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.