Free-Radical-Induced Oxidation and Reduction of 1-Arylazo-2-naphthol Dyes: A Radiation Chemical Study

The kinetics and transient optical absorption spectra of the intermediates formed in the reaction of • OH, N 3 •, 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) × 10 10 M -1 s -1 and (0.8−2.5) × 10 10 M -1 s -1 . The reaction of N 3 • with these azo dyes is more selective (ρ + = −2.0) with the rate constants varying between 0.1 × 10 9 and 8 × 10 9 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 N 3 • . The • OH radical reacts with the dyes by addition to form the • OH adducts, whereas N 3 • reacts with the dyes by direct one-electron transfer to form the one-electron oxidized dye radical. The p K 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.