Direct phototransformation of aromatic pesticides in aqueous solution

Abstract

The photochemical behaviour of many aromatic pesticides (mainly herbicides) are compared and the main reactions are separated in three different classes: 1- reactions involving carbon-halogen bond; 2 - other reactions involving the aromatic ring; 3 - reactions of the aliphatic moiety. It appears that the nature of the substituents and their relative positions on the ring play a major role in the orientation of the reaction. The molecular and ionic forms of ionisable molecules may have different photochemical behaviour. A wavelength effect is observed with some compounds. The case of mecoprop [2-(4-chloro-2-methylphenoxy)-propanoic acid] is presented as an example.The irradiation of herbicides in aqueous solution may lead to the formation of photochemical intermediates more toxic to microorganisms than the initial substrate.

Nitrite and nitrate induced photodegradation of monolinuron in aqueous solution

“…1,4-benzoquinone 8 appearing exclusively in the presence of oxygen (aerated solutions) is probably produced through a carbene intermediate as has been suggested previously (Boule et al 2002). As shown by their evolution curves (Fig.…”

“…Direct and induced photolyses of substituted phenylureas were reviewed by Boule et al (2002) and Burrows et al (2002) and reported to give as main degradation pathways: (i) photohydrolysis (ii) demethylation and/or demethoxylation of the substituted N -terminus, (iii) possible rearrangement and iv) formation of benzoquinone and analogues. Ring hydroxylation occurs in general under induced (radicalar) conditions and oligomerisation may be observed at high substrate concentration.…”

Direct photolysis mechanism of pesticides in water

“…In general, the heterolytic mechanism for forming hydroxylated derivatives is likely for 2-or 3-halogenated phenol, aniline and urea, while the formation of carbene has been proposed for 4-halogenoaromatics. 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.…”

“…149) In the aqueous photolysis of metobromuron in a buffer with a pH of 7, reductive debromination was more favorable than the formation of a 4-OH derivative, 4) while the latter product predominated in slow sunlight photodegradation in pure water. 37) The LFP study at 266 nm indicated the formation of the corresponding carbene via CBr cleavage 23,37,128) ; therefore, the proton concentration likely affects the product distribution of these herbicides. Both 4-H and 4-OH derivatives were formed from profenophos, 150) and a similar mechanism might be operative.…”

“…129) The photo-Fries rearrangement via the cleavage of an acyl C-N bond was reported for aromatic amides, 89) alkyl Narylcarbamates, 95) and phenylurea herbicides, 128) but only under exposure to UV light at <290 nm in organic solvents. Incidentally, oxazole derivatives were formed in the aqueous photolysis of cycloxydim 4,41) and sethoxydim; 120) the Beckmann rearrangement, followed by the intramolecular reaction of carbocation with the enol oxygen, is one possible mechanism.…”