Direct and induced phototransformation of mecoprop [2-(4-chloro-2-methylphenoxy)propionic acid] in aqueous solution

Mecoprop was irradiated under various conditions of pH, oxygenation and wav elengths in order to study the reactions involved in the phototransformatio n. Four main photoproducts were identified: 2-(4-hydroxy-2-methylphenoxy)pr opionic acid (I), o-cresol (II), 2-(5-chloro-2 -hydroxy-3-methylphenyl)prop ionic acid (III) and 4-chloro-a-cresol (IV). When the anionic form of mecop rop was irradiated between 254nm and 310nm (UV-C or UV-B), I was the main p hotoproduct. At 254nm its formation initially accounted for more than 80% o f the transformation. It has not previously been reported in the literature . The reaction results from a heterolytic photohydrolysis. Product II accou nted for only a low percentage of the transformation. The stoichiometry was different with the molecular form: the main photoproduct, III, resulted fr om a rearrangement after a homolytic scission. Products I, II and IV were a lso formed as minor photoproducts; Some other minor photoproducts were also identified. In contrast, IV was the main photoproduct under sunlight irrad iation or when solutions were irradiated in near-UV light (UV-A). This wave length effect is attributed to the involvement of an induced phototransform ation; IV is also the main photoproduct when the phototransformation is ind uced by Fe(III) perchlorate or nitrite ions. In usual environmental conditi ons the excitation of the molecular form is negligible and the phototransfo rmation is mainly due to induced photoreactions. (C) 2000 Society of Chemic al Industry.