Kinetics of oxidation of α-hydroxyisobutyric acid by aquomanganese (III) ions in aqueous perchlorate media

Abstract

The kinetics of the oxidation of α-hydroxyisobutyric acid by aquomanganese(III) ions have been investigated in aqueous perchlorate media using conventional and stopped-flow spectrophotometry. The acidity range is more restricted than that used for the oxidation of other organic substrates by Mn^III_aq as the Mn^II added to maintain the ionic strength at µ= 4.00 and to suppress the formation of Mn^IV has a small effect on the rate at [HClO₄]⩽ 1.0 M. The rate-determining step involves the decomposition of an intermediate complex, Mn³⁺(CH₃)₂COHCOOH_aq, and the overall energy E and entropy ΔS^* of activation are compared with E and ΔS^* for other organic substrates oxidized by Mn^III_aq: the high reactivity of α-hydroxyisobutyric acid arises from the high positive ΔS^* derived from chelation. A value is estimated for the protonation constant of α-hydroxyisobutyric acid.