Role of copper dimers and the participation of copper(III) in the copper-catalysed autoxidation of ascorbic acid. Part II. Kinetics and mechanism in 0.100 mol dm–3 potassium nitrate

Abstract

The Kinetics and mechanism of the copper(II)-catalysed oxidation of ascorbic acid by dissolved oxygen in the presence of nitrate ions have been reinvestigated in the pH range 2.0–3.5. The rate law for the disappearance of oxygen is as in (i) where L^2–is the ascorbate anion, and on the basis of this and on thermodynamic evidence a chain –d[O₂]/dt=k′[Cu²⁺][HL^–][O₂]^½(i) mechanism is proposed in which copper(II) ascorbate dimers are the reactive species. This mechanism involves a two-electron transfer to oxygen (the latter binding across a Cu–Cu binuclear site), as the primary electron-transfer process, and thus could be said to involve formal copper(III) intermediates. The catalytic activity of these copper dimers parallels that of the oxygen-binding sites of a number of copper-containing oxidases (e.g. ceruloplasmin, laccase, ascorbic acid oxidase, etc.) for which ascorbate is a natural substrate. The extension of the mechanism proposed for the model system to that of the oxidases is discussed, and it is suggested that electron transfer to oxygen might also occur as the primary process in the latter [likewise requiring the participation of what is formally copper(III) in the enzyme mechanism].