Cuprous Complexes and Dioxygen. Part 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] and O₂] and shows an inverse-square dependence on MeCN] as observed for the autoxidation of Cu. The pH dependence is complicated by the combination of the intrinsic pH effect on autoxidation with an 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, CuO. 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.