Valence-bond isomer chemistry. Part I. The valence-bond isomers of hexakis(trifluoromethyl)- and hexakis(pentafluoroethyl)-benzenes

Abstract

U.v.-irradiation of hexakis(trifluoromethyl)- and hexakis(pentafluoroethyl)-benzenes produces their valence-bond isomers. Hexakis(trifluoromethyl)benzene, in perfluoro-n-pentane solution, gives exclusively its benzvalene isomer with light of wavelength >270 nm.; with light of wavelength >200 nm., all three possible valence-bond isomers are produced, the benzvalene and para-bonded benzene isomers being formed reversibly and the prismane isomer essentially irreversibly in a process via the para-bonded benzene isomer. In contrast, hexakis(pentafluoroethyl)benzene gives only the para-bonded benzene with light of wavelength >270 nm., and the prismane isomer is also formed, as the major product, with light of wavelength >200 nm. These valence-bond isomers show very substantial thermal stability, with half-lives at 170° in hexafluorobenzene solution ranging from 135 hr. for the para-bonded isomer of hexakis(trifluoromethyl)benzene to 9 hr. for its benzvalene isomer. The benzvalene and para-bonded benzene isomers produce the parent benzenes, whereas the corresponding para-bonded benzene isomers are formed from the prismanes. The physical properties and the modes of formation of these valence-bond isomers are discussed.