The unimolecular acid hydrolysis of acetylglycine in aqueous acetic acid

Abstract

The hydrolysis of acetylglycine in aqueous acetic acid of low water content shows an ambiguous behaviour in that the relative rate sequence for an increasing water concentration becomes reversed on increasing the acid concentration. At the ‘high’ acid concentrations the rate is first-order with respect to the non-protonated acetylglycine concentration and the rate-controlling stage is consistent with the unimolecular reaction of the acetylglycine zwitterion to form an azlactone hydrate which is then rapidly hydrolysed to products. At ‘intermediate’ acid concentrations, the reaction is first-order with respect to both the acid and non-protonated acetylglycine concentrations, and there is presumably a rapidly established equilibrium between the acetylglycine and the azlactone and its hydrate and its solvate with acid. The rate-controlling step then becomes an acid-catalysed fission of the azlactone hydrate and solvate. At ‘low’ acid concentrations, the rate is less than that expected for the acid catalysis in the ‘intermediate’ range. The rate equation contains two terms, one corresponding to the ‘intermediate’ region, and the other is second-order with respect to the acid concentration and inversely proportional to the water concentration. This has been interpreted as a slow acid-catalysed solvation of the azlactone coupled with a slow acid-catalysed fission of the azlactone solvate. it is shown that the above interpretations are consistent with the ambiguous data reported for the hydrolysis of the dipeptides.