Kinetics and mechanism of the bleaching of a triarylmethane dye by hydrogen peroxide and water: evidence for intramolecular base catalysis

This work describes the ionisation of the triarylmethane dye Green S and the kinetics of its oxidation by hydrogen peroxide over a wide range of pH. Spectrophotometric titration yields pKa values for the equilibria D2− + H2O = D(OH)3− + H+, HD− = D2− + H+, and H2D = HD− + H+ of 11.72, 7.66, and 1.31 respectively. The forward rate constant for the first reaction is 1.3 × 10−4 s−1. The UV-visible spectrum of HD− indicates that the ortho-O− substituent is protonated. The rate constant for the reaction of D2− and HOO−, determined at high pH, is 0.31 dm3 mol−1 s−1. Rate constants for the reactions of D2− and HOO− associated with one, two and three protons, respectively, have been estimated from the pH dependence of the reaction. From the application of the transition-state pKa approach and consideration of the changes in the UV-visible spectra during the reactions, the rate constants have been assigned to the reactions of H2O2 and D2− (0.48 dm3 mol−1 s−1); H2O2 and HD− or H3O2+ and D2− (respective upper limits, 5.7 × 10−6 or 1.3 × 107 dm3 mol−1 s−1) and H3O2+ and HD− (5.1 × 102 dm3 mol−1 s−1). Comparisons between these rate constants and published rate constants for alkali and peroxide bleaching of similar triarylmethane dyes show the exceptionally high reactivity of D2− toward H2O and H2O2. This is attributed to intramolecular base catalysis by the ortho-O− substituent of D2− and is discussed in terms of the position of the catalytic site and the pKa of its conjugate acid. The relevance of this work to oxygen atom transfer catalysts is considered.