The Kinetics of the Acid Hydrolysis of some Dipeptides in Acetic acid

Abstract

Glycylglycine and a number of its C-methyl derivatives have been hydrolysed with excess perchloric acid in aqueous acetic acid as solvent, and by varying the water content of the medium its effect on the reaction rate has been determined. It has been found that small changes in the water concentration cause reversals in the relative rate sequences in both series of dipeptides studied. In aqueous acetic acid containing more than 2 per cent. water the relative rate sequence in the first series is glycylglycine > alanylglycine > α-aminoisobutyrylglycine, but at water concentrations less than 2per cent. α-aminoisobutyrylglycine reacts faster than glycylglycine. It is concluded that increasing methylation has produced a change in mechanism whereby α-aminoisobutyrylglycine reacts by a unimolecular heterolysis rather than by the bimolecular mechanism given in the previous paper (Martin 1957). For the second series of dipeptides, the relative rate sequence is glycylglycine > glycylalanine > glycyl-α-aminoisobutyric acid for water concentrations greater than 2 per cent. At water concentrations less than 2 per cent. the rate sequence is reversed, so that glycyl-α-aminoisobutyric acid > glycylalanine > glycylglycine. This reversal is attributed to changes in the polar effect of the terminal carboxyl group which is brought about by increasing methylation.