General base catalysis in the methanolysis of 4-nitrophenyl acetates

Abstract

The rates of the methanolysis of 4-nitrophenyl acetate, chloroacetate, and dichloroacetate have been measured in the presence of pyridines and imidazoles. Methyl substitution at the 2- and 6-positions in pyridine causes only a slight reduction in reactivity. A Brønsted β value of 0.48 has been evaluated for the catalysed methanolysis of 4-nitrophenyl chloroacetate by pyridines. The entropy of activation for the pyridine-catalysed methanolysis of 4-nitrophenyl dichloroacetate exhibits a substantial, negative value, –164 J mol^–1 K^–1. These results indicate that pyridine bases act as general base catalysts and not as nucleophiles in the methanolysis of 4-nitrophenyl acetates. Nucleophilic catalysis possibly occurs in the imidazole-catalysed methanolysis of 4-nitrophenyl acetate and chloroacetate. This suggestion is based on the steric effect caused by 2-methyl substitution of imidazole. The higher reactivity of imidazole as expected from the reactivity of pyridines confirms this conclusion.