Phototransformation of halogenoaromatic derivatives in aqueous solution

Abstract

Abstract. The photochemical behaviour of monohalogeno-phenols and -anilines is highly dependent on the position of the halogen on the ring, but most often it is not significantly influenced by the nature of the halogen (Cl, Br, F). Photohydrolysis is the main reaction observed with 3-halogenated and it is almost specific. With 2-halogenated, photohydrolysis and photocontraction of the ring compete, the latter being very efficient with 2-halogeno-phenolates.The photochemical behaviour of 4-halogeno-phenols and -anilines is more complex. It depends on both concentration and oxygenation. It was rationalized when it was experimentally proved by laser flash photolysis that the first step is the formation of a carbene (in the cationic form in the case of 4-halogenoaniline). p-Benzoquinone is the product of photooxidation of 4-halogenophenols, whereas the photooxidation of 4-chloroaniline leads to 4-amino-4 -chlorodiphenylamine.Chlorohydroquinone behaves differently, the formation of the main photoproducts involving a radical mechanism.With dihalogenoanilines photohydrolysis is initially almost quantitative whatever the position of halogen atom on the ring. It occurs more easily in meta than in ortho position and in ortho than in para position. In the case of 2,4-and 2,6-dichloroanilines aminophenoxazones are formed as secondary photoproducts. This reaction involves the intermediate formation of o-benzoquinone monoimine.Several halogenoaromatic herbicides were also studied (bromoxynil, chlorpropham, propanil, diuron, linuron, chlorbromuron, MCPA). The most frequent reaction is photohydrolysis. However some other reactions such as photoreduction and photo-demethoxylation were observed. In some cases a wavelength effect was observed particularly with MCPA and halogenophenylureas.

Isotope Fractionation Associated with the Photochemical Dechlorination of Chloroanilines

“…38,39,41 Formation of transient, dechlorinated species (3) is speculative due to the low quantum yield or short lifetime of excited state species. 39 Detection of 2-aminophenol and 2-aminophenoxazin-3-one suggest that pathway (i) was operational in our experiments.…”

Photochemical Behaviour of Carbamates Structurally Related to Herbicides in Aqueous Media: Nucleophilic Solvent Trapping versus Radical Reactions

“…Despite the presence of the O-aryl moiety, the photoinduced carbamate cleavage was not observed and only dehalogenated products, phenols 2a,b, and acetamides 3a,b, were isolated. Photohydrolysis to give phenols is the most frequent photoreaction of halogenated aromatic compounds in water and is reported to occur by a photoionization process, favoured by the medium, [18,19]. Accordingly, phenols 2a,b should derive from the radical cation 4 that can be trapped by water (pathway a, Figure 4) [18,19].…”

“…Photohydrolysis to give phenols is the most frequent photoreaction of halogenated aromatic compounds in water and is reported to occur by a photoionization process, favoured by the medium, [18,19]. Accordingly, phenols 2a,b should derive from the radical cation 4 that can be trapped by water (pathway a, Figure 4) [18,19]. The attack on the radical cation 4 by nitrogen of acetonitrile and subsequent hydrolysis should give compounds 3a,b (pathway b, Figure 4).…”

Direct photolysis mechanism of pesticides in water

“…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.…”

“…2-Chlorinated phenol and aniline, possible photoproducts of some pesticides, produced cyclopentadiene derivatives in their aqueous photolysis, and ketene was proposed as the key intermediate for 2-chlorophenol, based on LFP. 23) The aqueous photolysis of fenarimol produced a benzophenone derivative with the pyrimidyl ring migrating to one phenyl ring.…”