Photochemistry of 4-chloroaniline in solution. Formation and kinetic properties of a new carbene, 4-iminocyclohexa-2,5-dienylidene

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 1-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 Fl atom from aliphatic alcohols, giving an anilino radical cation, to add bromide, giving 4-bromoaniline quantitatively, and to react with H2O 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,N-dimethylaniline 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 il 4-chlorophenol, and this was traced to the fact that the iminoquinone oxide is able to add to 4-chloroaniline.