Thermodynamics of homogeneous oxidations: Part I. Thermodynamics of oxidation of ascorbic acid and cyclohexane by the Ru(III) analogue of the model peroxidase system: Ru(III)-EDTA-ascorbate-H₂O₂

Abstract

The homogeneous oxidation of ascorbic acid to dehydroascorbic acid and cyclohexane to cyclohexanol by hydrogen peroxide catalyzed by Ru(III)-EDTA and Ru(III)-EDTA-ascorbate complexes, respectively, has been investigated in the temperature range 288-313 K at μ=0.1 M KNO₃. In Ru(III)-EDTA catalyzed oxidation of ascorbic acid, an overall secondorder kinetics, first order in both [Ru(III)-EDTA] and [ascorbic acid] and zero order in hydrogen peroxide concentration, was observed in the temperature range studied. The oxidation of cyclohexane to cyclohexanol exhibits first-order dependence in [Ru(III)-EDTA], [ascorbic acid] and [H₂O₂] concentration. The dependence in [cyclohexane] was found to be fractional order. An inverse first-order dependence in hydrogen ion concentration was observed for both reactions. The activation parameters for the oxidation steps corresponding to the rate constants and thermodynamic parameters for the formation of kinetic intermediate complexes corresponding to the equilibrium constants were computed, and the mechanistic implications of the oxidations catalyzed by Ru(III)-EDTA and its thermodynamics are discussed. The thermodynamics of the Ru(III)-EDTA-ascorbate system is compared with that of the Fe(III)-EDTA-ascorbate system.