Kinetics in homogeneous solution

Once a transformation has been characterised, rate laws can be investigated. Sometimes, the kinetic study is simply to obtain rate data for technological reasons, and empirical rate laws may be sufficient. Fundamental knowledge of the reaction mechanism, however, generally offers better prospects for process optimisation. A simple kinetics study seldom allows identification of a single mechanism because different mechanisms may lead to the same rate law (see kinetic equivalence above and in Chapters 4 and 11). A mechanistic possibility may be rejected, however, if its predicted rate law is not in accord with what is observed experimentally. 

Discussions of results of rate studies permeate thisbookbecause kinetics investigations are the single most important group of techniques in mechanistic determinations. However, kinetics results have to be derived from measurements which are the outcome of experiments. Chapter 3 on conventional kinetics methods includes techniques which are generally applicable, and also current procedures for extracting rate constants (and, in some cases, equilibrium constants) from raw experimental data. 

In principle, any property of a reacting system which changes as the reaction proceeds may be monitored in order to accumulate the experimental data which lead to determination of the various kinetics parameters (rate law, rate constants, kinetic isotope effects, etc.). In practice, some methods are much more widely used than others, and UV-vis spectropho-tometric techniques are amongst these. Often, it is sufficient simply to record continuously the absorbance at a fixed wavelength of a reaction mixture in a thermostatted cuvette the required instrumentation is inexpensive and only a basic level of experimental skill is required. In contrast, instrumentation required to study very fast reactions spectrophotometrically is demanding both of resources and experimental skill, and likely to remain the preserve of relatively few dedicated expert users. 

A major recent development is the increasing exploitation of time-resolved IR spectropho -tometry for kinetics which has a major advantage over UV methods - in addition to kinetic data, it also provides readily interpretable IR spectroscopic information which allows some degree of structural characterisation of reactive intermediates. 

1 Example the kinetics of the capture ofpyridyl ketenes by n-butylamine 

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