cis-1, 2, 3, 4, 5, 5-Hexafluorobicyclo [2.1. 0] pentane and 1, 2, 4, 5-tetrafluorobicyclo [2.1. 0] pentane have been synthesized from hexafluorobenzene. The former hydrofluorocarbon, which exists entirely in the endo configuration, rearranges to cis-1, 2, 3, 3, 4, 5- hexafluorocyclopentene below room temperature (Ea= 21.9 kcal/mol, logA= 13.4). The latter undergoes degenerate ring inversion with extraordinary ease (ΔG‡= 6.8±0.2 kcal/mol at− ...