CUPROUS COMPLEXES AND DIOXYGEN .12. RATE LAW AND MECHANISM OF THE COPPER-CATALYZED OXIDATION OF ASCORBIC-ACID IN AQUEOUS ACETONITRILE

The copper-catalyzed oxidation of ascorbic acid (AscH2) has been studied with a Clark electrode in aqueous MeCN. Cu(I) or Cu(II) may be equally used as the source of metal ion, without influence on the rate law. At sufficiently high [MeCN], the rate of the overall reaction is essentially given by the rate of Cu(I) autoxidation: the reaction is of first order with respect to [Cu(tot)I,II] and [O2] and shows an inverse-square dependence on [MeCN] as observed for the autoxidation of Cu(aq)+. The pH dependence is complicated by the combination of the intrinsic pH effect on autoxidation with n additional term in the rate law which is directly proportional to [AscH-]. The latter term is explained by direct oxidation of the organic substrate by the primary dioxygen adduct of Cu(I), CuO2+. For [MeCN]<0.7M, a gradual and pH-dependent transformation of this rate law and deviation from the first-order dependence on [O2] is indicated.