Free-radical-induced oxidation and reduction of 1-arylazo-2-naphthol dyes: a radiation chemical study

Abstract

The kinetics and transient optical absorption spectra of the intermediates formed in the reaction of ^•OH, N₃^•, and e^-_aq with 1-arylazo-2-naphthol and its ortho- and para-substituted chloro, methyl and methoxy derivatives at pH 7.0 have been studied by pulse radiolysis. The rate constants for the reaction of ^•OH and the nucleophilic hydrated electron (e-_aq) with these compounds are (1.0-1.2) × 10¹⁰ M^-1 s^-1 and (0.8- 2.5) × 10¹⁰ M^-1 s^-1. The reaction of N₃^• with these azo dyes is more selective (ρ ⁺ = -2.0) with the rate constants varying between 0.1 × 109 and 8 × 10⁹ M^-1 s^-1 depending upon the substituent. The transient optical absorption spectra of the intermediates formed in the reaction of the dyes with ^•OH are different from the corresponding spectra of the intermediates observed in the reaction of the dyes with N₃^•. The ^•OH radical reacts with the dyes by addition to form the ^•OH adducts, whereas N₃^• reacts with the dyes by direct one-electron transfer to form the one-electron oxidized dye radical. The pK_A of the dye radical of the para-methoxy derivative was determined to be 5.2. This radical decays bimolecularly, predominantly by disproportionation, whereas the ^•OH adducts of the dyes also decay bimolecularly but predominantly by dimerization. The initially formed radical anion in the reaction of e^-_aq with the unsubstituted dye protonates rapidly to give the hydrazyl radical, which decays bimolecularly by disproportionation. Of the radicals studied, the ^•OH-induced oxidation of the dyes is the most effective process leading to decolorization of the dyes at pH 7.0.