Competitive phenylation of polyfluorobenzenes

Abstract

Benzoyl peroxide was allowed to decompose at 80°C in a range of polyfluorobenzenes, (i) in the absence of additives, (ii) in the presence of ferric benzoate, and (iii) in the presence of trichloroacetic acid. The yields of polyfluorobiphenyls and polyfluorophenyl benzoates from these reactions have been measured.

Biaryls arising from displacement of fluorine as well as hydrogen were observed, phenyldehydrogenation occurring more readily than phenyldefluorination in all solvents. Ferric benzoate was found to have only a minimal effect on the yields of biaryls and trichloroacetic acid was found to have no effect. The relative rates of displacement, as shown by the isomer distributions, were found to deviate from those predicted from partial rate factors for the phenylation of fluorobenzene. The effect is more pronounced with increase fluorine substitution.

The esters were found to arise from fluorine displacement only. Their yields decreased with added ferric benzoate and increased with added trichloroacetic acid.

The identity of some of the products from the residues arising from the decomposition of benzoyl peroxide in 1,3,5-trifluorobenzene and 1,2,3,4-tetrafluorobenzene was investigated.

Competitive phenylation reactions have been carried out in binary mixtures of polyfluorobenzenes. The relative susceptibilities of each substrate were measured by the relative yields of the appropriate biaryls. The orientation of attack was unaffected by the presence of a second substrate except when hexafluorobenzene was present. Then, the selectivity of phenylation, as judged by the change of orientation, apparently increased. However, self-consistent substituent rate factors were not obtainable for any set of competition reactions. Explanations have been suggested to account for these observations.

The steam-volatile products form the thermolysis of phenyl-azotriphenyl methane (PAT) at 80°C, in hexafluorobenzene have been investigated. Some products from the decomposition of benzoyl peroxide (80°) in benzene, pentafluorobenzene and hexafluorobenzene in the presence of pentafluoronitrosobenzene have also been studied.