The concerted mechanism problem

As emphasised recently by Hegarty and Jencks (1975), the interpretation of the third-order terms, kAp HAi[HAj][Af], in the rate law for acetone enolisation (4) has been the subject of much controversy and has played an important role in the development of ideas on the mechanism of acid-base catalysis in solution and at the active sites of enzymes . The problem has been not only to interpret this term, which can be accounted for by a concerted mechanism (17), but also to examine whether this mechanism, with H20 acting as an acid or a base, is valid for interpreting the other terms fcHA,[HAj] and fcA,lAj ]. 

This behaviour, which is unexpected if one considers only the successive two-step mechanisms for acid- and base-catalyses, means that two catalyst molecules (one molecule of acid and one of base) are present in the transition state, but does not necessarily imply that the concerted mechanism is correct. It was suggested (Dawson and Spivey, 1930 Rossotti, 1970) that this third-order term can also result from the existence of Af. HA, complexes (e.g. AcO. HOAc) which might act on their own. However, it was emphasised that such complexes would probably be less reactive than the free bases so that a downward curvature would have been observed (Hand and Jencks, 1975). 

Kinetic isotope effects and Bronsted exponents / A i Ha, for the third-order rate constants for a set of carboxylic acid-carboxylate ion buffers seem to be 

There have been attempts to interpret the kHA and kA terms as resulting only from a concerted mechanism in which Ar or HA, are replaced by water acting either as a base or an acid (Swain, 1950). Indeed, eqn (3) could be written as in (18). Arguments against such an interpretation have been put forward by 

Swain et al. (1958a,b), Bell (1973) and, more recently, by Hegarty and Jencks (1975). 

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