Kinetics of reaction of aquomanganese(III) ions with isobutyric acid and its protonated species in aqueous perchlorate media

Abstract

The slow oxidation of isobutyric acid by Mn^III in perchlorate media yields isopropanol, acetone and carbon dioxide; no formic acid is detected. The yield of acetone is very small in the absence of oxygen. The rate is the same with and without oxygen present, and is first order in both [Mn^III] and [isobutyric acid] and insensitive to changes in acidity in the range 0.5-3.7 M HCIO₄. The absence of spectral changes in Mn^III on adding isobutyric acid confirms that the rate-determining step is an outersphere interaction of the hexa-aquomanganese(III) ion with unprotonated and unionized isobutyric acid. Reaction mixtures rapidly initiate the polymerization of acrylonitrile, and a mechanism is proposed involving free radicals : peroxy radicals are involved in aerobic conditions. Values for the protonation equilibrium constant for isobutyric acid have been measured over a range of temperatures. The energy of activation E= 24.6 ± 0.4 kcal mol^–1 and the entropy of activation ΔS^*= 8.9 ± 1.4 cal deg.^–1 mol^–1 are compared with E and ΔS^* for the similar oxidations of secondary alcohols by the hexa-aquomanganese(III) ion.