The photochemistry of 4-chloroaniline and related 4-halogenoanilines was investigated in solution by means of nanosecond transient absorption spectroscopy and steady-state photoproduct analysis. The main emphasis was the study of the mechanism of photoinduced reactions in aqueous solutions, motivated by the presence of 4-chloroaniline in the environment, following biodegradation of chlorophenylurea-based herbicides. Additional measurements were carried out in some organic solvents (aliphatic alcohols, acetonitrile, and n-hexane) in order to clarify the mechanism. The first transient detectable at nanosecond time resolution in aqueous solution was assigned to a carbene, 4-iminocyclohexa-2,5-dienylidene, produced by photoinduced heterolytic dehalogenation. The spectral and kinetic properties of this previously unknown species, as well as the products of its reactions, were studied in detail; it was found to react with O 2 , giving an iminoquinone O-oxide, to abstract an H atom from aliphatic alcohols, giving an anilino radical cation, to add bromide, giving 4-bromoaniline quantitatively, and to react with H 2 O quite slowly. This behavior, characteristic of a carbene with a triplet ground state, is fully analogous to that found in the well-studied photochemistry of 4-halogenophenols. There are a few characteristic differences in this system, however, most conspicuous being that the amino (or imino) group can accept a positive charge. For this reason, contrary to 4-chlorophenol, photolysis of 4-chloroaniline also produced the carbene in nonprotic polar solvents, such as acetonitrile. Furthermore, contrary to 4-chlorophenol and 4-chloroanisole, the reaction mechanisms of 4-chloro-N,Ndimethylaniline and 4-chloroaniline are the same. Finally, the product distribution from 4-chloraniline in the presence of O 2 was found to be different from that of 4-chlorophenol, and this was traced to the fact that the iminoquinone oxide is able to add to 4-chloroaniline.

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.