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.

The peroxide bleaching reaction was found to be catalysed by a range of transition-metal salts. Kinetic experiments carried out at 25 °C and pH 3.0 and using up to 10^–4 mol dm^–3 concentrations of catalyst showed that the catalytic reaction was first order in each of malvin chloride, hydrogen peroxide and metal salt. The third-order catalytic rate constants decreased in the order [Co(NH₃)₅Cl]²⁺ > Mo^VI≈ Ti^IV > [MoO(O₂)₂(pyO)₂] > W^VI≈[WO(O₂)₂(pyO)₂]≈ V^V≈ Cr^VI, somewhat similar to the order found for the catalytic bleaching of phenolphthalein at pH 10. Use of trapping agents showed that neither free radical species nor singlet oxygen were involved in the catalytic process. The way in which the catalytic rate constant of K₂[MoO₄] decreased with increasing ionic strength showed that the flavylium ion I was reacting much more rapidly than species II. The various metal-peroxo species likely to be the active catalytic agents are discussed.