Effects of substituents on the hydrolysis of 4-nitrophenyl acetate catalysed by 2-substituted imidazoles

Abstract

The hydrolysis of 4-nitrophenyl acetate catalysed by a series of 2-substituted imidazoles has been studied in water at 30°. The substituents used are the alkyl groups Me, Et, Prⁱ, and Bu^t, and a few hydroxyalkyl groups. Activation parameters and the deuterium oxide solvent isotope effect have been determined for some of these imidazoles. In creased bulkiness of the 2-substituents results in decreased rates for the nucleophilic catalysis observed for most of the imidazoles studied. The results are analysed in terms of the Brönsted catalysis law assuming a common slope of 0.80 and an equal pK_a of 7.80, giving an extrapolated rate value to each imidazole. Based on the analysis, a measure of the steric effect of the individual imidazole relative to the parent compound is estimated. The mechanism by which 2-(1,1-dimethyl-2-hydroxyethyl)imidazole acts for the hydrolysis is judged as a partial general base catalysis.