Modeling the Substituent Effect on the Oxidative Degradation of Azo Dyes

In this paper, using DFT methods and reactivity indices, we show that the oxidative degradation of azo dyes occurs through the cleavage of the N−N bond following the hydroxyl radical addition to the chromophore. The geometries for two experimentally proposed reaction pathways regarding the cleavage of the C−N and N−N bonds for Cl- and CH₃-substituted azo dye 4-(4‘-sulfophenylazo)phenol are optimized at the B3LYP/6-31G(d) level of theory, and energies are further refined by single-point calculations at MPW1K/6-31+G(d,p)//B3LYP/6-31G(d). Potential energy surfaces (PES) are compared for the two mechanisms to determine the energetically more favorable mechanism as well as to explain the huge reactivity difference between the Cl- and CH₃-substituted derivatives. Reactivity indices are used in the search for competing reactions.