Studies on electron transfer reactions: oxidation of phenol and ring-substituted phenols by heteropoly 11-tungstophosphovanadate(V) in aqueous acidic medium

摘要

The kinetics of oxidation of phenol and a few ring-substituted phenols by heteropoly 11-tungstophos-phovanadate(V), [PV~VW_(11)O_(40)]~(4-) (HPA) have been studied spectrophotometrically in aqueous acidic medium containing perchloric acid and also in acetate buffers of several pH values at 25 °C. EPR and optical studies show that HPA is reduced to the one-electron reduced heteropoly blue (HPB) [PV~(IV)W_(11)O_(40)]~(5-). In acetate buffers, the build up and decay of the intermediate biphenoquinone show the generation of phenoxyl radical (ArO) in the rate-determining step. At constant pH, the reaction shows simple second-order kinetics with first-order dependence of rate on both [ArOH] and [HPA]. At constant [ArOH], the rate of the reaction increases with increase in pH. The plot of apparent second-order rate constant, k2, versus 1/[H~+] is linear with finite intercept. This shows that both the undissociated phenol (ArOH) and the phenoxide ion (ArO~-) are the reactive species. The ArO~--HPA reaction is the dominant pathway in acetate buffer and it proceeds through the OH~- ion triggered sequential proton transfer followed by electron transfer (PT-ET) mechanism. The rate constant for ArO~--HPA reaction, calculated using Marcus theory, agrees fairly well with the experimental value. The reactivity of substituted phenoxide ions correlates with the Hammett σ~+ constants, and ρ value was found to be —4.8. In acidic medium, ArOH is the reactive species. Retardation of rate for the oxidation of C6H5OD in D2O indicates breaking of the O-H bond in the rate-limiting step. The results of kinetic studies show that the HPA-ArOH reaction proceeds through a concerted proton-coupled electron transfer mechanism in which water acts as proton acceptor (separated-CPET).