Kinetics of Manganese(III) Acetate in Acetic Acid:  Generation of Mn(III) with Co(III), Ce(IV), and Dibromide Radicals; Reactions of Mn(III) with Mn(II), Co(II), Hydrogen Bromide, and Alkali Bromides

The reaction of cobalt(III) acetate with excess manganese(II) acetate in acetic acid occurs in two stages, since the two forms Co(IIIc) and Co(IIIs) are not rapidly equilibrated and thus react independently. The rate constants at 24.5 °C are k_c = 37.1 ± 0.6 L mol^-1 s^-1 and k_s = 6.8 ± 0.2 L mol^-1 s^-1 at 24.5 °C in glacial acetic acid. The Mn(III) produced forms a dinuclear complex with the excess of Mn(II). This was studied independently and is characterized by the rate constant (3.43 ± 0.01) × 10² L mol^-1 s^-1 at 24.5 °C. A similar interaction between Mn(III) and Co(II) is substantially slower, with k = (3.73 ± 0.05) × 10^-1 L mol^-1 s^-1 at 24.5 °C. Mn(II) is also oxidized by Ce(IV), according to the rate law −d[Ce(IV)]/dt = k[Mn(II)]²[Ce(IV)], where k = (6.0 ± 0.2) × 10⁴ L² mol^-2 s^-1. The reaction between Mn(II) and HBr₂^•, believed to be involved in the mechanism by which Mn(III) oxidizes HBr, was studied by laser photolysis; the rate constant is (1.48 ± 0.04) × 10⁸ L mol^-1 s^-1 at ∼23 °C in HOAc. Oxidation of Co(II) by HBr₂^• has the rate constant (3.0 ± 0.1) × 10⁷ L mol^-1 s^-1. The oxidation of HBr by Mn(III) is second order with respect to [HBr]; k = (4.10 ± 0.08) × 10⁵ L² mol^-2 s^-1 at 4.5 °C in 10% aqueous HOAc. Similar reactions with alkali metal bromides were studied; their rate constants are 17−23 times smaller. This noncomplementary reaction is believed to follow that rate law so that HBr₂^• and not Br^• (higher in Gibbs energy by 0.3 V) can serve as the intermediate. The analysis of the reaction steps then requires that the oxidation of HBr₂^• to Br₂ by Mn(III) be diffusion controlled, which is consistent with the driving force and seemingly minor reorganization.