Mechanisms of antioxidant action: reactions of phosphites with hydroperoxides

Abstract

The decomposition of 1-methyl-1-phenylethyl hydroperoxide by 4-methyl-2,6-di-t-butylphenyl o-phenylene phosphite (Ic) is a catalytic pseudo-first-order reaction which does not appear to involve free radicals. It is preceded by a faster, second-order reaction between the phosphite and hydroperoxide which corresponds to the conversion of the phosphite to the active catalytic species in a reaction which involves four molecules of hydroperoxide for each molecule of the phosphite. Triphenyl phosphite by contrast reacts in a strictly stoicheiometric reaction with 1-methyl-1-phenylethyl hydroperoxide but other cyclic phosphites, ethylene isopropyl and isopropyl propane-1,3-diyl phosphites (III) and (IV), behave similarly to the o-phenylene phosphites but give different first-order rate constants in the catalytic stage of the reaction. The implications of these results for the antioxidant behaviour of cyclic phosphites are discussed.