Phototransformation of halogenoaromatic derivatives in aqueous solution

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 2,4and 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.