Proton-induced disproportionation of superoxide ion in aprotic media

Abstract

The proton-induced disproportionation of superoxide ion (Of) in dimethylformamide and in acetonitrile has been studied by stopped-flow spectrophotometry, cyclic voltammetry, and controlled-potential voltammetry at a rotating-disk electrode. For strongly acidic substrates (HC104, HC1, and NH4C104) the disproprotionation reaction is second order with respect to Of, but too rapid to measure accurately, k > 107 M'1 s'1. With less protic substrates (ascorbic acid, phenols, catechols, alcohols, and water) the rate of disproportionation is first order with respect to Of and with respect to substrate; proton transfer is the rate-limiting step. In dimethylformamide the second-order rate constants range from k = 2 X 104 M"1 s"1 for ascorbic acid to k = 1 X 10'3 for water. On the basis of the kinetic data, a self-consistent mechanism for superoxide disproportionation by acidic substrates in nonaqueous solvents is proposed.

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