A range of alternative mechanisms can usually be postulated for most organic chemical reactions, and identification of the most likely requires detailed investigation. Investigation of Organic Reactions and their Mechanisms will serve as a guide for the trained chemist who needs to characterise an organic chemical reaction and investigate its mechanism, but who is not an expert in physical organic chemistry.Such an investigation will lead to an understanding of which bonds are broken, which are made, and the order in which these processes happen. This information and knowledge of the associated kinetic and thermodynamic parameters are central to the development of safe, efficient, and profitable industrial chemical processes, and to extending the synthetic utility of new chemical reactions in chemical and pharmaceutical manufacturing, and academic environments.Written as a coherent account of the principal methods currently used in mechanistic investigations, at a level accessible to academic researchers and graduate chemists in industry, the book is highly practical in approach. The contributing authors, an international group of expert practitioners of the techniques covered, illustrate their contributions by examples from their own research and from the relevant wider chemical literature. The book covers basic aspects such as product analysis, kinetics, catalysis, and investigation of reactive intermediates. It also includes material on significant recent developments, e.g. computational chemistry, calorimetry, and electrochemistry, in addition to topics of high current industrial relevance, e.g. reactions in multiphase systems, and synthetically useful reactions involving free radicals and catalysis by organometallic compounds.
Author(s): Howard Maskill
Edition: 1
Publisher: Wiley-Blackwell
Year: 2007
Language: English
Pages: 392
The Investigation of Organic Reactions and Their Mechanisms......Page 1
Contents......Page 7
Contributors......Page 17
Foreword......Page 19
Preface......Page 23
1.2 The nature of mechanism and reactivity in organic chemistry......Page 25
1.3.1 Product analysis, reaction intermediates and isotopic labelling......Page 26
1.3.1.1 Example: the acid-catalysed decomposition of nitrosohydroxylamines......Page 27
1.3.2 Mechanisms and rate laws......Page 28
1.3.3.1 Example: the acid- and base-catalysed decomposition of nitramide......Page 30
1.3.4 Kinetics in homogeneous solution......Page 31
1.3.4.1 Example: the kinetics of the capture of pyridyl ketenes by n -butylamine......Page 32
1.3.5 Kinetics in multiphase systems......Page 33
1.3.6 Electrochemical and calorimetric methods......Page 34
1.3.7 Reactions involving radical intermediates......Page 36
1.3.8 Catalysed reactions......Page 37
References......Page 40
2.1 Introduction and overview– why study organic reaction mechanisms?......Page 42
2.2.1 Quantitative determination of product yields......Page 45
2.2.2 Product stabilities, and kinetic and thermodynamic control of product formation......Page 47
2.3.1 Stereochemical considerations......Page 49
2.3.2 Use of isotopic labelling......Page 50
2.4 Mechanistic evidence from variations in reaction conditions......Page 51
2.6 Kinetic evidence from monitoring reactions......Page 56
2.6.1 Sampling and analysis for kinetics......Page 57
2.7.1 Product-determining steps in SN1 reactions......Page 58
2.7.2 Selectivities......Page 60
2.7.3 Rate– product correlations......Page 62
Bibliography......Page 67
References......Page 68
3.2.1 Reaction rate, rate law and rate constant......Page 70
3.2.3 Reaction mechanism, elementary step and rate-limiting step......Page 72
3.3 How to obtain the rate equation and rate constant from experimental data......Page 74
3.3.1 Differential method......Page 75
3.3.1.1 Example:reaction between RBr and HO-......Page 76
3.3.2 Method of integration......Page 77
3.3.2.2 Example: decomposition of N2O5 in CCl4......Page 78
3.3.3 Isolation method......Page 80
3.3.3.1 Example: oxidation of methionine by HOCl......Page 81
3.4.1 Rate constants for forward and reverse directions, and equilibrium constants......Page 82
3.5.1 Preliminary studies......Page 83
3.5.2.2 Temperature......Page 84
3.5.2.3 pH......Page 85
3.5.2.4 Solvent......Page 86
3.5.2.5 Ionic strength......Page 87
3.5.2.6 Other experimental aspects......Page 88
3.6.3 Continuous static monitoring......Page 89
3.7.1 Spectrometric methods......Page 90
3.7.1.1 Conventional and slow reactions......Page 91
3.7.1.2 Fast reactions......Page 93
3.7.1.3 Very fast and ultrafast reactions......Page 94
3.7.2 Conductimetry......Page 95
3.7.4 Potentiometry......Page 97
3.7.5 Dilatometry......Page 98
3.7.7 Chromatographic methods......Page 99
References......Page 100
4.1 Introduction......Page 103
4.2.1 Single-step unidirectional reactions......Page 104
4.2.2.1 Consecutive unimolecular (first-order) reactions......Page 105
4.2.2.3 Parallel (competitive) unimolecular (first-order) reactions......Page 107
4.2.3 Complex reaction schemes and approximations......Page 110
4.2.3.1 The steady-state approximation (SSA)......Page 112
4.2.3.3 The rate-determining step approximation......Page 113
4.2.3.4 The steady-state approximation,and solvolysis of alkyl halides and arenesulfonates......Page 114
4.3.1 Chlorination of amino compounds......Page 115
4.3.2 The Aldol reaction......Page 119
4.3.2.1 At low concentrations of aldehyde......Page 120
4.3.2.2 At high concentrations of aldehyde......Page 121
4.3.3 Hydrogen atom transfer from phenols to radicals......Page 122
4.3.4 Oxidation of phenols by Cr(VI)......Page 124
References......Page 127
5.1 Introduction......Page 128
5.2.1 Mass transfer coupled to chemical reaction......Page 129
5.2.1.1 Reaction too slow to occur within the diffusion film......Page 130
5.2.1.2 Reaction fast relative to the film diffusion time......Page 131
5.2.1.3 Interfacial reactions......Page 133
5.2.2 Phase-transfer catalysis (PTC)......Page 134
5.2.3 System complexity and information requirements......Page 136
5.3.1.1 Gas–liquid reactions......Page 137
5.3.1.2 Dispersed liquid–liquid systems......Page 138
5.3.1.3 Liquid–solid reactions in a stirred reactor......Page 139
5.3.2.1 Techniques based on the Lewis cell......Page 140
5.3.2.2 The rotated disc reactor......Page 141
5.3.2.3 Rotated diffusion cell......Page 142
5.3.2.4 Channel flow techniques......Page 143
5.3.2.5 The jet reactor......Page 144
5.3.2.6 Expanding drop methods......Page 145
5.3.2.8 Microelectrode techniques......Page 146
5.4 Information requirements and experimental design......Page 147
5.5 Summary......Page 148
References......Page 149
6.1 What is organic electrochemistry?......Page 151
6.2 The relationship between organic electrochemistry and the chemistry of radical ions and neutral radicals......Page 154
6.3 The use of electrochemical methods for investigating kinetics and mechanisms......Page 155
6.4.1 Two-electrode and three-electrode electrochemical cells......Page 156
6.4.2 Cells for electroanalytical studies......Page 157
6.4.3.1 The working electrode (W)......Page 158
6.4.5 The electronic instrumentation......Page 159
6.5 Some basics......Page 160
6.5.1 Potential and current......Page 161
6.5.2 The electrochemical double layer and the charging current......Page 162
6.5.3 Mass transport and current......Page 163
6.6.2 Mechanisms and rate laws......Page 165
6.7 The response curves for common electroanalytical methods......Page 166
6.7.1 Potential step experiments (chronoamperometry and double potential step chronoamperometry)......Page 167
6.7.2 Potential sweep experiments (linear sweep voltammetry and cyclic voltammetry)......Page 171
6.7.2.1 CV conditions......Page 175
6.7.2.3 Fitting simulated voltammograms to experimental voltammograms......Page 178
6.7.3 Potential sweep experiments with ultramicroelectrodes......Page 179
A.1 The preliminary experiments......Page 183
A.2 Preliminary studies by cyclic voltammetry......Page 184
A.3 Determination of the number of electrons, n (coulometry)......Page 186
A.4 Preparative or semi-preparative electrolysis, identification of products......Page 188
References......Page 189
7.1.1 General remarks......Page 191
7.1.2 Potential energy surfaces, reaction coordinates and transition structures......Page 192
7.1.3 Absolute and relative energies; isodesmic and homodesmotic equations......Page 194
7.2.1 Molecular mechanics......Page 196
7.2.3 Semiempirical methods......Page 197
7.2.4 Hartree–Fock theory......Page 199
7.2.5 Electron-correlation methods......Page 200
7.2.6 Density functional theory......Page 203
7.2.8 Basis sets......Page 205
7.3.1 The ethane rotational barrier and wave function analysis......Page 206
7.3.2 The nonclassical carbocation problem and the inclusion of solvent effects......Page 211
7.4 Matching computed and experimental data......Page 216
7.5 Conclusions and outlook......Page 217
References......Page 218
8.1 Introduction......Page 222
8.2 Investigation of reaction kinetics and mechanisms using calorimetry and infrared spectroscopy......Page 223
8.2.2 Types of reaction calorimeters......Page 224
8.2.2.2 Power-compensation calorimeters......Page 225
8.2.3 Steady-state isothermal heat-flow balance of a general type of reaction calorimeter......Page 226
8.2.4 Infrared and IR-ATR spectroscopy......Page 229
8.2.5.1 Experimental methods for isothermal calorimetric reaction data......Page 230
8.2.5.2 Experimental methods for isothermal infrared reaction data......Page 233
8.3.1 Calorimetric device used in combination with IR-ATR spectroscopy......Page 235
8.3.2.2 Results and discussion......Page 237
8.3.3 Example 2: sequential epoxidation of 2,5-di-tert-butyl- 1,4-benzoquinone......Page 240
8.3.3.2 Results and discussion......Page 241
8.3.4.1 Materials and methods......Page 246
8.3.4.2 Results and discussion......Page 247
8.4 Conclusions and outlook......Page 248
References......Page 249
9.1.1 Potential energy surfaces and profiles......Page 251
9.1.2 From molecular potential energy to rates of reaction......Page 253
9.2.1 Reaction classification......Page 255
9.2.2 Consequences of uncoupled bonding changes......Page 256
9.2.3 Sequences of basic reactions......Page 257
9.3.1 Direct observation......Page 258
9.3.2 Deductions from kinetic behaviour......Page 262
9.3.3 Trapping of intermediates......Page 266
9.3.4 Exploitation of stereochemistry......Page 270
9.3.5 Isotopic substitution in theory......Page 273
9.3.6 Isotopic substitution in practice......Page 276
9.3.7 Linear free energy relationships......Page 280
References......Page 282
10.1.1 Radical intermediates......Page 285
10.1.2 Some initial considerations of radical mechanisms and chapter overview......Page 286
10.2 Initiation......Page 288
10.3 Radical addition to alkenes......Page 290
10.5 Nitroxides......Page 292
10.5.1 Nitroxide-trapping experiments......Page 293
10.5.2 Alkoxyamine dissociation rate constant, kd......Page 294
10.5.3 The persistent radical effect (PRE)......Page 297
10.5.4 Nitroxide-mediated living/controlled radical polymerisations (NMP)......Page 299
10.6 Radical clock reactions......Page 300
10.7 Homolytic aromatic substitution......Page 304
10.8.1 Reductions with samarium di-iodide,SmI2......Page 308
10.8.2 SRN1 substitution......Page 311
Bibliography......Page 315
References......Page 316
11.1.1 Definitions......Page 317
11.2.1 Experimental demonstration......Page 318
11.2.2 Reaction flux and third-order terms......Page 321
11.2.3 Brønsted equations......Page 322
11.2.4.1 Cross-correlation effects......Page 323
11.2.4.2 The diffusion-controlled limit as a criterion of mechanism......Page 325
11.2.5.1 Stepwise proton transfer (trapping)......Page 326
11.2.5.2 Stabilisation of intermediates by proton transfer......Page 328
11.2.5.3 Preassociation......Page 330
11.2.5.5 Push–pull and bifunctional acid–base catalysis......Page 331
11.3.1 Detection of intermediates......Page 332
11.3.2 Non-linear free energy relationships and transient intermediates......Page 334
11.4 Enzyme Catalysis......Page 335
11.4.2 Enzyme assay......Page 336
11.4.3.1 Active-site titration......Page 337
11.4.3.2 Active-site directed irreversible inhibitors......Page 339
11.4.4 Kinetic analysis......Page 340
11.4.5 Reversible inhibitors......Page 341
11.4.6.1 Direct observation......Page 342
11.4.6.2 Structural variation......Page 343
11.4.6.4 Kinetics......Page 344
11.4.6.5 Trapping......Page 345
References......Page 346
12.1.1 The challenges inherent in the investigation of organic reactions catalysed by organometallics......Page 348
12.1.2 Techniques used for the study of organometallic catalysis......Page 350
12.1.3 Choice of examples......Page 351
12.2.1 Background and introduction......Page 352
12.2.2 The 31P{1H}NMR investigation of the Rh-catalysed asymmetric phenylation of cyclohexenone......Page 354
12.2.3 Summary and key outcomes from the mechanistic investigation......Page 357
12.3.1 Background and introduction......Page 358
12.3.2 Kinetic studies employing classical techniques......Page 359
12.3.3 ‘Atom accounting ’through isotopic labelling......Page 362
12.3.4 Observation of pro-catalyst activation processes by NMR spectroscopy......Page 365
12.3.5 Summary and mechanistic conclusions......Page 366
12.4.1 Background and introduction......Page 367
12.4.2 Early mechanistic proposals for the alkene metathesis reaction......Page 368
12.4.3 Disproving the ‘pairwise ’mechanism for metathesis......Page 369
12.4.4 Mechanistic investigation of contemporary metathesis catalysts......Page 372
12.4.5 NMR studies of degenerate ligand exchange in generation I and generation II ruthenium alkylidene pro-catalysts for alkene metathesis......Page 375
12.4.6 Summary and mechanistic conclusions......Page 376
References......Page 377
Index......Page 378
