MECHANISM OF OXIDATION OF AZO DYES BY A STERICALLY HINDERED ANIONIC OXOIRON(IV) PORPHYRIN IN AQUEOUS-SOLUTION

The oxidation of 1-(substituted phenylazo)-2-hydroxynaphthalene-6-sulf onate dyes by oxoiron(IV) tetra(2,6-dichloro-3-sulfonatophenyl)porphyr in ((OFeTDCSPP)-T-IV) in aqueous solution is first-order in the concen tration of the dye and of the oxoiron(IV) species over the pH range 6. 93-12.68. The pH dependence of the second-order rate constant, k(obs), which shows a minimum between pH 8.5 and 9.5, can be simulated using two pH-dependent equilibria [(OFeTDCSPP)-T-IV(OH2)/(OFeTDCSPP)-T-IV(OH ) and dye/dye anion] and the rate constants for three oxidations, dye by (OFeTDCSPP)-T-IV(OH2) and (OFeTDCSPP)-T-IV(OH) and dye anion by (OF eTDCSPP)-T-IV(OH). The fourth combination, the oxidation of the dye an ion by (OFeTDCSPP)-T-IV(OH2), makes an insignificant contribution to t he overall rate of reaction, The mechanisms of the three oxidation pro cesses have been studied using Hammett correlations of substituent eff ects and by comparisons with the oxidations of isomeric 1-aryl-4-hydro xynaphthalenesulfonate dyes, of an O-methylated dye and of a deuterate d 1-phenylazo-2-hydroxynaphthalene-6-sulfonate dye, bearing in mind th at the dyes in aqueous solution exist as an equilibrium between azo an d hydrazone tautomers. In strongly basic solution the dominant reactio n is an electron-transfer oxidation between the dye anion and (OFeTDCS PP)-T-IV(OH) whereas at neutral pH the major reaction is hydrogen atom -abstraction by (OFeTDCSPP)-T-IV(OH2) from the azo tautomer of the dye . The mechanism at the pH-rate minimum, between (OFeTDCSPP)-T-IV(OH) a nd dye, is less clear and alternatives are discussed.