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 wavelengths in order to study the reactions involved in the phototransformation. Four main photoproducts were identified: 2‐(4‐hydroxy‐2‐methylphenoxy)propionic acid ( I ), o ‐cresol ( II ), 2‐(5‐chloro‐2‐hydroxy‐3‐methylphenyl)propionic acid ( III ) and 4‐chloro‐ o ‐cresol ( IV ). When the anionic form of mecoprop was irradiated between 254 nm and 310 nm (UV‐C or UV‐B), I was the main photoproduct. At 254 nm its formation initially accounted for more than 80% of the transformation. It has not previously been reported in the literature. The reaction results from a heterolytic photohydrolysis. Product II accounted for only a low percentage of the transformation. The stoichiometry was different with the molecular form: the main photoproduct, III , resulted from a rearrangement after a homolytic scission. Products I, II and IV were also formed as minor photoproducts. Some other minor photoproducts were also identified. In contrast, IV was the main photoproduct under sunlight irradiation or when solutions were irradiated in near‐UV light (UV‐A). This wavelength effect is attributed to the involvement of an induced phototransformation; IV is also the main photoproduct when the phototransformation is induced by Fe( III ) perchlorate or nitrite ions. In usual environmental conditions the excitation of the molecular form is negligible and the phototransformation is mainly due to induced photoreactions. © 2000 Society of Chemical Industry