General base catalytic activity of 2-substituted imidazoles for hydrolysis of ethyl dichloroacetate

Abstract

The hydrolysis of ethyl dichloroacetate catalysed by a series of 2-substituted imidazoles has been studied in water at 30°. The substituents are Me, Et, and Prⁱ, and a few hydroxyalkyl groups. Activation parameters have been determined for some of the imidazoles. Niether general acid nor nucleophilic catalysis has been detected for the reactions, confirming the general base catalysis mechanism. The catalytic activities of the imidazoles are evaluated from the extrapolated rate values derived from the k_im values at 30° assuming a common Brönsted slope of 0.47 and an equal pK_a of 7.80. It is found that the sensitivity of the substituents to Taft steric constant parameters is rather small (0.26) and that the hydroxy-group in the substituent acts favourably for the catalysis (by a factor of 1.7). The latter is explained in terms of a microsolvent effect.