Kinetics and mechanism of disproportionation of Mn(VI)O42− induced by decomposition of intramanganato-polygalacturonate methyl ester coordination polymer precursor: Medium-substituent effects on reactivity

During the oxidation of polygalacturonate methyl ester polysaccharide by MnO4− in highly basic medium, the formation of the green polygalacturonate methyl ester (PGME-Mn(VI)O42−) and the blue (PGME-Mn(V)O43−) intermediates were followed spectrophotometrically. In this paper, the kinetics of the decay of the green (PGME-Mn(VI)O42−) intermediate, the second stage of the oxidation process, were followed spectrophotometrically at different concentrations of NaOH and temperatures ranging from 10 to 30 °C. The decay of the green intermediate (PGME-Mn(VI)O42−) follows pseudo-first-order kinetics, d[PGME-Mn(VI)O42−]/dt = kobs.[PGME-Mn(VI)O42−] where kobs. = k2[OH−]. The reaction was found to be base catalyzed. The obtained linear Michaelis–Menten kinetics proves the formation of intermediates during the decay process. A reaction mechanism involving a deprotonation pre-equilibrium step followed by a rate determining decay of the deprotonated green intermediate (PGME-Mn(VI)O42−) is proposed. Moreover, the activation parameters were consistent with the assumed reaction mechanism. The positive salt effect indicates the ionic character of the transient species. The decay and formation rates increase in the following order of polysaccharides: MC-Mn(VI)O42− < CMC-Mn(VI)O42− ≪ PGME-Mn(VI)O42−.