Oxidation of ascorbic acid by copper(II) and the ferrocenium ion in acetonitrile–water mixtures

Abstract

Measurements are reported on the rates of oxidation of ascorbic acid (H₂A) by copper(II) and the ferrocenium cation [Fe(C₅H₅)₂]⁺ in mixtures of water and acetonitrile. The stabilisation of Cu⁺ by acetonitrile results in a rapid increase in E^⊖ for Cu²⁺–Cu⁺ and a simple, irreversible reaction between copper(II) and ascorbic acid to form copper(I) and dehydroascorbic acid upon addition of small amounts of acetonitrile to water. The acid dependence of the rate constant is consistent with the involvement of complexes of the monoanion, [Cu^II(HA)]⁺, and the dianion [Cu^IIA], the former becoming relatively more important with increasing acetonitrile content of the solvent. The solvent dependence of the rate constant is discussed in relation to the solvation of copper(II) and copper(I) in the mixtures and it is suggested that the dominant influence is the solvation of copper(I) formed in the initial electron-transfer process. Redox potentials, E for [Fe(C₅H₅)₂]⁺–[Fe(C₅H₅)₂], and rates of oxidation of ascorbic acid by ferrocenium, a typical outer-sphere oxidant, have been measured under the same conditions and are compared with those of the copper(II)–copper(I) system.