Stepwise kinetics examples

Although the overlap of terms is great, many exceptions exist. For example, the formation of polyurethanes typically occurs through stepwise kinetics. The polymers are classified as condensation polymers, and the backbone is heteratomed, yet no byproduct is released when the isocyanate and diol are condensed. The formation of nylon 6, a condensation polymer, from the corresponding internal lactam occurs through chain-growth kinetics. 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

One of the important mechanistic uses of isotopic substitution is that it can selectively affect different steps in a stepwise mechanism and so help resolve mechanisms and pathways. For example, in reactions where there is hydride transfer coupled to C—C bond formation, the measurement of both 13C/12C and D/H kinetic isotope effects can distinguish whether the steps are concerted or stepwise. Consider, for example, the oxidative decarboxylation of malate catalyzed by the malic enzyme, which could occur either via an intermediate mechanism (2.81) or in a concerted mechanism (2.82).59... 

The detection of intermediates depends upon the relative values of the rate constants for their formation and decay (see Chapter 4). An example is the imidazole-catalysed hydrolysis of aryl acetates where the concentration of acetylimidazole builds up and decays subsequently by hydrolysis. A stepwise process is manifestly obvious if, during a kinetic study, an intermediate species is observed to accumulate and then decay to give products (Fig. 11.6A)[12,13]. The nature of the measuring device is not relevant to the argument but is likely to be spectroscopic (see Chapters 2 and 9). The direct observation of an intermediate depends on a build-up of its concentration to a measurable level and this requires that the decay to the product is relatively slow. The simplest possible case of a stepwise process is shown in Scheme 11.15 and this also happens to be one of the most generally applicable mechanisms (see Chapter 4). 

Since the SHE chemistry is correlated with one-atom-at-a-time chemistry, one may ask if it is meaningful to carry out experiments with a single atom. From a theoretical point of view, as it was demonstrated above, for a 1 1 stoichiometry reaction the mass action law and the kinetics laws are valid. However, as there is no macrocomponent consumption, such reaction appears as of pseudo first order. Note that reactions with a 1 1 stoichiometry include all reactions between the microcomponent and a single macrocomponent. This concept can also be extended to stepwise reactions such as successive formation of metal complexes (hydrolysis, halide complexation) for example ... 

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