Kinetics and mechanism of the perchloric acid-catalysed hydrolyses of para-substituted benzoic anhydrides

Abstract

The kinetics of the perchloric acid-catalysed hydrolysis of benzoic anhydride and its p-methoxy-, p-t-butyl-, p-methyl-, p-chloro-, and p-bromo-derivatives in 60:40 (v/v) dioxan–water mixture have been investigated at different temperatures over the acidity range from 0·260M to 4·86M. The effect of lithium perchlorate upon the spontaneous and acid hydrolysis of these anhydrides has also been studied at various temperatures. The interpretation of the kinetic data by different mechanistic criteria leads to the conclusion that the acid hydrolyses occur, in the most general case, by two simultaneous mechanisms, A1 and A2. The contributions of both the reaction mechanisms to the total acid hydrolyses depend on the nature of the substituent group, the acidity and the temperature. At a given acidity and temperature, the contribution of the A1 mechanism is more significant for electron-releasing groups than for electron-withdrawing ones. Moreover, the A1 mechanism is more favoured by high acidities and temperatures. It is also concluded that the shift of the reaction mechanism toward the A1 type as the acidity increases is due to the electrolyte effects of perchloric acid.