The Hammett Equation

Press Published by the Syndics of the Cambridge University 2DB Bentley House, 200 Euston Road , Lond on NWl N.Y. 10022 American Branch: 32 East 57th Street, New York, ~~Cambridge

University Press 1973

140 Libra ry of Congress Catalogue Card Num ber: 72-93 ISBN : 0 521 20138 1 Printed in Grea t Britain by r and Beccles William Clowes & Sons Limited, Lond on, Colcheste

Contents

Preface

page vii

The Hammett op relationship 1.1. Introduction 1.2. The Hammett equation 1.3. The reaction constant p 1.4. The substituent constant a 1.5. The a 0 scale 1.6. The effect of solvent on a values 1.7. Problems

2 Elucidation of reaction mechanisms 2.1. Introduction 2.2. Modified substituent constants 2.3. The a- substituent constant 2.4. The a+ substituent constant 2.5. Diagnosis of reaction mechanisms 2.6. Aromatic nucleophilic substitution 2.7. Aromatic electrophilic substitution 2.8. Nucleophilic aliphatic substitution 2.9. Condensations of carbonyl compounds with amine derivatives 2.1 0. Free radical and multicentre reactions 2.11. Conclusion 2.12. Problems

1 I I

7

n

17 20 24 27 27 27 28 31 33 38 40 49 57 63 66 67

3 The separation of inductive, resonance and steric effects; application of the Hammett equation to aliphatic systems 3.1. Introduction 3.2. The evaluation of inductive effects 3.3. Esterification and ester h_vdrolysis: the Taft equation 3.4. The evaluation of resonance effects

69 69 70 75 83 V

3.5. The Yukawa-Tsuno equation 3.6. Some attempts to evaluate o-substituent constants 3.7. Problems 4

Application of the Hamme tt equation to data other than side chain reactivities of substitu ted benzenes 4.1. Heteroaromatic systems 4.2. The heteroatom as reaction site 4.3. The heteroatom as substituent 4.4. The heteroatom as part of the communicating system between reaction site and substituent 4.5. Five-rnembered ring hcteroaromatic compounds; the extended selectivi(v treatmen t 4.6. Polycyclic systems: biphenyl 4.7. Polycyclic systems: naphthalene 4.8. Non-bonding molecular orbital theory 4.9. Application of the Hammet t equation to spectral measurements 4.1 0. Problems

5 Thermo dynamic aspects of the Hamme tt equation 5.1. Introduction 5.2. Basic thermodynamic formula e 5.3. The H ammett equation: a linear free energy

5.4. 5.5. 5.6. 5.7.

relationship The isokinetic relationship Internal and external contributions to L1H and L1S The H ammond postulate Problems

86 92 94 96 96 96 99 103 104 108 Ill

111 126 130 133 133 138 142 144 150 152 159

Problem discussion

161

References

188

Index

193

vi