Proton-coupled electron transfer reactions: kinetic studies on the oxidation of dihydroxybenzenes by heteropoly 10-tungstodivanadophosphate in aqueous acidic medium

Abstract

Kinetic studies on the oxidation of hydroquinone and catechol by the heteropoly 10-tungstodivanadophosphate anion, [PVVVVW10O40]5−, have been carried out in aqueous acidic medium at 25 °C by UV–visible spectrophotometry. The oxidation of hydroquinone shows simple second-order kinetics overall, with first-order dependence of the rate on both [oxidant] and [hydroquinone] at constant [H+]. For catechol oxidation, the order of the reaction with respect to [oxidant] is unity, while the order with respect to [catechol] is variable; this reaction shows Michaelis–Menten-type kinetics at constant [H+]. The rate of the reaction is insensitive to [H+] in the pH range 1.2–1.7. Rate retardation for deuterated hydroquinone and catechol (C6H4(OD)2) in D2O indicates breaking of the –OH bond in the rate-limiting step. Based on the observed kinetic isotope effect and calculated ground-state free energy change (∆G 0) values, a hydrogen atom transfer mechanism is suggested for the reaction; i.e., in the rate-limiting step, one electron and one proton are transferred from the reductant to the oxidant in a concerted manner. Rates of oxidation of hydroquinone by this oxidant in neat acetonitrile at 25 °C have also been measured. By applying the Marcus equation, the self-exchange rate constant of the oxidant (VVVIVOH/H·) in acetonitrile has been evaluated.