The kinetics and mechanism of the mineral acid-catalysed hydrolysis of carboxylic acid anhydrides in water and in dioxane–water mixtures. Application of the excess acidity function to these systems

Abstract

We report a kinetic study of the mechanism of the mineral acid-catalysed hydrolysis of some benzoic anhydrides, and of pivalic anhydride, in water and in dioxane–water mixtures. Employing a wider range of acid concentrations than used hitherto, we have considered the effects of acid concentration on the rate of hydrolysis via the excess acidity approach, and have examined the effects of substituents and of temperature. Contrary to previous suggestions, we find that benzoic and p-toluic anhydrides hydrolyse in water predominantly by the A1 mechanism. For these anhydrides, and for the p-methoxy and p-chloro derivatives, in 60%(v/v) dioxane–water as solvent, a change in mechanism (A2 to A1) is suggested by all the criteria used, but only the A1 mechanism is detectable for mesitoic anhydride. Pivalic anhydride, contrary to previous conclusions, exhibits the A2 and the A1 mechanisms in both water and dioxane–water. The advantage of dioxane–water mixtures over pure water as solvent for detecting mechanisms of acid catalysis, and the value of the excess acidity method for detecting changes in mechanism in the former type of solvent, are both illustrated.