High-Temperature Reactions of Hexafluorobenzene

The direct replacement of aromatic fluorine in hexafluorobenzene has hitherto been possible only by the use of nucleophilic reagents. In this investigation, the replacement of nuclear fluorine by nonnucleophilic, or weakly nucleophilic, reagents was achieved by reaction at relatively high temperatures, 300 to 850 °C. For example, the reaction of hexafluorobenzene with such reagents as bromine, chlorine, and tetrafluoroethylene gave as major products bromopentafluorobenzene, chloropentafluorobenzene, and octafluorotoluene. In addition, pentafluorohalobenzenes can also be produced by passage of hexafluorobenzene over the appropriate alkali or alkaline earth-metal halides at elevated temperatures. The mechanism of the pyrolytic reactions of hexafluorobenzene and the nonionic coreactants are considered to involve free-radical intermediates. The reaction of hexafluorobenzene with the ionic coreactants may proceed by an ionic mechanism similar to that advanced for the usual, relatively low-temperature, nucleophilic substitution reactions of aromatic systems. However, a more complicated free radical-ionic process cannot be ruled out for these reactions.