Intramolecular catalysis. Part 9.1 The hydrolysis of p-nitrophenyl acetate catalysed by imidazoles having proximate carboxylate groups

Abstract

The rate coefficients for the hydrolysis of p-nitrophenyl acetate catalysed by a series of imidazoles, 4,5-diphenylimidazoles, benzimidazoles and perimidines in 30 mol% aqueous dimethyl sulfoxide at 30.0 °C have been measured, as have the pK_a values of the catalysts in the same medium. The effect of a 2-(2-carboxyphenyl) substituent in each system has been studied. The pK_a values of the imidazolium and related ions show that the 2-carboxylate group increases their acidity by 1.03 to 1.32 pK_a units. The same effect on the nucleophilicity of the corresponding imidazole and related compounds gives rise to rate increases of a factor of 1.4 to 3.2. These effects are related to the structure of the imidazoles, indicating the importance of the environment of carboxylate groups.