Kinetics of electron-transfer reactions of hydroquinones and ascorbic acid with 1-phenyl-3-pyrazolidone radicals

Abstract

The kinetics of oxidation of four hydroquinones and ascorbic acid by 1-phenyl-3-pyrazolidone radicals have been examined in aqueous solution from pH 6.5 to 9.5. For the hydroquinones, the kinetics are markedly autocatalytic unless sulfite is present in the solutions. The autocatalysis is apparently due to accumulation of a significant quantity of semiquinone formed via reproportionation of the quinone product with unreacted hydroquinone. If present in sufficient concentration, sulfite eliminates autocatalysis by scavenging quinone, and the kinetics are first-order in each reactant. The kinetic results suggest that the electron transfer from hydroquinone to the radical to form semiquinone is rate-limiting. In the case of ascorbic acid, autocatalysis is not observed, and the kinetics are first-order in oxidant and reductant. The kinetic dependence on pH permits the resolution of bimolecular rate constants for oxidation of the fully ionized and the singly protonated reductants. The correlation of these rate constants with thermodynamic driving force is in agreement with the Marcus theory.