Arrhenius parameters for the alkaline hydrolysis of esters in aqueous solution. Part III. Methyl betaine methyl ester

Abstract

The kinetics of the alkaline hydrolysis of methyl betaine methyl ester have been studied using a pH-stat in aqueous solution, over a range of hydroxide ion concentrations at 0, 25, and 42°, and at an ionic strength of 0·1M. The results show the presence of a very small but detectable water rate at 25 and 42° but not at 0°. The effect of ionic strength has been studied. Rate constants and the energy and entropy of activation all show the expected dependence on ionic strength. Methyl betaine methyl ester has been shown to hydrolyse approximately three times faster at 25° than ethyl betaine ethyl ester. The Arrhenius parameters for both reactions are discussed. A comparison of ethyl betaine ethyl ester with glycine ethyl ester shows the effect of introducing both charge and substituent, while a comparison with the reaction of the protonated glycine ethyl ester shows the effect of substitution only. Such comparisons indicate that, though the introduction of a charge on the N atom has a major effect on the rate, substitution at this centre has little effect. This is also discussed with reference to the Arrhenius parameters for the relevant reactions.