A kinetic study of the oxidations of 2-mercaptoethanol and 2-mercaptoethylamine by heteropoly 11-tungsto-1- vanadophosphate in aqueous acidic medium

Abstract

The kinetics of oxidation of 2-mercaptoethanol and 2-mercaptoethylamine by the heteropoly 11-tungsto-1- vanadophosphate anion, [PVVW11O40]4−, have been studied spectrophotometrically in aqueous perchloric acid at 25 °C. EPR and optical studies show that [PVVW11O40]4− is reduced to the one-electron reduced heteropoly blue, [PVIVW11O40]5−, whilst the thiols are oxidized to the corresponding disulphides, RSSR. Spectrophotometric titrations show that the stoichiometry of both reactions is 1:1. At constant pH, the reactions show simple second-order kinetics with first-order dependence of rate on both [oxidant] and [thiol]. At constant [thiol], the rate of the reaction increases with increasing pH. Plots of kobs/[thiol]t versus 1/[H+] are linear with finite intercepts, showing that both the undissociated thiol (RSH) and thiolate ion (RS−) are reactive species. Generation of RS· from RSH proceeds via a separated-concerted proton–electron transfer mechanism. The reaction of thiolate ion is a simple outer-sphere electron transfer reaction. By applying the Marcus theory, the self-exchange rate constants for the couples HOCH2CH2S·/HOCH2CH2S− and H3N+CH2CH2S·/H3N+CH2CH2S− were evaluated as 3 × 109 and 2.2 × 108 M−1 s−1, respectively, at 25 °C.