Theoretical mechanistic studies on the degradation of alizarin yellow R initiated by hydroxyl radical

The degradation of azo dyes has attracted many research efforts not only due to the resulting environmental problems but also because the azo compounds with various substituents may show different degradation mechanism. It has been computationally found here, for the first time, that the HO center dot initiated cleavages of C-N and N-N bonds of alizarin yellow R with carboxyl group are kinetically competitive. In view of the formation of HO center dot adducts, the C-N and N-N bond cleavages of the hydrazone tautomer of alizarin yellow R are also kinetically competitive, but the former is more thermodynamically favorable. This result is different from that previously reported for the hydrozone tautomers of Acid Orange 7 and Acid Orange 8 containing hydroxyl and azo groups in neighboring positions, which are favorable to follow C-N bond cleavage mechanism both kinetically and thermodynamically. The decarboxylation occurs via an attack of HO center dot to the benzene ring carbon connecting to the carboxyl group rather than a direct attack of HO center dot to the carboxyl carbon atom. The anion form has higher reactivity than the neutral form in all of the reactions investigated. In addition, a water molecule as a proton relay reagent could significantly reduce the energy barrier for the N-N bond cleavage of alizarin yellow R. Copyright (c) 2014 John Wiley & Sons, Ltd.