Homolytic pathways to aromatic bromo-compounds

Abstract

The reactions of some polybromoanilines with pentyl nitrite in benzene at 80° has been studied. 2,3,4,5,6-Pentabromobiphenyl was prepared by this method and also by the Ullmann reaction between iodobenzene, pentabromoiodobenzene, and copper powder. The yield of biaryl formed from homolytic arylation of benzene decreases as o-bromoaniline, 2,4,6-tribromoaniline or pentabromoaniline are the sources of the aryl radicals. This decrease is paralleled by an increase in the yield of the benzene derivative arising from the formal protodeamination of the parent aniline derivative. Evidence was also found of a heterolytic reaction accompanying the main homolytic arylation reaction which gives hexabromobenzene as a side-product in the reaction of pentabromoaniline and pentyl nitrite in benzene.

Photochemical bromination reactions of some benzene derivatives, using molecular bromine as the source of bromine atoms, were performed. Both nuclear addition and nuclear substitution products were formed, the relative amounts of which vary with the benzene derivative since both polar and steric factors influence these reactions. However, it was found that the ratio of nuclear substitution to nuclear addition increased with increase in temperature from ca. 25 to ca. 50°. Evidence (i.e. the photochemical decomposition of benzene hexabromide and of 1,2,3,4,5,6-hexabromochlorocyclohexane to give some di-halo-arenes) was adduced which suggests an addition-elimination mechanism as an interpretation of the formation of bromo-arenes in the photochemical bromination of benzene derivatives. Direct abstraction of hydrogen from a species such as C₆H₆Br· seems unlikely by Br ₂ on thermochemical grounds, but possible by Br· or by a photochemically excited species Br₂*. Partial rate factors for free radical bromination of biphenyl have been determined and discussed, although it is unlikely that they are capable of simple interpretation. The isomer distribution of bromo-arenes found here was quite different from the corresponding ratio reported for heterolytic reactions. The meta-positions of certain benzene derivatives, which are deactivated towards electrophiles in ionic reactions, were found to be readily attacked by bromine atoms.