Kinetics of reaction of aquomanganese(III) ions with sec-butanol and cyclohexanol and their protonated species

Abstract

The reactions of Mn(III) with sec-butanol and cyclohexanol in aqueous perchlorate media are first order in [Mn(III)] and first order in [alcohol], and one molecule of ketone is formed for every two Mn(III) consumed. No intermediate complex is detected. The observed bimolecular rate constants are insensitive to changes in acidity in the range 1.0–3.70 M HClO₄, and the rate-determining step is the attack of the hexaaquomanganese(III) ion on the unprotonated alcohol. The insensitivity to changes in [HClO₄] arises from the hydrolysis equilibrium constant for Mn(III)∼ the protonation equilibrium constant of the alcohols. The latter were determined for a range of temperatures, and show a maximum at ∼25°C similar to that found previously for methanol and isopropanol. The activation energies and entropies of activation are compared with those for the oxidation of isopropanol, and discussed in terms of the simultaneous accommodation of an electron in the Mn atom and a proton in the bulk solvent.