Kinetic and equilibrium solvent isotope effects on proton transfer between thiols and amines in aqueous solution

Abstract

Proton transfer from the thiol groups in mercaptoacetate and 2-mercaptobenzoate to amines in aqueous solution occurs with rate coefficients which are two orders of magnitude below the diffusion limit. The S–H ⋯ O^– hydrogen bond in 2-mercaptobenzoate is weak and has negligible effect on the rates of proton transfer. Solvent isotope effects on the equilibrium constants for reaction of mercaptoacetate with four amines are in the range K(H₂O)/K(D₂O)= 0.52–0.59. Kinetic solvent isotope effects on the forward [k_f(H₂O)/k_f(D₂O)= 2.4 ± 0.4] and reverse [k_r(H₂O)/k_r(D₂O)= 4.3 ± 0.7] rate coefficients are large, but show no evidence for a maximum with changing base strength of the amines over a limited range. The behaviour of thiols is compared with that of oxygen and nitrogen acids.