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The Investigation of Organic
Reactions and Their Mechanisms
Edited by
Howard Maskill
Sometime lecturer
University of Newcastle upon Tyne
and visiting professor
University of Santiago de Compostela
Spain
631353834.001.png
2006 by Blackwell Publishing Ltd
C
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accordance with the Copyright, Designs and Patents Act 1988.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any means, electronic, mechanical, photocopying,
recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act
1988, without the prior permission of the publisher.
First published 2006 by Blackwell Publishing Ltd
ISBN-13: 978-1-4051-3142-1
ISBN-10: 1-4051-3142-X
Library of Congress Cataloging-in-Publication Data
The investigation of organic reactions and their mechanisms / edited by Howard Maskill.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-1-4051-3142-1 (hardback : alk. paper)
ISBN-10: 1-4051-3142-X (hardback : alk. paper)
1. Chemistry, Physical organic. 2. Chemical reactions. 3. Chemical processes.
I. Maskill, Howard.
QD476.I558 2006
547 .2—dc22
2006012267
A catalogue record for this title is available from the British Library
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Contents
Contributors
xv
Foreword
xvii
Preface
xxi
1
Introduction and Overview H. Maskill
1
1.1 Background
1
1.2 The nature of mechanism and reactivity in organic chemistry
1
1.3 The investigation of mechanism and the scope of this book
2
1.3.1 Product analysis, reaction intermediates and isotopic labelling
2
1.3.1.1 Example: the acid-catalysed decomposition of
nitrosohydroxylamines
3
1.3.2 Mechanisms and rate laws
4
1.3.3 Computational chemistry
6
1.3.3.1 Example: the acid- and base-catalysed decomposition
of nitramide
6
1.3.4 Kinetics in homogeneous solution
7
1.3.4.1 Example: the kinetics of the capture of pyridyl ketenes
by n -butylamine
8
1.3.5 Kinetics in multiphase systems
9
1.3.6 Electrochemical and calorimetric methods
10
1.3.7 Reactions involving radical intermediates
12
1.3.8 Catalysed reactions
13
1.4 Summary
16
Bibliography
16
References
16
2
Investigation of Reaction Mechanisms by Product Studies T. W. Bentley
18
2.1 Introduction and overview – why study organic reaction mechanisms?
18
2.2 Product structure and yield
21
2.2.1 Quantitative determination of product yields
21
vi
Contents
2.2.2 Product stabilities, and kinetic and thermodynamic control of
product formation
23
2.3 Mechanistic information from more detailed studies
of product structure
25
2.3.1 Stereochemical considerations
25
2.3.2 Use of isotopic labelling
26
2.4 Mechanistic evidence from variations in reaction conditions
27
2.5 Problems and opportunities arising from unsuccessful experiments or
unexpected results
32
2.6 Kinetic evidence from monitoring reactions
32
2.6.1 Sampling and analysis for kinetics
33
2.7 Case studies: more detailed mechanistic evidence from product studies
34
2.7.1 Product-determining steps in S N 1 reactions
34
2.7.2 Selectivities
36
2.7.3 Rate–product correlations
38
Bibliography
43
References
44
3
Experimental Methods for Investigating Kinetics M. Canle L., H. Maskill
and J. A. Santaballa
46
3.1 Introduction
46
3.2 Preliminaries
46
3.2.1 Reaction rate, rate law and rate constant
46
3.2.2 Reversible reactions, equilibrium and equilibrium constants
48
3.2.3 Reaction mechanism, elementary step and rate-limiting step
48
3.2.4 Transition structure and transition state
50
3.3 How to obtain the rate equation and rate constant from
experimental data
50
3.3.1 Differential method
51
3.3.1.1 Example: reaction between RBr and HO −
52
3.3.2 Method of integration
53
3.3.2.1 Data handling
54
3.3.2.2 Example: decomposition of N 2 O 5 in CCl 4
54
3.3.3 Isolation method
56
3.3.3.1 Example: oxidation of methionine by HOCl
57
3.4 Reversible reactions and equilibrium constants
58
3.4.1 Rate constants for forward and reverse directions, and
equilibrium constants
58
3.4.1.1 Example: cis-trans isomerisation of stilbene
59
3.5 Experimental approaches
59
3.5.1 Preliminary studies
59
3.5.2 Variables to be controlled
60
3.5.2.1 Volume
60
3.5.2.2 Temperature
60
3.5.2.3 pH
61
Contents
vii
3.5.2.4 Solvent
62
3.5.2.5 Ionic strength
63
3.5.2.6 Other experimental aspects
64
3.6 Choosing an appropriate monitoring method
65
3.6.1 Periodic monitoring
65
3.6.2 Continuous on-line monitoring
65
3.6.3 Continuous static monitoring
65
3.7 Experimental methods
66
3.7.1 Spectrometric methods
66
3.7.1.1 Conventional and slow reactions
67
3.7.1.2 Fast reactions
69
3.7.1.3 Very fast and ultrafast reactions
70
3.7.1.4 Magnetic resonance spectroscopy
71
3.7.2 Conductimetry
71
3.7.3 Polarimetry
73
3.7.4 Potentiometry
73
3.7.5 Dilatometry
74
3.7.6 Pressure measurements
75
3.7.7 Chromatographic methods
75
3.7.8 Other techniques
76
Bibliography
76
References
76
4
The Relationship Between Mechanism and Rate Law J. A. Santaballa,
H. Maskill and M. Canle L.
79
4.1 Introduction
79
4.2 Deducing the rate law from a postulated mechanism
80
4.2.1 Single-step unidirectional reactions
80
4.2.2 Simple combinations of elementary steps
81
4.2.2.1 Consecutive unimolecular (first-order) reactions
81
4.2.2.2 Reversible unimolecular (first-order) reactions
83
4.2.2.3 Parallel (competitive) unimolecular (first-order)
reactions
83
4.2.2.4 Selectivity in competing reactions
86
4.2.3 Complex reaction schemes and approximations
86
4.2.3.1 The steady-state approximation (SSA)
88
4.2.3.2 The pre-equilibrium approximation
89
4.2.3.3 The rate-determining step approximation
89
4.2.3.4 The steady-state approximation, and solvolysis of alkyl
halides and arenesulfonates
90
4.3 Case studies
91
4.3.1 Chlorination of amino compounds
91
4.3.2 The Aldol reaction
95
4.3.2.1 At low concentrations of aldehyde
96
4.3.2.2 At high concentrations of aldehyde
97

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