Polyhalogenoallenes. Part V. Perfluoropenta-1,2-diene

Abstract

Perfluoropenta-1,2-diene can be obtained by vigorous dehydrohalogenation with alkali-metal hydroxides of 2H,2H-3-chloro-octafluoro-1-iodopentane, 2H-3-chloro-octafluoropent-1-ene, or 2H,2H-nonafluoro-1-iodopentane. The second precursor is prepared by mild basic dehydroiodination of the first, which is synthesised from 1,1-difluoroethylene and 1-chlorohexafluoro-1-iodopropane, obtained, together with a small amount of the isomeric 2-chlorohexafluoro-1-iodopropane, by photolysis of a mixture of trifluoroiodomethane and chlorotrifluoroethylene. Pyrolysis of 2H,2H-3-chloro-octafluoro-1-iodopentane over activated charcoal yields 2H-3-chloro-octafluoropent-1-ene, trans-2H-1-chloro-octafluoropent-2-ene, and cis- and trans-2H-nonafluoropent-2-ene; similar treatment of 2H-3-chloro-octafluoropent-1-ene gives the same products. At 20°, perfluoropenta-1,2-diene dimerises rapidly to perfluoro-(1,2-diethyl-3,4-dimethylenecyclobutane), perfluoro-(1-ethyl-2-methylene-3-propylidene-cyclobutane), and unidentified compounds.

One of the by-products of the conversion of 2H,2H-3-chloro-octafluoro-1-iodopentane or 2H-3-chloro-octafluoropent-1-ene into perfluoropenta-1,2-diene with molten alkali-metal hydroxides is believed to be 3-chloroheptafluoropenta-1,2-diene.