Ascorbic acid copper dioxygen complexes

The radical form 9.4 has an unpaired electron and may undergo fast reactions with redox partners that also undergo one-electron processes. Such a redox partner is the triplet radical, dioxygen. The copper complex of ascorbic acid undergoes rapid aerial oxidation to give the dione, dehydroascorbic acid, which may be viewed as being derived by electron loss from the radical (Fig. 9-4). 

Figure 9-4. The reaction of dioxygen with the copper complex of ascorbic acid generates a copper complex of dehydroascorbic acid.

Recently Jameson and Blackburn (14,15,16) have suggested an alternate mechanism for the copper-catalyzed autoxidation of ascorbic acid, involving the formation of a binuclear Cu(II) complex (17) of the ascorbate anion, and the subsequent formation of an intermediate peroxo type Cu( II)-dioxygen-ascorbate complex (18). Their kinetic data suggested a variety of rate behavior depending on the nature of the supporting electrolyte. Formula 17, which was postulated for nitrate... 

In order to explain the half-order dependence on dioxygen concentration it has been found necessary to propose rather exotic complexes between copper(ii) and L-ascorbic acid ... 

Figure 7.14 The dioxygen molecule complex intermediate in the oxidation of L-ascorbic acid catalysed by copper( ii) ions...

FIGURE 6. Ascorbic acid as a prooxidant. Copper, Cu (II), can catalyze the production of hydrogen peroxide and superoxide. The copper-ascorbic acid-dioxygen complex (an enediol-like complex) may be enhanced by the superoxide-containing resonance form (Taqui-Khan, 1967). This oxidation leads to a dicarbonyl product. Adapted from Martell (1980). 

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