CHEMICAL KINETICS

EDITED BY

C. H. BAMFORD

M.A., Ph.D., Sc.D. (Cantab.), F.R.I.C, F.R.S. Campbell-Brown Professor of Industrial Chemistry,

University of Liverpool

AND

C. F. H. TIPPER

Ph.D. (Bristol), D.Sc. (Edinburgh) Senior Lecturer in Physical Chemistry,

University of Liverpool

VOLUME 8

PROTON TRANSFER

ELSEVIER SCIENTIFIC PUBLISHING COMPANY AMSTERDAM - OXFORD - NEW YORK

1977

Contents

Preface vii

Chapter 1 (A. V. Willi)

Homogeneous catalysis of organic reactions (mainly acid—base) 1

1. Introduction 1

2. Dependence of the rate of acid—base catalyzed reactions on pH and buffer concentration 2 2.1 General 2 2.2 Selection of the pH scale 2 2.3 Acid—base catalyzed reactions with interfering acid—base equilibria

of the Substrate 3 2.4 pH dependence of acid—base catalyzed reactions with changing

rate-determining step 5 2.5 Experimental studies of general acid—base catalysis 7

3. Investigation of the mechanisms of acid—base catalyzed reactions . . . 8 3.1 Types of mechanism and nomenclature 8 3.2 Experimental criteria for the mechanisms of acid—base catalysis . . 10

3.2.1 General catalysis 10 3.2.2 Volume change on activation 12 3.2.3 Entropy change on activation 16 3.2.4 Zuker—Hammett hypothesis and Bunnett criterion . . . . 19 3.2.5 Solvent isotope effect 21

4. Energy barriers and isotope effects in proton transfer reactions . . . . 24 4.1 A semi-empirical model of the energy barrier 24 4.2 Primary isotope effects and reacting bond orders 27

5. Survey of mechanisms of acid—base catalyzed reactions 30 5.1 Mechanismsofaeid catalysis 30 5.2 Mechanisms of base catalysis 32 5.3 The rate-determining step 34

6. Acid—base catalyzed formation and hydrolysis of epoxides 35 6.1 Introduction 35 6.2 Formation of epoxides 36 6.3 Hydrolysis of epoxides and related reactions 37

6.3.1 Rate coefficients of ring-opening reactions of epoxides . . . 37 6.3.2 Mechanism of basic hydrolysis of epoxides 39 6.3.3 Mechanisms of aeidie hydrolysis of epoxides 39 6.3.4 Mechanisms of uncatalyzed hydrolysis of epoxides . . . . 41

7. Acid catalyzed hydrolyses of acetals, ketals, and orthoesters 42 7.1 Introduction 42 7.2 Acetals 44 7.3 Ketals 48 7.4 Cyclic acetals and ketals 49 7.5 Orthoesters 52

8. Acid catalyzed hydrolysis of aliphatic diazo Compounds 55 8.1 Introduction 55 8.2 Ethyl diazoacetate 56

X

8.3 Trifluorodiazoethane, primary diazoketones and diazosulfones . . 58 8.4 Diphenyldiazomethane, 9-diazofluorene and other diazo substituted

hydrocarbons 65 8.5 Secondary diazo Compounds 67 8.6 Diazoacetate ion 69

9. Acid catalyzed aromatic decarboxylation 72 9.1 Introduction 72 9.2 Decarboxylation of aromatic acids with methyl, methoxy and

hydroxy substituents 74 9.3 Decarboxylation of aromatic amino acids 78 9.4 Change of the rate-determining step in strongly acidic Solutions . . 79

9.4.1 Aromatic amino acids 79 9.4.2 Azulene-1-carboxylic acid 82 9.4.3 Polyhydroxybenzoic acids 83

10. Nucleophilic catalysis 85

11. Catalysis of organic reactions by metal ions 86

12. Polyfunctional catalysis and intramolecular catalysis 88 12.1 Polyfunctional catalysis 88 12.2 Intramolecular catalysis 89

References 89

Chapter 2 (F. Hibbert)

Proton transfer to and from carbon 97 1. Introduction 97 2. Experimental methods 98

2.1 Direct measurement of rates of proton transfer 99 2.2 Rate-determining proton transfer steps 101

2.2.1 Hydrogen isotope exchange 102 2.2.2 Halogenation of carbon acids 106 2.2.3 Proton transfer to olefins 107 2.2.4 Acid catalyzed hydrolysis of diazo Compounds 109 2.2.5 The ElcB mechanism of 1,2-eliminations 112

3. Rates of proton transfer reactions 113 3.1 Normal acids 113 3.2 Carbon acids 116

3.2.1 Water catalyzed proton transfer 116 3.2.2 Proton transfer to bases 119

4. Rate—equilibrium correlations for classes of carbon acids 125 4.1 Ketones 125 4.2 Nitroparaffins 130 4.3 Cyanocarbons 135 4.4 Acetylenes 142 4.5 Sulphones 145 4.6 Halogen substituted carbon acids 153 4.7 Proton transfer to unsaturated carbon 161 4.8 Proton transfer from weak carbon acids in non-aqueous solvents . . 168

5. Discussion of results for proton transfer to and from carbon 173 5.1 Mechanism of proton transfer to and from carbon 173 5.2 Marcus'theory cf proton transfer 179

xi

6. Kinetic isotope effects in proton transfer to and from carbon . . . . 182 6.1 Primary kinetic hydrogen isotope effects 183 6.2 H20—D2 O solvent isotope effects 185

References 190

Chapter 3 (J. E. Crooks)

Proton transfer to and from atoms other than carbon 197 1. General aspects of fast proton-transfer reactions 197

1.1 The mobility of the proton 197 1.2 Theoretical calculations of the rate of a diffusion-controlled

reaction 201 1.3 The calculation of rates of reactions from relaxation times . . . . 204 1.4 The influence of molecular structure on rates of proton-transfer

reactions 209 1.5 Proton transfers between acids and bases in general, and the

Br0nsted catalysis law 213 2. Carboxylic acids 219

3. Alcohols, phenols and indicator acids 227

4. Amines 232

5. Enols 237

6. Electronically excited acids and bases 238

7. Proton-transfer reactions in aprotic solvents 242

References 248

Index 251