Proton ionisation constants and kinetics of base hydrolysis of some α-amino-acid esters in aqueous solution

Abstract

The kinetics of base hydrolysis of sixteen α-amino-acid methyl and ethyl esters have been studied at 25° and l= 0·1M in water over a range of pH. There is a reasonable correlation between the rates of hydrolysis and Taft's polar substituent constant σ*. In the case of the valine and isoleucine esters, where chain branching occurs near the reaction centre, specific steric effects operate. Enthalpies and entropies of activation determined for the hydrolysis of a selected number of the esters obey an isokinetic relationship. The proton ionisation constants of the α-amino-acid esters are roughly 2 pK units lower than those of the corresponding amino-acids. The reasons for this behaviour are discussed in terms of the relevant enthalpy and entropy data.