The mechanism of ascorbic acid oxidation by Cu(II)-poly-4-vinylpyridine complexes

Abstract

The mechanism of ascorbic acid (DH 2) oxidation with molecular oxygen catalysed by the polynuclear complex of Cu 2+ with poly-4-vinylpyridine (PVP), partially quaternized by dimethylsulphate, has been studied. The half-conversion time of the reaction of DH 2 with Cu(II) PVP under anaerobic conditions is independent of [Cu 2+]. At pH 3.5, t 0.5 (sec) = 0.8 + 5 × 10 −4 [DH 2]. The formation of an intermediate cupric-ascorbate complex is suggested ( K c ≈ 10 4 M −1). Free radicals of ascorbic acid are detected by the ESR-method combined with a flow technique. The small steady-state concentration of radicals indicates that their decay occurs inside the macromolecular complex. The rate constant of the PVP Cu(II) DH − ternary complex dissociation is ≈0.4 sec −1 (pH 3.5). The reaction of Cu(I) PVP with O 2 is not accompanied by formation of O 2 − outside the macromolecule bulk. The rate constant of this reaction is 1.3 ± 0.15) × 10 2 M −1 sec −1 (pH 3.5). The cyclic mechanism of the catalytic reaction is suggested to include interchange of the redox state of copper-ions. About 2 3 of the total copper ion exists in the form Cu(I) PVP during the reaction at pH 3.5. The rate of DH 2 oxidation under these conditions is limited by the rate of Cu(I) PVP reaction with O 2. At pH 4.5 the overall reaction rate is limited by the rate of interaction of Cu(II) PVP with DH −.