Kinetics of oxidation of benzene-1,2-diol derivatives by cobalt(III) in aqueous acidic perchlorate media

Abstract

The kinetics of reaction of benzene-1,2-diol, (–)-4-[1-hydroxy-2(methylamino)ethyl]benzene-1,2-diol, 3-methylbenzene-1,2-diol, 4-methylbenzene-1,2-diol, 2,3-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, and 3,4-dihydroxybenzonitrile with Co^III in aqueous acidic perchlorate media have been investigated with the stopped-flow method, at different temperatures (7.5–35 °C), acidities(0.20–2.00 mol dm^–3 HClO₄), and I= 2.0 mol dm^–3(Na[ClO₄]). The reactions are first order in the concentrations of both reactants and the dependence of the second order rate constants on acidity is given by the expression k₀=A+B[H⁺]^–1. This behaviour is explained by assuming the formation of two differently protonated activated complexes. The substitutents largely effect the contribution of the path which is independent of acidity and involves Co³⁺(aq), and a linear relation between ΔG^‡ and ΔG^⊖ suggests an outer-sphere mechanism. However, the path involving [Co(OH)]²⁺(aq) is not affected by the nature of the organic substrate: because of its high rate with respect to water substitution, an outer-sphere limiting mechanish is advanced.