Types of mechanism and nomenclature

Investigation of the mechanisms of acid- base catalyzed reactions 3.1 TYPES OF MECHANISM AND NOMENCLATURE 

In this chapter, a reaction is considered acid or base catalyzed if its rate is proportional to the concentration of acid or base, respectively. According to Ostwald s definition of catalysis, however, it is required that the acid or base be not consumed in the reaction [12]. A true catalyst combines with the substrate to form a reactive intermediate, and the catalyst is regenerated in one of the final steps of the mechanism. In Bell s definition, a catalyst appears in the rate expression to a power higher than that to which it appears in the stoichiometric equation [1]. On the other hand, it merely depends on the acidity or basicity of the products (and also on the pH of the solution) whether or not the catalyst will be regenerated at the end. Therefore, it is not essential for the classification of reaction type and mechanism whether the acid or base is a true catalyst according to the more restricted definition, or a reactant which is consumed [12]. In both cases, the formation of an intermediate from substrate (S) and acid or base opens a low free energy pathway for the reaction. 

The sequence of essential steps in an acid catalyzed reaction may be formulated as (see also Vol. 2, pp. 351-355) 

If kn fe I proton transfer from H30+ to S is slow and ratedetermining. This mechanism is designated A-SE2 [13] (acid catalysis, bimolecular electrophilic substitution). If fen k lt the intermediate SH + is formed relatively fast in a pre-equilibrium step, and the second step is slow and rate-determining. (In most cases, the pre-equilibrium is far on the 

Actually, there are two kinds of A2 mechanisms [13]. In one of them, the attacking reagent, X, acts as a nucleophile in the other, X acts as a base and abstracts a proton. An example of the first kind is the acid catalyzed hydrolysis of carboxylic esters which involves attack of H20 onto the carbonyl carbon of the carbonyl oxygen protonated ester [4, 15], viz. 

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