General acid catalytic activity of 2-substituted imidazoles for hydrolysis of diethyl phenyl orthoformate

Abstract

Hydrolysis of diethyl phenyl orthoformate catalysed by a series of 2-substituted imidazoles has been studied in 50% dioxane–water (v/v) at 30 °C. The substituents are H, Me, Et, Prⁱ, Bu^t, 2-hydroxypropyl, and 1,1-dimethyl-2-hydroxyethyl groups. General acid catalysis is observed for hydrolysis by these imidazoles but not general base catalysis. The rate data are analysed in terms of the second-order rate constants for imidazoles versus the Taft steric constant parameter, E_s. The steric effect of substituents is small. Activation parameters determined for a few imidazoles contain large negative entropies. These results and a D₂O solvent isotope effect indicate that solvent water molecules participate in the transition state of the reaction.