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 (AscH 2 ) 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 [O 2 ] 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 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   2 + . For [MeCN] < 0.7 M , a gradual and pH‐dependent transformation of this rate law and deviation from the first‐order dependence on [O 2 ] is indicated.