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–3potassium chloride

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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 CuII. 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 CuIII.

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