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, [PV^VW₁₁O₄₀]⁴⁻, have been studied spectrophotometrically in aqueous perchloric acid at 25 °C. EPR and optical studies show that [PV^VW₁₁O₄₀]⁴⁻ is reduced to the one-electron reduced heteropoly blue, [PV^IVW₁₁O₄₀]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 k_obs/[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.