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

Abstract

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 pK_a 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 pK_a 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 pK_a of its conjugate acid. The relevance of this work to oxygen atom transfer catalysts is considered.