Kinetics and mechanisms of reactions of pyridines and imidazoles with phenyl acetates and trifluoroacetates in aqueous acetonitrile with low content of water: nucleophilic and general base catalysis in ester hydrolysis

Abstract

Reactions of pyridines and imidazoles with substituted phenyl acetates and trifluoroacetates have been studied in acetonitrile and in water-acetonitrile containing 0.56 mol dm^–3 of water. The water isotope effects, steric effects, the effect of water in the reaction medium, and the derived Brønsted β and Hammett ρ values have been used as mechanistic criteria. Pyridines and imidazoles catalyse the hydrolysis of phenyl trifluoroacetates by general base catalysis while imidazole acts as nucleophile toward 4-nitrophenyl and 2,4-dinitrophenyl acetates. As indicated by the second-order dependence on amine concentration beside the first-order term in amine, the reaction of imidazole exhibits general base catalysis in the case of both phenyl acetates and phenyl trifluoroacetates. This reaction obviously is general base-catalysed nucleophilic reaction of imidazole. The activation parameters ΔH^‡ and ΔS^‡ derived for the reactions of pyridine and imidazole with 4-nitrophenyl trifluoroacetate and for the reaction of imidazole with 4-nitrophenyl acetate are consistent with the proposed reaction mechanisms.