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 th e kinetics of its oxidation by hydrogen peroxide over a wide range of pH. S pectrophotometric 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 respectiv ely. The forward rate constant for the first reaction is 1.3x10(-4) s(-1). The UV-visible spectrum of HD- indicates that the ortho-O- substituent is p rotonated. The rate constant for the reaction of D2- and HOO-, determined a t high pH, is 0.31 dm(3) mol(-1) s(-1). Rate constants for the reactions of D2- and HOO- associated with one, two and three protons, respectively, hav e been estimated from the pH dependence of the reaction. From the applicati on of the transition-state pK(a) approach and consideration of the changes in the UV-visible spectra during the reactions, the rate constants have bee n assigned to the reactions of H2O2 and D2- (0.48 dm(3) mol(-1) s(-1)); H2O 2 and HD- or H3O2+ and D2- (respective upper limits, 5.7x10(-6) or 1.3x10(7 ) dm(3) mol(-1) s(-1)) and H3O2+ and HD- (5.1x10(2) dm(3) mol(-1) s(-1)). C omparisons between these rate constants and published rate constants for al kali and peroxide bleaching of similar triarylmethane dyes show the excepti onally high reactivity of D2- toward H2O and H2O2. This is attributed to in tramolecular base catalysis by the ortho-O- substituent of D2- and is discu ssed in terms of the position of the catalytic site and the pK(a) of its co njugate acid. The relevance of this work to oxygen atom transfer catalysts is considered.