Reactions of keten acetals. Part II. The hydrolysis of 2-dichloromethylene-1,3-dioxolan: general acid catalysis and rate and product hydrogen isotope effects

Abstract

The acid-catalysed formation of 2-hydroxyethyl dichloroacetate from 2-dichloromethylene-1,3-dioxolan (DMD) shows the same general kinetic features as the hydrolysis of cycanoketen dimethyl acetal. The parameters of the Brønsted catalysis law for both protio- and deuterio-acids and the effects of changing the deuterium content of the medium on the reaction catalysed by acetic acid and the effects of changing the deuterium content of the medium on the reaction catalysed by acetic acid and by hydrogen ion have been established. The results can be interpreted on the same lines as the corresponding results for cyanoketen dimethyl acetal and support the conclusions reached from the study of that compound.

In addition it is possible to study in this case the isotopic composition of the product of the hydrogen ion-catalysed reaction in H₂O–D₂O mixtures. Taken in conjunction with measurements of the isotopic rate ratio (k_H/k_D), the results permit an evaluation of the transfer contribution to the solvent isotope effect. It is concluded that this effect must be small relative to the exchange effect for reactions of this type.

The problem whether it is correct to assume that the Brønsted exponent is a quantitative measure of the degree of proton transfer in the transition state is discussed with reference to the finding that the rate isotope effect (k_H/k_D) for the present reaction is less than that for the hydrolysis of cyanoketen dimethyl acetal, despite the fact that the Brønsted exponent for DMD is almost exactly one-half.