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

Abstract

The kinetics of the copper(II)-catalysed oxidation of ascorbic acid by molecular oxygen have been investigated in the range pH 1.80–5.00 using 0.100 mol dm^–3 KCl as background electrolyte. The reaction was followed both by continuous measurement of oxygen consumption (using a Clark-type oxygen-sensitive electrode) and by direct spectrophotometric measurement (on samples) of the unchanged ascorbic acid. At constant [H⁺] the rate law conforms to (i) where [L]_T is the total concentration of ascorbic acid and [Cu]_T is the total concen-, –d[L]_T/dt=–d[O₂]/dt=k[Cu]_T[L]_T^½[O₂]^½(i) tration of Cu^II. On the basis of this, and on thermodynamic evidence, a chain mechanism is proposed in which a copper–copper binuclear species (containing both ascorbate and chloride ligands) is the reactive species. The dependence on [H⁺] is too complex, however, to be fully explained. Both one- and two-electron transfer mechanisms are discussed and preference is shown for that involving an initial two-electron transfer to the dioxygen and consequently to the participation of what is formally Cu^III.