The behaviour of perchloro-3,4-dimethylenecyclobutene,-pentafulvene and- pentafulv-alene under electron-impact

The mass spectra of 1,2-dichloro-3,4-bis(dichloromethylene)cyclobutene (IV) and of hexachloropentafulvene (II) have been studied. Compound IV cannot be an intermediate in the formation of II from octachloro-1,2-dimethylenecyclobutane (III) under electron-impact, as previously suggested. In the mass spectra of II and IV the species C₆]⁺ and C₅]⁺ occur, obviously through cleavage of the semicyclic C-C bond. The mass spectrum of pentachlorofulvalene (VI) shows strikingly that successive elimination of an even number of CI atoms is preferred over that of an odd number of CI atoms; probably corresponding C-CI bonds in the two rings are broken simultaneously. Amongst the fragments, the species C₁₀]⁺ and C₇]⁺ and possibly also C₈]⁺ and C₉]⁺ have been observed.