Mechanism of bleaching by peroxides. Part 4.—Kinetics of bleaching of malvin chloride by hydrogen peroxide at low pH and its catalysis by transition-metal salts

Abstract

The kinetics of the oxidation of malvin chloride [3,5-bis(glucosyloxy)-4′,7-dihydroxy-3′,5′-dimethoxyflavylium chloride] by hydrogen peroxide have been studied in aqueous solution over the pH range 1.5 to 4.0 and at temperatures from 25 to 45 °C. The rates were found to be first order both in malvin chloride and in hydrogen peroxide and independent of the ionic strength. Use of free radical and singlet oxygen trapping agents ruled out the involvement of these species in the reaction. The variation of the rate constant with pH showed that the reaction took place between molecular hydrogen peroxide and both the flavylium ion I and the carbinol pseudo-base II, with I reacting more rapidly than II. At pH 3.0 the activation energy was 65.7 kJ mol–1 and the entropy of activation –78 J K–1 mol–1.