Kinetics and electrochemical studies on superoxide

Abstract

Rate constants for the reaction of superoxide O^- ₂ with various substrates were obtained through stationary electrode polarography theory and technique. In solvent acetonitrile, the substrate and the rate constants of the reaction O^- ₂ + AH^{- k2}→Product, are, AH = isopropanol (k₂ < 0.01 M^-1 s^-1); ethanol (k₂ = 1.42 × 10² M^-1 s^-1); methanol (k₂ = 1.1 × 10⁷ M^-1 s^-1), H₂O (k₂ = 1.0 × 10⁵ M^-1 s^-1). In MeCN, O^-2 was found to be rather unreactive towards glucose and acetone but it reacts with fructose and sucrose catalytically. However, in DMF2, O^- ₂reacts with glucose and fructose with k2 order of 105 M-1 s-1. The mechanism of the reaction of O^- ₂ with the substrates (AH) is proposed as O^- ₃ + AH k₂O, AH^k2 _k-1 k′ [O₂H + AH]^-, _k-2→O₂H + A^- with k₁ = 10⁹ M^-1 s^-1 and k^-1 = 10⁸ -10⁹ s^-1. With these values of k_-1 and k₁, k′ k₂(obs). The reversible E_1/2 for O₂ + e O^- ₂ in various solvents: MeCN, acetone, isopropanol, methanol, H₂O were obtained either directly from the reversible voltammogram or from experimental voltammograms and the rate constants obtained (as above) using stationary electrode polargraphy theory; E_1/2 being -0.82 (MeCN),-0.85 (acetone),-0.72 (isopropanol);-0.66 (MeOH),-0.56 (H2O) vs SCE.