Phototransformation of metoxuron [3‐(3‐chloro‐4‐methoxyphenyl)‐1,1‐dimethylurea] in aqueous solution

Abstract

UV irradiation of metoxuron in aerated aqueous solution at 254 nm or between 300 and 450 nm led initially to an almost specific photohydrolysis of the C-Cl bond, resulting in the formation of 3-(3-hydroxy-4-methoxyphenyl)-1,1-dimethylurea (MX3) and hydrogen chloride. The quantum yield was determined to be 0.020 (+/- 0.005) in solutions irradiated at 254 nm. Five minor photoproducts were also identified, in particular the dihydroxydimethoxybiphenyl derivatives resulting from the phototransformation of MX3. Irradiation increased the toxicity of an aqueous solution of metoxuron to the marine bacterium Vibrio fischeri.

Direct phototransformation of aromatic pesticides in aqueous solution

“…This wavelength effect is related to the presence of NH − CO group since it is also reported with other halogenophenylureas [13] With p-halogenated phenols and anilines it is experimentally proved that the first step of the reaction is the formation of a carbene. The same type of reaction is observed with metobromuron [3-(4-bromophenyl)-1-methoxy-1-methylurea] where carbene formation accounts for about 45% of the sum of reactions of metobromuron [15]. This carbene can then be hydrolysed that explains the formation of the p-hydroxylated derivative reported by Rosen, Strusz, Nick and Schöler [16,17].…”

“…This carbene can then be hydrolysed that explains the formation of the p-hydroxylated derivative reported by Rosen, Strusz, Nick and Schöler [16,17]. In the presence of oxygen, the main transformation pathway of the carbene is its oxidation into p-benzoquinone through the intermediate formation of an unstable quinone-imine [15]. The carbene may also be reduced (in the presence of H-donor substances), or, in more concentrated solutions, lead to the formation of adducts.…”

“…It is the substitution of a halogen atom by OH involving a molecule of water. This reaction is almost quantitative with meta-halogenated phenylureas such as chlorotoluron [10] and metoxuron [11], and with meta-halogenated phenylcarbamates (chlorpropham [6], chlorbufam [10]). It is also the main pathway with many other herbicides such as bromoxynil and chlorophenoxyacetic acids in the anionic form.…”

“…N-methoxylated phenylureas such as metobromuron, linuron and chlorbromuron are commonly used as herbicides. One of the main pathways of phototransformation is the elimination of methoxy group released as formaldehyde [13,15] Demethylation and oxidation of methyl group also occur, but they are minor pathways in aqueous solution.…”

Direct photolysis mechanism of pesticides in water

“…128) In the latter case, either reductive dehalogenation or OH substitution is possible, depending on the solvent. The aqueous photolysis of chlorpropham 129) and metoxuron 130) produced the corresponding OH derivatives via heterolytic C-Cl bond cleavage. The effect of oxygen on the photolysis rate and product distribution together with the LFP study indicated the involvement of both S* and T* in the aqueous photolysis of chlorsulfuron at 280 nm, and either homolytic or heterolytic C-Cl bond cleavage was proposed.…”