Saturated Fluorocarbons, Fluorocarbon Hydrides, and Fluorocarbon Halides

BY R. E. BANKS

1 Fluorocarbons

The so-called carbon monofluoride, [CFx]n, discovered about forty years ago by Ruff and his co-workers during their studies on the direct fiuorination of various forms of carbon and currently attracting attention as a lubricant and as a cathode component for high-energy batteries, can now be obtained commercially (see ref. 2). The grades available include the snow-white variety (x [plus or minus] 1), poly(carbon monofiuoride), which can be produced by direct halogenation of graphite either in a fluorine plasma (fluidized bed reactor, gas temperature < 150 °C) or at high temperature (600 °C and 1 atm) or pressure (20 °C and 15 atm); the plasma method produces material with a fluorine:carbon ratio as high as 1.19:1. Full details of refinements to the old Riidorff structure of carbon monofluoride are to be published soon.

Poly(carbon monofluoride) is a hydrophobic, electrically non-conductive, white powder, claimed to be inert towards hydrogen at 400 °C and stable indefinitely at 600 °C and for short periods at 800 °C. Carbon monofl.uoride of stoichiometry CF, however, is said to decompose suddenly at about 600 °C, giving carbon and low-molecular-weight fluorocarbons; other decompositions reported for this material include those involving hydrogen at 450 — 500 °C (-> C + HF), potassium iodide at 360 — 500 °C (-> KF + I2 + C), and potassium carbonate at ca. 310 °C (-> KF + CO2 + C), study of which has led to the value 448.5 kJ mol-1 for the C — F bond energy [the C — F bond energy in poly(carbon monofluoride) of stoichiometry CF is reported to be ca. 480 kJ mol-1]. Obviously, thermal decomposition of carbon monofluoride in the presence of suitable substrates might prove an interesting, if not useful, source of some fluorinated organic, organometallic, or organometalloidal compounds.

Full details are now available of Japanese work on the preparation (via thermal fluorination of wood charcoal, binder carbon, graphite fibre, carbon black, petroleum cokes, or natural graphite) and properties of carbon monofluoride samples with stoichiometries in the range CF0.91 &mdahs CF0.98 Kinetic data for the fluorine-carbon reaction are also to hand, as are instructions for preparing the known black 'tetracarbon monofluoride' in essentially quantitative yield via admission of fluorine to a cold (room temp.) pre fluorinated nickel or monel Parr bomb containing graphite and pressurized with hydrogen fluoride (a rapid exothermic reaction ensues). This fluorination technique produces material in the stoichiometry range from C4.05[plus or minus]0.5F : HF of 1 :0.005 to C4.35 [plus or minus]0.5F : HF of 1 :0.0037, characterized by X-ray structural parameters apparently in conflict with previous data. In situ fluorination of expendable carbon electrodes during plasma-jet melt electrolysis of calcium fluoride can be used to produce either carbon tetrafluoride and hexafluoroethane or tetrafluoroethylene, depending on whether the gaseous cell product is cooled slowly or rapidly.

Application of the low-temperature solid-substrate direct florination techniques developed at Rice University" to hexamethylethane has provided the first sample of the highly branched fluorocarbon (CF3)3 C·C(CF3) 3, albeit in only low yield (9 %); coupled fluorine combustion-gas chromatography has been investigated as a method for organic elemental analysis, carbon being determined as CF4.

Saturated fluorocarbon production via cobalt trifluoride fluorination has featured in work on (i) a new synthesis of polyperfluorocyclobutene (2) (see Scheme 1), (ii) compounds derived from photo-isomerization of perfluorocyclohexene (see Scheme 2 and p. 55),(iii) polyfluoro-n-octanes and -n-hexadecanes (see Scheme), (iv) the fluorination of alkyladamantanes, 20 and (v) work on polyfluorobicyclo[2,2,1]heptanes [see Schemes 13 (p. 15) and 15 (p. 17)]. Definite compounds were not isolated in the penultimate study (iv) [note that electrochemical fluorinations (Simons' process) were also performed, using electrolytes comprising emulsions of alkyladamantanes in hydrogen fluoride dosed with alkali-metal fluoride conductivity aids], but C, H, and F values quoted for material (m.p. -55, b.p. 205 °C) obtained by passage of nitrogen-diluted 1-ethyladamantane vapour over cobalt trifluoride at 350 °C indicated that it was essentially fluorocarbon in nature; commercial interest in such materials as chemically inert hydraulic fluids or dielectric coolants stems from their unusually wide liquid-state temperature ranges.

The Birmingham group, famed for its work with cobalt trifluoride, has at last published in full its ideas about the mechanism of fluorination of organic substrates with high-valency transition-metal fluorides. Substrate oxidation via electron or hydrogen-atom removal is believed to initiate a sequence of events that may include all of steps (a)-(e) of Scheme 4 when an alkane or cycloalkane is involved; aromatization of hydroaromatic substrates (e.g. cyclohexane or tetrahydrofuran) may take precedence over C-F bond formation, which then proceeds via quenching of radicakations by fluorine atom (the favoured mode, at present) or :fluoride ion (see Scheme 5). Applications of these basic proposals are mentioned later (see p. 260).

Radical-cations may also play a role in HF-catalysed XeF2 :fluorination of aromatic substrates (see p. 352) and in Simons-type electrochemical fluorination, e.g. see Scheme 6. However, both Italian and Russian groups working on the mechanism of electrochemical fluorination believe that anodic complexes of the type first proposed by Burdon and Tatlow are involved; the latter group, following its analysis of anode deposits and demonstration that preliminary treatment of nickel electrodes with fluorine or preliminary anodic polarization of the electrodes in anhydrous hydrogen fluoride appreciably shortens the well-known induction period, suggest that the primary act is formation of a film of nickel fluoride at the anode succeeded by the steps shown in Scheme 7.25 A new review of electrochemical fluorination \now available emphasizes commercial aspects; 27 perhaps before the next one is written the mystery still surrounding the mechanism of this old technique will have been solved.

The results of radiolysis (60Co γ-rays and neutrons) experiments on CF4-C2F8* mixtures encapsulated in aluminium [the predominant products were C3F8, i-C4F10, i-C5F12, and (CF3) CP.CF(CF33); oxygenated products, e.g. (C2F5) O, presumably arose through attack on aluminium oxide] have been quoted in support of the contribution of fluorine-abstraction reactions to the mechanisms of radiation-induced fluorocarbon transformations [for example: [FORMULA NOT REPRODUCIBLE IN ASCII] γ-Radiolysis (60Co) of the perfluoroalkanes [FORMULA NOT REPRODUCIBLE IN ASCII] and [FORMULA NOT REPRODUCIBLE IN ASCII] has been used to procure isomers of molecular formula for gas-liquid chromatographic studies; only eight of the nine possible isomers seem to have been obtained, the neo-form [FORMULA NOT REPRODUCIBLE IN ASCII] failing to appear in the product from i-C6F14

Hot-atom chemistry of a number of fluorocarbons [perfluoro-n-hexane, -cyclohexane, -(methylcyclohexane), -cyclohexene, -benzene, and -toluene] has been studied, the substrates being activated by means of the nuclear reactions 19F(n,2n)18F and 19F(γ,n)18F a large number of 18F-labelled compounds were obtained, but, except in the case of perfluoro-n-hexane(suggested displacement reactions: C6F14 + 18F· -> C6F1318F + F·; C6F14 + 18F•-> C5F11·18F + CF3·;C6F14+18F· -> C4F918F + C2F5·), most of the radioactive products were not recognized.

Publications have appeared which deal with (i) the determination of collision rate constants of perfluoroalkane ions by ion cyclotron resonance; (ii) ionization of perfluorocyclobutane by electron impact; (iii) dissociative electron capture and dissociative ionization in fluorocarbons(CF4,C2F4, C3F6,CF3·CF:CF·CF3, and cyclo-C4F8); (iv) mass-spectrometric study of the system CF4-CH4; (v) use of a time-of-flight mass spectrometer to determine the composition of the mixture (main component, C2F4) obtained by decomposition of carbon tetrafluoride in an electric discharge; (vi) negative ion-molecule reactions in perfluoropropane; (vii) fast-flow microwave discharge studies of the system CF4-CCl4; (viii) the production of cotton with enhanced water repellency but unchanged porosity by subjecting it to an electrodeless discharge in a carbon tetrafluoride atmosphere; (ix) a pulsed atomic fluorine laser employing He-CF4, He-C2F6, or He-SF6 mixtures; (x) rapid evaluation of heat-transfer characteristics of gaseous coolants for electrical appliances (data given for CF4, C2F6, C3F8, cyclo- C4F8, CF3Cl, CF2Cl2, CHF2Cl, and CHF3); (xi) electric strengths of fluorocarbon liquids [including perfiuorodecalin and perfluoro(methyldecalin)], and flammability limits for hydrogen-oxygen mixtures containing CF4, C3F8, cyclo- C4F8, or CF3Br as diluent; (xii) fluorocarbons as solvents for studies in the vacuum-u.v. region [the usefulness of perfluoro-n-hexane and perfluoro(methyl-cyclohexane) extends to 151.0 and 154.0 nm, respectively, in a 25 µm cell with CaF2 windows]; (xiii) calculations of the entropies of vaporization of twelve saturated hydrocarbons and the corresponding fluorocarbons [FORMULA NOT REPRODUCIBLE IN ASCII]; and CF2·(CF2)3CF2 according to the 'Hildebrand rule' the entropy of vaporization of each fluorocarbon exceeds that of its hydrocarbon analogue by ca. 4 kJ K-1 mol-1 [as anticipated from the earlier work on the pair n-C7F16/n-C7H16 (difference, 6.7 kJ K-1 mol-1)], probably owing to the smaller free volume of a fluorocarbon}; (xiv) a new treatment for the estimation and correlation of solubilities [includes data for solubilities of iodine in perfluoro (methylcyclohexane), perfluoro-n-heptane, perftuorotributylamine, and 1,1,2-trichlorotrifluoroethane]; (xv) the critical constants of binary mixtures of perfluoro-compounds with alkanes [C3H8 with [FORMULA NOT REPRODUCIBLE IN ASCII]or [FORMULA NOT REPRODUCIBLE IN ASCII]with n-C6F14 or C6F6; and n-C3H7·CH:CH2 with [FORMULA NOT REPRODUCIBLE IN ASCII] (xvi) the removal of olefinic impurities from saturated fluorocarbons by passing their vapours over hot Al2O3-MOH (M = alkali metal) catalysts; (xvii) uncoupling of mono-oxygenation and electron transport by perfluoro-n-hexane in liver microsomes;(xviii) the possibility of degradation of perfluoro-n-pentane by 'nascent' iron, i.e. a clean Fe film produced by evaporation of the metal in vacuo (evidence for the occurrence of a reaction at 10-9 Torr and ambient temperature apparently rests solely on differences in the mass spectrum of a sample of n-C5F12 before and after contact with the iron); (xix) the conversion of polyftuoroalkanoyl fluorides into alkanes via decarbonylation with antimony pentafluoride [e.g. C2F5·COF at 125°C gives C2F6 and traces of (CF3)2C0]; (xx) the electronic structure of carbon tetrafluoride; (xxi) the crystal structure of carbon tetraftuoride; and (xxii) MO calculations for the hypothetical ion CF62-.

The molecular structures of perfluoro-cyclopropane and -cyclobutane have been determined by electron diffraction [geometrical parameters: (i) for [FORMULA NOT REPRODUCIBLE IN ASCII], dihedral angle = 17.4°, tilt angle for CF2 = -5.4°]; in each perfluorocycloalkane the C — F bond length is in close agreement with the values reported for carbon tetrafluoride (132.3 [plus or minus] 0.5 pm) and hexafluoroethane (132 [plus or minus] 1 pm), and the C — C bond length is fractionally shorter than that in the corresponding hydrocarbon. Consideration of the FCF angle in perfluorocyclopropane has led to the conclusion that the carbon hybrid atomic orbitals involved in the C — F and C — C bonds are sp2.65 and sp3.42 in nature, respectively (cf. cyclopropane: [FORMULA REPRODUCIBLE IN ASCII]

2 Hydrides

Kinetic and mechanistic aspects of direct gas-phase fluorination of methane, its fluorides, and related compounds have received further attention. Gas-phase photodifluoroamination of methyl fluoride with tetrafluoro- hydrazine has been shown to yield NN-difluorofluoromethylamine (mainly) and 1,2-difluoroethane [FORMULA NOT REPRODUCIBLE IN ASCII] but no cyanogen fluoride [FORMULA NOT REPRODUCIBLE IN ASCII]; cf. the unimolecular elimination of HF from vibrationally excited MeNF2 during photodifluoroamination of methane), possibly owing to the capacity of a C — F bond to store excess vibrational energy. The results of a kinetic study of the photochlorination of 2,2-difluoropropane dissolved in carbon tetrachloride [products: [FORMULA NOT REPRODUCIBLE IN ASCII] and [FORMULA NOT REPRODUCIBLE IN ASCII] have been discussed in terms of inductive, resonance, and steric effects. Hydrogen-atom abstraction from fluorocarbon hydrides by fluoro-sulphonyloxyl radicals has been investigated (see p. 256).

Details have been published of the preparation of: (i) 1H-tridecafluoro-n-hexane via decarbonylation of the alkanoyl fluoride CHF2·(CF2)5;·COF with antimony pentafluoride at elevated temperatures; (ii) fluoroethanes via electrofluorination ['medium-temperature' (72 — 95 °C) F2 generator, KF,2HF electrolyte, substrate fed in through porous carbon anode], e.g. C2H6 -> CH2F·CH2F, MeCHF2, and EtF; (iii) polyfluoroalkane anaesthetics via fluorination of alkanes with cobalt trifluoride, e.g. n-C4H10 140 — 230 °C -> CHF2·CF2·CHF·CF3; (iv) a complex mixture containing at polyfluorocyclohexanes [C6F11H, C6F10H2 (1H2/H and 1H/4H), and C6F9H3(1H,4H/2H, 1H/2H,4H and 1H,2H/4H)] via fluorination of benzene with manganese trifluoride at 300 °C (see also p. 44); (v) gem-difluoro-compounds via treatment of carbonyl compounds with molybdenum hexafluoride in the presence of boron trifluoride, e.g. [FORMULA NOT REPRODUCIBLE IN ASCII] (53%); (vi) fluoroalkanes via hydrogenation of fluoroethylenes with sodium borohydride, e.g. [FORMULA NOT REPRODUCIBLE IN ASCII] (51 %), CF3·CH2·CCl:CF2 (41%), CF3·CH2·CCl:CHF (~ 2 %); (vii) polyfluorides connected with an investigation of the pyrolysis of the ketone (CHF2)2CO (see p. 161)[FORMULA NOT REPRODUCIBLE IN ASCII] and [FORMULA NOT REPRODUCIBLE IN ASCII] and (viii) polyfiuoroalkylpyridines, e.g. (6), via treatment of appropriate N-oxides with polyfluoroalkenes.