Kinetics of the Formation and Decay of Benzidine Mono Radical Cation in Aqueous Solution

Abstract

Abstract The oxidation of benzidine by Ce(IV); the oxoanions MnO4− and Cr2O72−; peroxides, namely potassium peroxomonosulfate (KHSO5), peroxodisulfate (S2O82−) and H2O2; and halogens viz., Cl2, Br2, and I2, to the benzidine mono radical cation (benzidine•+) and further oxidation to the benzidine diradical cation (+•benzidine•+) by the above-mentioned oxidants, except for Br2 and I2, were investigated by a stopped-flow technique. The kinetics of both reactions, the formation and decay of benzidine•+, were monitored by following the absorption of the radical cation at λmax = 603 nm. The reactions were found to follow a total second-order kinetics, first-order each with respect to [benzidine] or [benzidine•+] and the [oxidant]. The effects of the pH and temperature on the formation and decay of benzidine•+ were also investigated. The kinetic and transition-state parameters were evaluated with a suitable reaction mechanism. The radical cation benzidine•+ was converted back to the parent compound, benzidine, by such reducing agents as sulfite (SO32−), thiosulfate (S2O32−), dithionite (S2O42−), and disulfite (S2O52−). The rate constants for these reactions were also estimated. The experimentally determined rate constants for the oxidative electron-transfer reactions were correlated theoretically using Marcus theory; the observed and calculated rate constants show good agreement.