Saturated Fluorocarbons, Fluorocarbon Hydrides, and Fluorocarbon Halides

1 Fluorocarbons

Two new reviews of the physical properties and associated applications of saturated fluorocarbons have been published. One of these concentrates on outlets for such materials in the electronics industry, while the other seems to have been inspired by current interest in medical and biological circles regarding the use of fluorocarbons and fluorocarbon ethers or amines in blood substitutes (see also p. 98). The use of fluorocarbons as working fluids for heat engines has also been reviewed.

A direct calorimetric determination of the heat of reaction of a poly(carbon monofluoride) of stoicheiometry CF1.12 with fluorine to yield carbon tetrafluoride has enabled the values - 46.7 [plus or minus] 1.0 kcal mol-1 and ~ 115 kcal mol-1 to be calculated for the heat of formation of [CFl.12]n and the C-F bond energy in this material, re~pectively.~ Using this heat of formation and those of polytetrafluoroethylene, perfluoro-n-heptane, hexafluoroethane, and carbon tetrafluoride, it has been found that the heat of formation of 'saturated' carbon fluorides can be expressed by the equation ΔHt°(CFx) = - (44x + 3x2) [plus or minus] 3 kcal mo1- l. The results of an assessment of poly(carbon monofluoride) as a solid lubricant have become available.

More thorough investigation of the 1740 A xenon-sensitized photolysis of perfluorocyclobutane has provided no evidence for the formation of xenon difluoride and shown that the organic products include the fluorocarbons CF4, C2F4, C2F6, C3F6, C3F8, C2F5 • CF:CF,, and trans-CF3•CF:CF•CF3 (the main product) and polytetrafluoroethylene.6 A detailed account has been published of gas-phase pyrolysis of perfluorocyclopropane (-> C2F4 [plus or minus] :CF2), perfluorocyclobutane (->2C2F4), and perfluorocyclohexane (- >C2F4, C3F6, cyclo-C4F6, cyclo-C4F8) in single-pulse shock tubes.

No convincing evidence has been found for insertion of carbon-11 from nuclear recoil reactions into C-F bonds of saturated fluorocarbons, in contrast to the situation with alkanes, for which C-H insertion is the most prominent reaction. Recoil 18F attacks perfluorocycloalkanes to yield products derived from F-for-F substitution, e.g.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

(*translational energy, [dagger]vibrational excitation) and, preferentially , C-C fission (l8F* + cyclo-CnF2n -> n-CnF2n18F)

The results of further studies on fluorocarbons, particularly perfluorocyclohexane, as electron scavengers during y-radiolysis of hydrocarbons have been published. γ-Irradiation of solid perfluorocyclohexane at 77 K has been shown by e.s.r. spectroscopy to produce mainly perfluorocyclohexyl radicals ; the spectrum given by perfluoro(methylcyc1ohexane) under the same conditions was not identified, although above 160-170 K the radical [FORMULA NOT REPRODUCIBLE IN ASCII]• was definitely observed. A microwave conductivity detector has been used to follow electron disappearance in pulse-irradiated (2.8 MeV electrons from a Van de Graaff accelerator) gaseous C1 — C4 perfluoro-n-alkanes and related compounds (CF3Cl, CHF3, CH2F2, CH3F, CF3•CH3, C2H5F, C2F4), and radiation damage in orientated polytetrafluoroethylene caused by a pulsed electron beam has been investigated by means of e.s.r. Regarding the effect of radiation on fluorocarbon polymers, it is interesting to note that Mariner IV, in which the thermal shields contained Teflon FEP film and all interconnecting wire and cable was insulated with Teflon, performed flawlessly throughout its 7½ month flight to photograph Mars.

Two interesting variations of Simons' electrochemical fluorination technique for the preparation of fluorocarbon materials are under investigation. One involves electrolysis of solutions of inorganic fluorides in anhydrous organic solvents using nickel anodes [FORMULA NOT REPRODUCIBLE IN ASCII] and the other involves the introduction of gaseous alkanes, alkenes, or alkyl halides (see p. 14) into a porous carbon anode of a medium-temperature fluorine generator (72-95 °C, electrolyte KF,2HF) (e.g. [FORMULA NOT REPRODUCIBLE IN ASCII]). Electrolysis of a solution of potassium bifluoride in acetic acid with platinum electrodes yields no fluorinated product, but similar electrolysis of a solution of naphthalene in acetonitrile containing triethylamine hydrofluoride apparently gives α-fluoronaphthalene (4 — 5 % yield) and other unspecified fluorinated The mechanism shown in Scheme 1 has been proposed to account for the electrochemical fluorination of acetic acid-KHF2 at a nickel anode.lS Kolbe electrolysis of heptafluoro-n-butyric acid has been shown to give perfluoro-n-hexane in ca. 20% yield.

Skeletal rearrangements have been found to occur during the exhaustive fluorination of bicyclo [2,2,2]octane and bicyclo [3,2,1 ]octane at 320 °C with cobalt trifluoride; thus, both these bridged systems yield a mixture of the perfluorobicyclo-octanes (l), (2), and (3) in the ratio of ca. 60:30:10 and cu. 50:40:10, respectively. By contrast, similar fluorination of bicyclo [3,3,0]octane gives only its fluorocarbon counterpart (1). It has been pointed out that these results can be rationalized on the basis of the generation of carbonium ions early on in the reactions via attack of cobalt trifluoride on starting material or intermediates of low fluorine content. Skeletal rearrangements do not appear to occur during CoF3-fluorination of polyfluorobicycIo[2,2,2]octenes (e.g. see p. 9, in keeping with the difficulty of gaining access to cationic species in polyfluorinated systems.

2 Hydrides

Further examination of the complex mixture obtained by vapour-phase fluorination of benzene with cobalt trifluoride at 150 — 200 °C has resulted in the isolation and identification of three hitherto-unknown minor constituents : a 1H,2H,3H,5H-octafluorocyclohexane, 1H,2H,4H/3H-octafluorocyclohexane, and 1H,2H:4H,5H/-hexafluorocyclohexene. As part of this work, all six possible 1H,2H,3H,4H-octafluorocyclohexanes were synthesized via photochemical addition of chlorine to 3H,4H/- and 3H/4H-octafluorocyclohexene followed by reduction of the resultant dichloro-octafluorocyclohexanes with lithium aluminium hydride. Fluorination of benzene at 280 °C with potassium tetrafluorocobaltate(III) [prepared from fluorine and potassium...