Oxidation of ferrous and ferrocyanide ions by peroxyl radicals

Alkylperoxyl and arylperoxyl radicals were produced by pulse radiolysis in aqueous solutions, and their reactions with ferrous and ferrocyanide ions were studied by kinetic spectrophotometry. Oxidation of Fe(CN)₆^4- took place with rate constants that varied from <1 × 10⁵ to 5 × 10⁷ L mol^-1 s^-1, depending on the electron-withdrawing effects of the substituents on the peroxyl radical and presumably reflecting variations in reduction potential of the peroxyl radical, as expected for outer-sphere electron transfer. Oxidation of Fe_aq²⁺, on the other hand, took place by an inner-sphere mechanism controlled by the rate of dissociative interchange of the water ligand. The rate constants were nearly the same for all the peroxyl radicals examined (k = (0.5-1.1) × 10⁶ L mol^-1 s^-1) and involved the formation of a transient intermediate, RO₂^-Fe³⁺, which later decomposed to yield Fe_aq³⁺. The decomposition was accelerated by H⁺ and by Fe²⁺. The proposed mechanism is a modification of a previously suggested reaction scheme.