One-electron oxidation and reduction of different tautomeric forms of azo dyes: A pulse radiolysis study

To characterize the relative reactivities of different dye tautomeric forms of model azo dyes, pulse radiolysis studies were made of their reactions with either oxidizing radicals (hydroxyl radical ((OH)-O-.) and N-3(.)) or reducing species (e(aq)(-) and the 2-hydroxy-2-propyl radical) in the pH range 3 - 12. Methyl orange, 2-(arylazo)-1-naphthol-3,6-disulfonate, and orange I or its O-methylated derivative undergo direct one-electron oxidation with N-3(.) and one-electron reduction with either eaq- or the 2-hydroxy-2-propyl radical. The efficiency of one-electron oxidation of the dyes is strongly dependent upon their different tautomeric forms in the order common ion > hydrazone > azo. In contrast, the ease of reduction of the different tautomeric forms of the dyes is in the order protonated azo greater than or equal to hydrazone > azo > common ion but shows a weaker dependence than that seen for oxidation. Radical intermediates formed either from one-electron oxidation or from one-electron reduction of the dyes mainly decay bimolecularly to give product(s) through radical-radical disproportionation. The reactivity of the (OH)-O-. is independent of the tautomeric forms of the dyes, forming (OH)-O-. adducts that decay bimolecularly to give addition product(s) by radical-radical combination (dimerization) at high radical concentrations. However, intermediates formed from orange I and its O-methylated derivative exhibit a competing hydroxyl ion elimination to give the one-electron-oxidized species, which decay by disproportionation, a less efficient process than bimolecular dimerization. Thus, the efficiency of removal of the dye by the (OH)-O-. is critically dependent on the pH, which governs the competition between the bimolecular dimerization of the (OH)-O-. adducts and the first-order water elimination pathway.