The oxidation of azo dyes by peroxy acids and tert-butyl hydroperoxide in aqueous solution catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrin: product studies and mechanism

Abstract

Product studies on the oxidation of two 1-arylazo-2-hydroxynaphthalene-6-sulfonate dyes by peroxy acids and by tert-butyl hydroperoxide catalysed by iron(III) 5,10,15,20-tetra(2,6-dichloro-2-sulfonatophenyl)porphyrin in solution have been carried out. In single turnover experiments at pH 11.88, 9.30 and 6.93, dye bleaching by peroxy acids is a two-step process; an initial fast reaction, during which the oxoiron(IV) porphyrin is also formed, is followed by a slow phase involving the regeneration of the iron(III) porphyrin from the oxoiron(IV) porphyrin. The peroxy acid∶dye stoichiometry is 1∶1 for the initial fast dye bleaching and 2∶1 for the slow phase; the latter is equivalent to a 4∶1 stoichiometry of oxoiron(IV) porphyrin to dye. In multiple turnover reactions the dye bleaching occurs in two sequential oxidations by oxoiron(IV) porphyrin π-cation radical, each with a peroxy acid∶substrate stoichiometry of 1∶1.

Evidence is presented to show that, in multi-turnover reactions at pH 11.88 where the substrate is the dye anion, the initial product arises from the hydroxylation of the 1-position of the naphthol ring which in the second step is oxidised further to give an azoxy compound. At pH 9.30, where the dyes exist predominantly as the hydrazone tautomers, the initial oxidation generates an azoxy compound which following a Wallach rearrangement is further oxidised on the naphthol ring. More extensive oxidation results in the cleavage of the N–N bond.

The iron(III) porphyrin-catalysed dye oxidation with excess of tert-butyl hydroperoxide at pH 6.93, which occurs by hydrogen atom-abstraction, has a hydroperoxide∶dye stoichiometry approaching 2∶1 and gives the same azoxy product as obtained at pH 11.88 with the peroxyacids.

Mechanisms to account for the observed products and stoichiometries are discussed.