Photolysis of Diuron

The major photoproducts observed in the photolysis of diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] (2) in aqueous solution resulted from a heterolytic substitution of chlorine by OH (photohydrolysis). A wavelength effect was observed: at 254 nm the formation of 3-(4-chloro-3-hydroxyphenyl)-1,1-dimethylurea (3) accounted for more than 90% of the conversion, whereas when the solution was irradiated in ‘black light’ (85% of photons emitted at 365 nm, about 7% at 334 nm), the major photoproduct was 3-(3-chloro-4-hydroxyphenyl)-1,1-dimethylurea (4). The presence of methanol favoured the photoreduction into 3-(3-chlorophenyl)-1,1-dimethylurea (5). Completely different reactions were observed when 2 was irradiated in dry aerobic conditions on silica. They resulted from elimination or oxidation of methyl groups. The main photoproducts initially formed were 3-(3,4-dichlorophenyl)-1-methyl urea (6) and 3-(3,4-dichlorophenyl)-1-formyl-1-methylurea (7). In the second stage (6) was transformed into (3,4-dichlorophenyl)-urea (8) and 3-(3,4-dichlorophenyl)-1-formylurea (9); some other minor products such as monuron (1) were also identified. The formation rate of 6 and 7 was much slower on clay (montmorillonite or kaolin) than on silica. In contrast with products 6 and 8, the formation of 7 and 9 needed the presence of oxygen: they did not appear when diuron was irradiated in deoxygenated C2Cl3F3. It can be concluded that the photolysis of diuron is highly dependent on the conditions of irradiation.