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United States Patent Office 3,251,390 United States Patent Office Patented May 17, 1966 2 That the pyrolysis reactions which are the subject of 3,251,890 PRODUCTION OF HEXAFLUOROBENZENE the present invention could be carried out was wholly Robert Neville Haszeldine, “Windyridge, Lyme Road, Surprising in that it was to be expected that pyrolysis of Disley, England, and John Michael Birchall, Flat 3, a fluoro- or fluorochloro olefin would yield either a 77 Palatine Road, Withington, Manchester 20, England cyclobutane, or an acyclic compound, likely a dimer, No Drawing. Filed Oct. 30, 1961, Ser. No. 149,818 trimer or tetramer. Thus, for example, it is well known Claims priority, application Great Britain, Nov. 17, 1959, that the pyrolysis of CF:CFC1 yields either the cyclo 38,961/59 butane 4 Claims. (C. 260-650) 0. This application is a continuation-in-part of our co CF-CFCQF-GFCI pending application Serial No. 37,842, filed June 22, 1960, now abandoned. or the dimer CFC1 CFC-CF: CF. Again, the fluoro This invention relates to a new chemical process and chloro olefin CFCFCFCI on pyrolysis gives a linear in particular to a new process for the preparation of dimer, trimer or tetramer. Yet again, perfluoroethylene hexafluorobenzene. when pyrolysed at 200 C. yields perfluorocyclobutane Hexafluorobenzene is of great interest in organic and at 500 C. gives perfluoroisobutylene and also per fluorine chemistry as it is analogous to benzene in class fluoropropene. ical aromatic organic chemistry. It is valuable as an in One Way of conducting the pyrolysis reaction is to pass termediate in the preparation of pharmaceutical com 20 a vapor stream of the starting compound through a heated pounds and of polymers, such as the polyethers derived tube in the absence of moisture, air or oxygen. An inert from pentafluorophenol which have excellent thermal and diluent gas such as nitrogen may be used with the vapor. chemical stability, and the polyesters of the perfluoro The products of the reaction are then condensed in cooled phthalic acids, especially the fluorine analogues of com traps and hexafluorobenzene is recovered by distillation, pounds important as raw materials for the production 25 vapor phase chromatography or other conventional puri of artificial fibers. - fication techniques. It can also be converted to potassium pentafluoro Alternatively a batch technique may be used as by phenate, CFOK by treatment with caustic potash in heating the starting material in an autoclave. tert-butanol. This compound, which is a powerful fungi The pyrolysis reactions of the invention can be carried cide, can be reacted with hexafluorobenzene to form the out using either batch or flow techniques at tempera halogenated diphenyl ethers C6F5" O'C6F5 and tures ranging from about 400 to about 1000 C. Pres Sures are not critical and may range from 0.001 to 1000 CF. OCF. O'CF5 atmospheres, absolute. Reaction times may be from say which are useful as heat exchange media and transformer one second to several hours. oils. When the starting material is an ethylene of the class Hitherto the most favorable method preparing hexa defined the temperature is normally in the range from fluorobenzene has involved either (a) the reaction of 400 to 800° C., preferably from 500 to 700° C. The benzene with cobalt trifluoride to give CFH which pressure may be normally from say 0.001 to 100 atmos compound is then subjected to dehydrofluorination or (b) pheres and the reaction time from 0.4 minute to one hour. the pyrolysis of fluorotribromomethane according to the 40 When the starting material is dichlorofluoromethane reaction 6CFBr3)CF6-9Br2. Method (a) is tedious the temperature is normally from say 450° C. to 1000 C., and expensive and the yields are low. Method (b), preferably from 550° C. to 800° C. for a flow process and while giving satisfactory yields, involves the liberation of 450 C. to 900 C. for a batch process. The pressure a large amount of bromine which must be recovered. is normally from say 0.01 atmosphere to 1000 atmos There is, therefore, a need for an improved route to hexa pheres, preferably 0.1 to 100 atmospheres. Contact time fluorobenzene. is on the order of 1 second to 10 minutes, preferably 10 It has now been discovered that hexafluorobenzene can seconds to 5 minutes for a flow process or 15 to say 200 i. be prepared in a more economical manner by a method minutes for a batch process. The reaction is preferably which involves the pyrolysis of trifluoroethylene, 1,2- carried out in apparatus constructed of inert material, difluorochloroethylene, 1,2-difluorobromoethylene, or di 50 e.g. platinum or carbon, although nickel or stainless steel chlorofluoromethane. The first three pyrolysis reactions can be used, particularly for 1,2-difluorochloroethylene can be represented as follows: and 1,2-difluorobroethylene. In repeated experiments involving the pyrolysis of these compounds in a platinum lined nickel tube no attack on the platinum or the oc where X is fluorine, chlorine, or bromine, but preferably 55 currence of carbonisation has been observed. In the chlorine or bromine. pyrolysis of dichlorofluoromethane any of the above ina The course of the pyrolysis of dichlorofluoromethane terials, monel, inconel or even iron may be used. to form CFs is not fully known. However, it is con For any particular pyrolysis apparatus, using a flow sidered possible that CHFC undergoes coupling to technique, optimum yields of hexafluorobenzene may be form CHFC-CHFC which then loses HCl to form Secured by appropriate correlation of the factors of the CHFFCFCI. In the production of CFs from the last flow rate, temperature and pressure. Appropriate cor compound, perfluoroacetylene, C2F2, may be an inter relation of these factors may, of course, be readily mediate. achieved by a few routine tests and satisfactory yields of The invention thus comprises a method for the prep hexafluorobenzene thereby obtained. aration of perfluorobenzene which comprises subjecting 65 Apart from the fact that satisfactory yields of hexa a compound selected from the group consisting of di fluorobenzene can be obtained in the pyrolysis reaction of chlorofluoromethane and compounds having the general the invention, another advantage of the method resides formula in the use as starting materials, of the simple halogenated CHF-scCFX methane CHFC and ethylenes, CHF=CFC and 70 CHF-CFBr, which can be readily prepared. Thus, for where X is selected from the group consisting of fluorine, example, the aforementioned ethylene may be prepared chlorine and bromine, to pyrolysis. by any of the methods represented below. 3,251,890 3 4. 1(a) Hydrogen Zinc dust (160 g.) and ethanol (110 ml.). The reaction CFX': CFX -- HY halide CHFX.CFX/Y proceeds Smoothly and on condensation and distillation -----as' of the volatile product, 31 g, of a fraction boiling in the addition temperature range -15 C. to -12 C. are obtained and (b) CHFX'. CFXY dehalogenation CHF:CFX this is identified as chloro-1,2-difluoroethylene. (c) The pyrolysis of this compound CHF=CFC), is where, in the above formula, when X' represents chlorine, then carried out to give hexafluorobenzene. 28 g. of Y represents chlorine or bromine and when X" repre chloro-1,2-difluoroethylene are passed through platinum Sents bromine, Y also represents bromine. The hydrogen lined nickel tube maintained at 650 C., at such a rate halide addition step in the above synthesis may be carried that the contact time with the tube is 0.4 minute. The out at a temperature in the range from 20-250° C. and O products are condensed in cooled traps and distilled to a pressure in the range from 1-100 atmospheres. Ultra give a number of fractions including one boiling in the violet light is often a convenient means for facilitating temperature range 87.5-81.1 C., in which the presence of the addition of HBr to the olefin. The dehalogenation hexafluorobenzene was detected by infra-red spectroscopy. step may be carried out at a temperature in the range Gas-liquid chromatography indicates that this compound from 20-100° C. and a pressure in the range from 1-100 is the major component of the fraction. A yield of atmospheres in the presence of a halogen acceptor such 6.7% is estimated. as zinc, iron, magnesium or sodium amalgam. Prefera A series of pyrolysis experiments are carried out under bly, a dehalogenation reaction is carried out in the pres similar conditions to those in (c) above but with different ence of a solvent, e.g. an alcohol such as ethanol. 20 reaction variables. The results of these experiments are 20a) CFX'.CFX' hydrogenation tabulated below. - CFIFX'.CEFX' where both X' atoms are either chlorine or bromine. Chromato Wt. of CECIF Tempera- Contact Conver- graphic Such a hydrogenation reaction may, for example, be Pyrolysed, g. ture, C. time, sion, Estimation of carried out in an autoclave at a temperature in the range 25 min. percent Yield (C6F), from 20-200 C. under a pressure in the range from 5-100 percent atmospheres and in the presence of a hologenation 650 0.4 35 6-7 catalyst. 620 ... 1 55 7-8 590 2.3 56 14.2 (b) CEIFX'. CHFX' dehydrohalogenation CFX'-CHF 600 2.3 6. 17.2 -HY 620 2.3 71. 10.6 such a dehydrohalogenation can be effected by use of an 600 2.3 62-------------- alcoholic alkali such as ethanolic KOH, or by use of a 600 2.3 61 13, 6 tertiary organic base such as quinoline. 3(a) halogenation Example 2 CHF:CHF CHFX'. CHFX Bromo-1,2-difluoroethylene, CHF=CFBr, is pyrolysed dehydrohalo under conditions similar to those used for chloro-1,2-di (b) genation fluoroethylene, CHF=CFCI, in Example 1.
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