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

Abstract

The thermodynamic proton ionisation constants, pK^T _a,A for the amino-groups of the acids, A, α-alanine, α-amino-n-butyric acid, norvaline, norleucine, valine, isoleucine, β-alanine, γ-amino-n-butyric acid, and serine have been determined at 25 °C and I= 0·1 M. The corresponding pK^T _a,E values for the ionisation of the methyl esters, E, have also been obtained. The temperature-dependence of pK^T _a,A and pK^T _a,E has been studied at the additional temperatures of 37 and 50 °C for α-alanine, α-amino-n-butyric acid, β-alanine, γ-amino-n-butyric acid, and serine. Values of ΔH° and ΔS°₂₉₈ have been determined for the ammonium ionisation in each case, and the results obtained for the acids and their esters compared and discussed. The base hydrolysis of the protonated (EH⁺) and unprotonated (E) forms of the methyl esters of α-alanine, α-amino-n-butyric acid, norvaline, norleucine, valine, and β-alanine has been studied at 25 °C and I= 0·1M. Values of ΔH^‡ ₂₉₈ and ΔS^‡ ₂₉₈ have been obtained for the E and EH⁺ forms of the methyl esters of α-alanine, α-amino-n-butyric acid, β-alanine, and for the unprotonated form of serine.