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 MnIIIaq as the MnII added to maintain the ionic strength at µ= 4.00 and to suppress the formation of MnIV 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)2COHCOOHaq, and the overall energy E and entropy ΔS* of activation are compared with E and ΔS* for other organic substrates oxidized by MnIIIaq: 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.