Photodehalogenation, aryl halides

The reductive photodehalogenation of aryl halides has been actively investigated in recent years. Special attention has been given to (poly)halobenzenes and (poly)halo-biphenyls. The reactions are of interest in view of their mechanisms, and because of the importance of chlorinated aromatic hydrocarbons as environmental pollutants and the possibility of their photoinduced degradation. The photochemistry of aryl halides and related compounds in general14 and the photochemistry of polyhaloarenes in particular18 have been reviewed. 

In view of the many reports which apptear each year on photodehalogenation of aryl halides, it is noteworthy that photochemistry is also used to synthesize such arenes. Thus Williams and co-workers describe the photobromination of benzene and halogenobenzenes to give polyhalogenocyclohexanes which themselves are photodecomposed to bromobenzene, dibromobenzenes, and bromochlorobenzenes. The same workers also report that the photobromination of biphenyl in carbon tetrachloride gives the 2-, 3-, and 4-bromoarenes in respective yields of 24, 27, and 49%. 

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