Reactions of Carbon Dioxide Radical Anion with Substituted Benzenes

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY J. Phys. Org. Chem. 2001; 14: 300–309 DOI:10.1002/poc.365

Reactions of carbon dioxide radical anion with substituted benzenes Janina A. Rosso,1 Sonia G. Bertolotti,2 Andre M. Braun,3 Daniel O. MaÂrtire1* and MoÂnica C. Gonzalez1* 1 Instituto de Investigaciones FisicoquõÂmicas TeoÂricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina 2 Universidad Nacional de RõÂo Cuarto, CoÂrdoba, Argentina 3 Lehrstuhl fuÈr Umweltmesstechnik, UniversitaÈt Karlsruhe, D-76128 Karlsruhe, Germany

Received 09 November 2000; revised 26 December 2000; accepted 03 January 2001

ABSTRACT: We report mechanistic studies on the reactivity of CO2. radicals towards substituted benzenes, XPh, with X = NO2, COOH, COH, CH3, H, Cl and HO. Vacuum ultraviolet (VUV, 172 nm) irradiation of aqueous solutions containing formate was used as a method of CO2. radical generation for the study of the reaction products, which were determined by gas chromatography–mass spectrometry and high-performance liquid chromatography. The nature of some of the reaction intermediates was studied by laser flash photolysis of aqueous solutions of peroxodisulfate in the presence of formate. The observed products and intermediates of reaction are discussed and rationalized by a dual reactivity of the CO2. radical anion with substituted benzenes: a one-electron reduction and radical addition to the aromatic ring. For substituted benzenes with X = NO2, COOH and COH, the observed products support an electron transfer from CO2. to the substrates as the primary reaction channel, while the reactions with toluene follow the channel of radical addition. Benzene and chlorobenzene react with CO2. by both routes, electron transfer and radical addition. Our experimental results indicate that H abstraction by the CO2. radical is negligible under the chosen experimental conditions. Copyright  2001 John Wiley & Sons, Ltd. KEYWORDS: carbon dioxide radical anion; vacuum ultraviolet irradiation; electron transfer; radical addition; phenol; toluene; benzene; chlorobenzene; benzaldehyde; benzoic acid

INTRODUCTION Carbon dioxide radical anions, CO2. , are commonly used in aqueous chemistry as a reducing agent for metalloporphyrins or as intermediate in the formation of superoxide anion. CO2. has been reported to undergo efficient electron transfer reactions with methyl viologen,1 quinones,2 alkyl halides,3 fumarates,4 nitro and nitrosobenzenes5,6 and chlorinated benzaldehydes.7 With nitrobenzenes and chlorinated benzaldehydes, electron attachment occurs on the nitro and aldehyde groups, respectively. CO2. radicals have also been reported to add to some unsaturated compounds such as acrylamide8 and pyridin-3-ol.9 Efficient hydrogen abstraction from mercaptobenzenes have also been reported.10,11 *Correspondence to: D. O. Ma´rtire or M. C. Gonzalez, Instituto de Investigaciones Fisicoquı´micas Teo´ricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina. E-mail: [email protected]; [email protected] Contract/grant sponsor: Fundacio´n Antorchas, Agencia Nacional de Promocio´n Cientı´fica y Technolo´gica (ANPCyT). Contract/grant sponsor: State of Baden-Wu¨rttemberg. Contract/grant sponsor: Deutsche Akademische Austauschdieinst (DAAD). Contract/grant sponsor: International Office of BMBF. Copyright  2001 John Wiley & Sons, Ltd.

The exact determination of the reduction potential, E°, of the couple CO2/CO2. was difficult because of the irreversibility of the electron transfer process due to the change in structure from linear to bent.12 Results of different approximations and experimental methods agree in a value of E° (CO2/CO2. )  1.9 V.12,13 Despite the fact that the CO2. radical is characterized by a more negative reduction potential than O2. radicals [E°(O2/O2. )  0.16 V], in agreement with its efficient reaction with oxygen,2 the reaction of CO2. with sulfur radical cation complexes14 is slower by a factor of 2.6. The trend observed in reactivity is in line with theoretical calculations15 predicting a high internal reorganization energy ( = 447 kJ mol 1) for the couple CO2/CO2. than for the couple O2/O2. ( = 88.6 kJ mol 1). The estimated value for the reduction potential of the couple CO2. , H‡/HCO2 is 1.07 V at pH 7.12 The fact that CO2. is also able to oxidize hydrogencarbonate to CO3. (k = 2  103 l mol 1 s 1)16a although the reduction potential for the couple CO3. /CO32 is E° = 1.5 V,16b is an indication that CO2. radicals may oxidize some classes of substrates by hydrogen abstraction and/or by an addition–elimination pathway. Dimerization to oxalate, reaction (R1), seems to be the J. Phys. Org. Chem. 2001; 14: 300–309

REACTIONS OF CO2 RADICAL ANION WITH BENZENES

main reaction of CO2. substrates.5

in the absence of added

CO2
‡ CO2
! C2 O4 2

…R1†

The reaction channels of CO2. radicals in the presence of organic substrates depend strongly on the particular substrate and need still to be investigated in detail. In this paper we report mechanistic studies on the reactivity of CO2. radicals towards substituted benzenes, XPh, with X = HO, CH3, H, Cl, COH, COOH and NO2. CO2. radicals show absorption in the UV (max = 235 nm) with a relatively high extinction coefficient [e (235 nm) = 3000 l mol 1 cm 1], independent of pH between pH 3 and 13.17 The conjugate acid of CO2. , i.e., HCO2., has a pKa value of 1.4.2 For our investigations, CO2. radicals were generated in aqueous systems by two different ways: (i) by reaction of HO. radicals and H atoms with formic acid (pKa = 3.75) or formate ions, reactions (R2–R5):2

301

CO2. radicals. Reaction of CO2. radicals with S2O82 ions,22 reaction (R7), yields again SO4. radicals as reaction products, and its main effect is that of depleting formate and peroxodisulfate from the reaction system. SO4
‡ HCO2 ! SO4 2 ‡ CO2
‡ H‡ k6 ˆ 1:1  108 l mol

1

s

1

…R6†

S2 O8 2 ‡ CO2
! SO4
‡ SO4 2 ‡ CO2 k7 ˆ 1  105 l mol

1

s

1

…R7†

The reaction products obtained from the reaction of CO2. with the aromatic substrates with reaction (R8) were determined by gas chromatography–mass spectrometry (GC–MS) and high-performance liquid chromatography (HPLC). The nature of some of the reaction intermediates was studied by laser flash photolysis. CO2
‡ XPh ! products

…R8†

HO
‡ HCO2 ! H2 O ‡ CO2
k1 ˆ 3:2  109 l mol

1

s

1

…R2†

HO
‡ HCO2 H ! H2 O ‡ HCO2
k2 ˆ 1:3  108 l mol

1

s

1

…R3†

1

…R4†

H
‡ HCO2 ! H2 ‡ CO2
k3 ˆ 2:1  108 l mol

1

s

H
‡ HCO2 H ! H2 ‡ HCO2
k4 ˆ 6:1  107 l mol

1

s

1

…R5†

Vacuum ultraviolet (VUV) photolysis of liquid water (exc 70 is required for a scavenging reaction of HO. radicals.23 If a similar ratio is expected for formate, the required concentration of formate ion in the reaction system should be >0.3 M. In order to check the validity of this estimation, and to establish the experimental conditions under which the added organic substrate, XPh, will not react with HO. radicals and H atoms, a series of experiments were performed to investigate the depletion rate of XPh as a function of the analytical initial concentration of formate [HCO2 ]0. Toluene was used as the organic probe, since it efficiently reacts with HO. (k = 5.1  109 l mol 1 s 1)25 and H. (k = 2.6  109 l mol 1 s 1)26 and is expected to show relatively low reactivity towards the electron-rich CO2. radicals. In the absence of formate, mainly reactions with HO. radicals and H atoms deplete toluene. In the presence of formate, both toluene and formate compete for HO. and H.. Under conditions of high formate to toluene molar ratios, the reactive intermediates are quantitatively scavenged by formate and toluene reacts primarily with CO2. . Figure 1 shows the depletion of toluene obtained Copyright  2001 John Wiley & Sons, Ltd.

from saturated aqueous solutions of pH 4 containing different [HCO2 ]0. The depletion rates of toluene observed for [HCO2 ]0 0.2 M are coincident, within experimental error. This observation indicates that under these conditions, the CO2. formation rates are independent of [HCO2 ]0, and HO. and H. formed by water homolysis are quantitatively scavenged by formic acid/formate ions, in agreement with our previous discussion (see above). The choice of [HCO2 ]0 = 1 M then guarantees that HO. and H atoms are quantitatively scavenged by HCO2H/HCO2 , even in the presence of efficient HO. and H. scavengers and for long irradiation periods, where even after a 50% consumption of [HCO2 ]0 the depletion rates of toluene are not affected. Owing to the high concentrations of formic acid/ formate ions used, VUV absorption by these substances may be suspected to compete with that of water. In fact, formic acid is known to absorb in the gas phase at wavelengths >180 nm, e.g. "200 nm = 40 l mol 1 cm 1, and below 180 nm several Rydberg transitions have been found leading to an ionization potential of 11.33 eV (corresponding in energy to light of 109 nm).27–29 Photolysis of formic acid in the gas phase has been reported to undergo two main reaction channels [e.g. reactions (R9–R11)], of which reaction (R9) is the most important process for irradiation wavelengths from 190 to 234 nm.28,29 HCO2 H ‡ h ! HO
‡ H
CO

…R9a†

H
CO ! CO ‡ H


…R9b†

HCO2 H ‡ h ! H
‡
CO2 H

…R10†

CO2 H ! CO2
‡ H‡

…R11†




Photolysis at 253.7 nm of formate/formic acid in aqueous solutions has been reported to lead to dissociation producing HO. and H.CO, reaction (R9a). Formyl radicals, H.CO, efficiently dissociate to CO and H atoms, reaction (R9b).30 Thus, electronic excitation of formic acid/formate results in the final formation of CO2. also involving reactions (R2–R5). Our experiments show no effect on the decay rate of toluene when [HCOO ]0 is increased to >0.2 M. This result indicates that similar CO2. concentrations are formed. If photolysis of formate/formic acid were to contribute significantly to the formation of CO2. , light should be quantitatively absorbed by formic acid already at [HCO2 ]0 = 0.2 M and, consequently, the formate/ formic acid absorption coefficient at 172 nm should be much higher than 2.7  104 l mol 1 cm 1 owing to the even lower local concentrations of formate/formic acid near the surface of the lamp. We therefore conclude that formate/formic acid photolysis is not a significant process under our experimental conditions. J. Phys. Org. Chem. 2001; 14: 300–309

REACTIONS OF CO2 RADICAL ANION WITH BENZENES

303

Table 1. Reaction products Substrate Nitrobenzene Benzaldehyde Benzoic acid Benzene Toluene Chlorobenzene Phenol a b

Reaction products Nitrosobenzene, aniline, N-phenylformamide, azoxybenzene, azobenzene Benzyl alcohol Benzaldehyde Benzaldehyde, 1,4-cyclohexadiene, phenol, benzoic acida, biphenyl 2-Methylphenol, 3,3'-dimethylbiphenyl Phenol, 2-chlorophenol, benzaldehyde –b

Determined by HPLC. Identified by its retention time and UV absorption spectrum. No products were detected by GC. One unidentified product was detected by HPLC.

In the absence of aromatic substrates and for [HCO2 ]0 = 1 M, diminution of [HCO2 ] follows a zero-order decay of (8  3)  10 5 mol l 1 s 1 for experiments made with 400 ml of solution. Consequently, 3  10 5 mol s 1 of formate are consumed by its reactions with HO. and H atoms near the surface of the lamp. Taking into account that 2 mol of formate are consumed per mole of water homolysed and that

ΦH2O = 0.42 mol einstein 1 at 172 nm, P0 is estimated to be (1.7  0.7)  10 7 einstein s 1 cm 2, as expected for Xe lamps of this type (see above). As CO2. radicals are formed from formate/formic acid due to reactions (R2–R5), 1.4  10 7 mol cm 2 s 1 of CO2. radicals are produced near the surface of the lamp. Depletion of CO2. in the primary volume of reactions is governed by their second-order recombina-

Scheme 1 Copyright  2001 John Wiley & Sons, Ltd.

J. Phys. Org. Chem. 2001; 14: 300–309

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J. A. ROSSO ET AL.

tion, reaction (R1),31 and by reaction with the organic matter, reaction (R8), hence severely reducing the steady concentration of CO2. which under such conditions cannot diffuse far out of this primary volume. In order to understand the complex kinetic process taking place in the primary (irradiated) volume of reactions, a differential equation system should take into account the diffusion of CO2. and of the organic matter to and from the bulk solution (secondary volume), the rate of CO2. formation and reactions (R1) and (R8). Assuming the steady-state condition for CO2. radicals and the organic matter in the primary volume of reactions, the product k8  [CO2. ]ss is estimated to be of the order of 6  10 2 s 1 for 109 >k8 (l mol 1 s 1) >106. The macroscopic depletion rate of aromatic substrates is then controlled by the rate of reaction (R8) in the primary volume of reaction and diffusion of the organic substrates from the bulk solution to the neighbourhood of the lamp surface. Under such conditions, substrate depletion rates of the order of 10 6 mol s 1 are expected, as observed experimentally for all the substrates investigated, except for phenol, for which an even lower depletion rate is found. The reaction products formed after the reaction of CO2. with the substituted benzenes were determined either by GC–MS or HPLC under the described experimental conditions. Products were analysed for different irradiation times up to 10 min and for