Studies in general acid and general base catalysis. Rate–acidity profiles for prototropic processes

Abstract

A theoretical study has been made of the effect of changing acidity on the rate of aqueous prototropic reactions. These processes, typified by ketone enolization, are catalyzed by general acids and bases and their log rate–pH profiles may take a number of different shapes. In the absence of buffer catalysis there are four reaction paths available to a substrate Z: ZH+ + HO−, Z + HO−, ZH+ + H2O, and Z + H2O. Equations have been derived that express the contributions each of these paths makes to the observed rate in terms of the experimental catalytic constants, the proton activating factor (paf), the deprotonating factor (dpf), and the basicity of the substrate as expressed by KZH+. For ideal systems the relative rates of the kinetically equivalent paths Z + H2O and ZH+ + HO− (though one does not know which is the larger) is given by the quantity [Formula: see text] where [Formula: see text], and [Formula: see text] are the experimental catalytic constants. It is argued in the case of the real substrate acetone that (a) this quantity underestimates the rate difference, and (b) the more important path is Z + H2O. Where paf is available from rate data determined in buffers an unequivocal choice can be made between the kinetically equivalent paths, provided paf is a constant for the particular substrate.