Process for the preparation of polychloro-fluoro-cyclo-pentenes

A process for the preparation of polychloro-fluoro-cyclopentenes having from 4 to 6 fluoro atoms by reaction of halogen-containing cyclic compounds having a 5-membered ring with hydrogen fluoride in the gas phase over a catalyst selected from the group consisting of catalysts wherein the essential active compound is a bismuth, lanthanide, or chromium containing compound, wherein hexachlorocyclopentadiene is employed as a starting material, hydrogen fluoride is present in an amount from about 7-40 mol per mol of halogen-containing cyclic compound, the reaction is carried out in the presence of from about 0.1-2 mol of Cl.sub.2 per mol of starting material, and at a temperature in the range of from 300.degree. to 450.degree. C.