Kinetics and mechanism of the hydroxylation of toluene catalyzed by a ruthenium(III) analogue of the udenfriend system: ru(III)-edta-ascorbate-molecular oxygen

Abstract

The hydroxylation of toluene to cresols by molecular oxygen is catalyzed by the Ru(III) analogue of the model Udenfriend system: Ru(III)-EDTA-ascorbic acid. The hydroxylation was investigated at 30°C and μ=0.1 M KNO₃ in a 50% (v/v) mixture of 1,4-dioxane and water in the pH range 1.50-2.50. The kinetic study of the reaction shows a first-order dependence in the concentrations of Ru(III)-EDTA, ascorbic acid, molecular oxygen and the substrate toluene. The rate of reaction was found to increase linearly with an increase in pH, and is inverse first order in hydrogen ion concentration in the pH range 1.50-2.50. The kinetic and stability parameters of the complicated reaction system were determined by potentiometric, spectrophotometric, manometric and gas Chromatographie techniques. The reaction proceeds via an oxygen-dependent pathway by the formation of an intermediate Ru(IV) dioxygen complex. A mechanism for hydroxylation is proposed on the basis of these results. The system Ru(III)-EDTA-ascorbic acid-molecular oxygen selectively oxidizes toluene to o- and p-cresols.