged reaction times. Very likely, siloxane bonds were broken. c. Reaction with Thionyl Chloride. Boehm and Schneider (188) and Schneider (192)used the reaction with thionyl chloride for the determination of silanol groups. For each hydroxyl group, one chlorine atom was retained on the surface. It was assumed that surface silanol groups were replaced by chlorine according to \

-Si--OH

\ + SOCl, + --Si-Cl

+ SO, + HCI

/ The silica sample is refluxed with pure SOCl, for a t least 8 hours. It is advisable t o use as little SOC1, as possible, just enough to form a stiff jelly with the silica, and to heat in an oil bath. Otherwise, violent /

230

H. P. BOEHM

bumping cannot be avoided. After distilling off the surplus SOCI,, the reaction product is outgassed in a high vacuum (10-5 mm Hg) a t 200" for a t least 48 hours. Samples are weighed with careful exclusion of moisture, covered with sodium hydroxide solution, and heated. Afterwards, chlorine can be titrated in the usual manner. Other reaction techniques using sealed tubes are described in ( 17 4 ) . The reaction of silanol groups with SOC1, was originally reported by Deuel for clay minerals (193) and for silica gel (194). However, no quantitative measurements are possible with narrow-pore silica gel, since the SOC1, molecule is rather large and excess reagent as well as hydrogen chloride cannot be removed completely by outgassing. The method works well with fine particle size silicas like Aerosil (Degussa), Cab-0-ail (Cabot Corp.), or Ludox (Du Pont de Nemours and Co.). It was doubted by Uytterhoeveii and Naveau (195)whether all of the surface silanol groups react with thionyl chloride. The authors found that less chlorine was bound than active hydrogen was fourid. However, the possible occurrence of strongly adsorbed water was not taken into account. The results of the thionyl chloride method agree very well with many other reactions of the silanol groups, e.g., esterification reactions or the determinations by No11 et al. (183).There exists no definite proof as yet that Si-Cl bonds are formed in the reaction. Attempts a t consecutive reactions, e.g., with metal organic compounds, brought no unambiguous results. It is conceivable that hydrogen chloride is very strongly adsorbed on the surface sites that had been occupied by water. However, in this case two HC1 molecules must be adsorbed in the place of one H,O molecule in order to account for the stoichiometry. Infrared spectra taken by Folman (196) with porous Vycor glass, which is essentially silica, showed disappearance of the OH adsorptions after treatment with thionyl chloride. This observation speaks for the presence of Si-CI groups. There was apparently formation of Si-NH, groups on subsequent reaction with ammonia. A marked decrease in the chlorine uptake was observed with increasing temperature of pretreatment of the samples. Very little chlorine was bound when the surface silanol groups were protected by esterification (see Section III,A,l,f and Table XVII). d . Active Hydrogen. Fripiat and Uytterhoeven (197)determined active hydrogen in Aerosil by using a modified Zerewitinoff method. This technique had been originally applied by Deuel and Huber (198)to the determination of surface silanol groups on clay minerals. Fripiat and Uytterhoeven obtained identical results with methylmagnesium iodide and with methyllit

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