Unexpected Superoxide Dismutase Antioxidant Activity of Ferric Chloride in Acetonitrile

The azobis(isobutyronitrile)-initiated autoxidation of -terpinene in acetonitrile at 50 C yields only p-cymene and hydrogen peroxide (1:1) in a chain reaction carried by the hydroperoxyl radical, HOO (Foti, M. C.; Ingold, K. U. J. Agric. Food Chem. 2003, 51, 2758-2765). This reaction is retarded by very low ( M) concentrations of FeCl3 and CuCl2. The kinetics of the FeCl3-inhibited autoxidation are consistent with chain-termination via the following: Fe3+ + HOO [FeIV-OOH]3+ and [FeIV-OOH]3+ + HOO Fe3+ + H2O2 + O2. Thus, FeCl3 in acetonitrile can be regarded as a very effective (and very simple) superoxide dismutase. The kinetics of the CuCl2-inhibited autoxidation indicate that chain transfer occurs and becomes more and more important as the reaction proceeds, i.e., the inhibition is replaced by autocatalysis. These kinetics are consistent withreduction of Cu2+ to Cu+ by HOO and then the reoxidation of Cu+ to Cu2+ by both HOO and the H2O2 product. The latter reaction yields HO radicals which continue the chain.