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

摘要

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 form 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 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.