Consecutive reactions, classification

Complex reaction mechanisms can conveniently be grouped within the following classification consecutive reactions, parallel reactions and reversible reactions. Parallel reactions are those in which the same species participates in two or more competitive steps. Consecutive reactions are characterised by the product of the first reaction being a reactant in a subsequent process, leading to formation of the final product. Reversible reactions are those in which the products of the initial reaction can recombine to regenerate the reactant. 

To this point we have focused on reactions with rates that depend upon one concentration only. They may or may not be elementary reactions indeed, we have seen reactions that have a simple rate law but a complex mechanism. The form of the rate law, not the complexity of the mechanism, is the key issue for the analysis of the concentration-time curves. We turn now to the consideration of rate laws with additional complications. Most of them describe more complicated reactions and we can anticipate the finding that most real chemical reactions are composites, composed of two or more elementary reactions. Three classifications of composite reactions can be recognized (1) reversible or opposing reactions that attain an equilibrium (2) parallel reactions that produce either the same or different products from one or several reactants and (3) consecutive, multistep processes that involve intermediates. In this chapter we shall consider the first two. Chapter 4 treats the third. 

There are, however, also many examples of mixed domino processes , such as the synthesis of daphnilactone (see Scheme 0.6), where two anionic processes are followed by two pericydic reactions. As can be seen from the information in Table 0.1, by counting only two steps we have 64 categories, yet by including a further step the number increases to 512. However, many of these categories are not - or only scarcely - occupied. Therefore, only the first number of the different chapter correlates with our mechanistic classification. The second number only corresponds to a consecutive numbering to avoid empty chapters. Thus, for example in Chapters 4 and 6, which describe pericydic and transition metal-catalyzed reactions, respectively, the second number corresponds to the frequency of the different processes. 

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