Hydrolysis of phthalic and 3,6-dimethylphthalic anhydrides

Abstract

Phthalic and 3,6-dimethylphthalic anhydrides give flat pH profiles for hydrolysis in aqueous solution over the range from 4M-hydrochloric acid to pH 5·2. The addition of electrolytes or 1,4-dioxan has the expected rate-retarding effect and 3,6-dimethylphthalic anhydride is slightly more sensitive to such changes than phthalic anhydride. The results of a study of the variation of equilibrium constant for the 3,6-dimethylphthalic anhydride–diacid equilibrium with temperature are reported and the activation parameters for the forward and reverse reactions and for the hydrolysis of phthalic anhydride are given. These values are consistent with a mechanism suggested for the hydrolysis of these anhydrides. At 25° phthalic anhydride is hydrolysed nearly eight times faster than the 3,6-dimethyl compound.