Saturated Fluorocarbons, Fluorocarbon Hydrides, and Fluorocarbon Halides

BY R. E. BANKS

1 Fluorocarbons

More information concerning the preparation, properties, and applications (lubricants, seals, and bearing; cathode components for high-energy batteries; g.s.c. stationary phase; precursor of diamonds and fluorinated diamonds) of poly(carbon monofluoride) ('graphite fluoride') is now available. Detailed experimental procedures for the direct fluorination of regular or pyrolytic graphite using autoclave, fluidized-bed, or normal flow techniques are now to hand; production of snow-white superstoicheiometric poly(carbon monofluoride), [CF1.12[plus or minus]0.03]n, by the flow or fluidized-bed method demands a reaction temperature range of 627 [plus or minus] 3 °C, outside which either carbon tetrafluoride plus soot (at > 630 °C) or black-to-grey substoicheiometric material (e.g. black [CF0.68]n at 540 °C) are formed. At atmospheric pressure, fluorine does not appear to attack graphite at temperatures below 450°C, but at pressures greater than 225 lbf in-2, reaction occurs spontaneously at 20°C and can lead to a violent explosion if the rate of introduction of fluorine into the autoclave is not regulated carefully. No decomposition of superstoicheiometric poly(carbon monofluoride) appears to occur at 650 — 700 °C in the presence of fluorine. More structural data for poly(carbon monofluoride) are available following an X-ray powder diffraction study on material pressed at 20 kbar and 150 °C and determination of the n.m.r. absorption mode second moment of commercial Fluorographite samples (see Vol. 2, p. 1, ref. 2) of stoicheiometry CFl.06 and CF1.15; the results of the latter study indicate that the most plausible layer structure comprises an infinite array of cis-trans-linked cyclohexane boats .

Direct fluorination (flow method) of 'graphite oxide', [C8O2(OH)2]n (from graphite/KMnO4-NaN03-H2SO4 at 66 °C) at 20 °C and 1 atm yields a powdery, pale grey, thermally (above 50 °C) and hydrolytically unstable 'oxyfluoride' (16.25 — 22.4% F; λmax 1095 cm-1 (C — F stretch); c[florin].1 [C4F]n 1090 cm-1, [CF1.12[plus or minus]0.03]n 1217s ([??]C — F), 1342m, 1072w (peripheral CF2) cm-1}.1

Full details have been published of the direct fluorination of hydrocarbon polymersl6 by the so-called LaMar procedure, the principal feature of which is inhite dilution initially with helium or nitrogen followed by gradient changes of fluorine concentration ; with substrate particle sizes greater than lo0 mesh a hydrocarbon core is retained, and large fabricated items such as polyethylene bottles can be given a fluorocarbon skin of thickness ca. 0.2. Complete, or almost complete, fluorination of finely powdered (< 100 mesh) polyethylene, polypropylene, poly(ethylene-co-propylene), polyisobutylene, polyacrylonitrile, polystyrene, and poly-p-xylylene can readily be achieved at room temperature, the nitrile {->[CF•CF(CF2•NF2)]n} and benzenoid polymers undergoing fluorine addition as well as hydrogen substitution (see Scheme 1); polyacrylamide17 and phenol-formaldehyde resins or prepolymers (resols, novolacs) seem to suffer cleavage of pendant groups whilst undergoing change to fluorocarbon systems, and all the linear polymers appear to become cross-linked.

Instructions for the conversion of polynuclear arenes (coronene, anthracene, decacyclene, naphthacene, naphthalene, pentacene, ovalene, 9,1 O-benzphenanthrene, 1,2-benzanthracene, 1,3,6,8-tetraphenylpyrene) into the corresponding perfluoroalicyclic compounds and of 1,4-dichlorobenzene into perfluorocyclohexane by 'LaMar' controlled-concentration direct-flow fluorination at room temperature and atmospheric pressure are now available in the patent literature, and precise details of the use of the method, in conjunction with a cryogenic reactor, to convert neopentane into perfluoroneopentane in low yield (10 % after g.l.c. isolation) are also to hand. The results of kinetic studies on the direct fluorination of methane, [2H2]methane, halogenomethane, and olefins are likewise in print. Treatment of the perfluoropropene dimers (CF3)2C:CF•CF2•CF3 and trans(CF3)2CF•CF:CF•CF3 with fluorine at - 78 °C yields perfluoro-(2-methylpentane) quantitatively, while fluorination of a mixture of the trimers [(CF3)2CF]2C:CF•CF3 and (CF3)2C:C(CF2•CF3)•CF(CF3)z provides the corresponding nonane [(CF3)2CF]2-CF•CF2•CF3 (~95 %) at 30 °C but mainly its isomer (CF3)2CF•C(CF3)(CF2 • CF3)2 at 100 °C, plausibly via the rearrangement depicted in Scheme 2. Direct fluorination of the trimer (CF3)2(CF•CF:C(CF3)•CF2•CF2•CF3 [from CF3•CF:CF2/(CF3•CHF•CF2•O•CH2•CH2)3N/DABCO in DMSO at 36 — 38 °C] gives the perfluorononane (CF3)2CF•CF2•CF(CF3)•CF2•CF2•CF3, while the tetramer [(CF3)2CF]2C:C(CF3)•CF(CF3)2 [from oligomerization of CF3•CF:CF2 as in Scheme 2 but at higher temperatures], at 75 °C (no reaction occurs at 20 °C),undergoes cleavage with formation of [(CF3)2CF]2CF•CF2•CF3, CF3•CF2•CF2•-CF(CF3)• CF(CF3)2. and C3F8. Treatment of syn- perfluorooctamethyltricyclo[4,2,0,0]octa-3,7-diene (from dimerization of perfluorotetramethylcyclobutadiene) or of i s valence isomer perfluoro-octamethylcyclo-octatetraene (see p. 96) with fluorine at - 78 to 163 °C in an attempt to provide chemical evidence of structure (double bond 'co2nt') gave complex mixtures which were not investigated.

Data provided by a kinetic study of the thermal (280 — 450 °C) fluorine-perfluorocyclobutane reaction [activation energy: 170 [plus or minus] 2 kJ mol-1(40.5 [plus or minus] 0.5 kcal mol-1); products: CF4, C2F6, C3F8, n-C4Fl0] have been discussed in terms of initiation by SH2 attack of fluorine atom on ring carbon followed by the sequence presented in Scheme 3 (* indicates a thermally excited species). The possibility that the Cl — C3 fluorocarbons arose via fluorinolysis of perfluoro-n-butane formed first was excluded by lack of reaction in a separate experiment between perfluoropropane and fluorine at 477 °C for 30 h, conditions under which perfluorocyclohexane likewise fails to suffer attack, the difference in reactivity between these fluorocarbons and perfluorocyclobutane presumably arising from ring-strain effects ; consideration of...