Photoinduced transformation processes of 2,4-dichlorophenol and 2,6-dichlorophenol on nitrate irradiation.

2,4-Dichlorophenol (2,4-DCP) and 2,6-dichlorophenol (2,6-DCP) undergo oxidation, nitrosation and nitration in the presence of nitrate under UV irradiation. Nitration is favoured under acidic conditions, most likely because HNO(2) is formed on nitrate photolysis. The most likely photonitration pathway is the reaction between radiation-excited dichlorophenols (DCP*) and HNO(2). HNO(2) is also able to nitrate DCP in the dark with elevated yields. Irradiation also causes DCP direct photolysis, which is more efficient for the dichlorophenolate anions. The photolysis of the dichlorophenols and that of the dichlorophenolates also produce different intermediates, by dechlorination in the former and ring contraction in the latter case.