Photochemical bromination of simple arenes

Abstract

Photochemical bromination of benzene, fluorobenzene, chlorobenzene, t-butylbenzene, α,α,α-trifluorotoluene, and (in tetrachloromethane) biphenyl and naphthalene gives substitution products and adducts such as 1,2,3,4,5,6-hexabromocyclohexane (1). The decomposition of (1) and of the analogous chlorobenzene adduct (3) under photochemical conditions gives the parent arene, the mono-brominated halogenobenzene, and bromine which may be scavenged by toluene to give benzyl bromide or by benzene to give bromobenzene and dibromobenzenes. Addition is a kinetically controlled process, so that the mechanism of formation of these aryl bromides must be largely through the reversible formation of these adducts. This is consistent with the unusual orientation of apparent attack by bromine upon the arene substrates, since the relative amounts of the isomeric aryl bromides is a consequence of the relative stabilities and ease of elimination of HBr and Br₂ from a family of adducts.

The range of isomer distribution found within the reaction of each arene with bromine is consistent with two competing processes involved in the formation of the aryl bromides; one of these might be the, direct homolytic substitution by bromine atoms upon the arene.