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 fluorine atom displacement on the fluorine of a carbon-fluorine bond in such systems is unreasonable in view of the bond energies involved [F — F 125 (37); av. C — F 485 kJ mol-1 (116 kcal mol-1)].

Fluorinations of polyfluorobiplienyls with cobalt trifluoride (see Scheme 4), hexafluorobenzene with cobalt trifluoride (->tcyclo-C6F12, cyclo-C6F10 at 100 — 106 °C; ->cyclo-C6F10, perfluorocyclohexa-1,4-diene at 50 °C in the presence of CaCl2), polynuclear aromatics with potassium tetrafluorocobaltate(III) [e.g. (see also p. 57) pyrene -> perfluoroperhydropyrene at 36 — 410 °C, decafluorobiphenyl -> perfluorobicyclohexyl (ca. 64 % yield) at 260 °C], benzene with silver difluoride (->cyclo-C6Fl2, cyclo-C6Fl0, and polyfluorocyclohexanes at 220 — 380 °C), potassium tetrafluoroargentate(III) (->cyclo-C6F12, cycloC6F12, and polyfluorocyclohexanes at 180 — 380 °C), or potassium hexafluoronickelate(IV) (->cyclo-C6F12, polyfluorocyclo-hexanes and -hexenes, 3,3,6,6tetrafluorocyclohexa-1,4-diene and fluorobenzenes at 100 — 250 °C), hexafluorobenzene, undecafluorocyclohexane, or decafluorocyclohexene with silver difluoride and with potassium tetrafluoroargentate(III) (->cyclo-C6F12 at 250 °C), and cyclohexanone or tetrahydrofuran with silver difluoride and with potassium tetrafluoroargentate(III) (-> complex mixtures of fluorohydrocarbons) have been investigated using stirred-bed flow reactors. No problems were encountered in the use of silver difluoride despite the previous claim that a low-melting eutectic mixture of the difluoride and the monofluoride may be produced, although the tubular nickel reactor was attacked, resulting in contamination of the fluorinating agent with nickel difluoride. The new reagents KAgF4 and K2NiF6 also behaved nicely as active components in the usual fluorine transfer-regeneration cycle, except that the former slowly became contaminated with the latter owing to attack on the nickel reactor. The products obtained by fluorination of benzene with the bright red hexafluoronickelate K2NiF6 (which becomes reduced to the yellow tetrafluoronickelate K2NiF4) do not differ significantly from those produced with cobalt trifluoride, but the former is the more vigorous reagent; the results of further work with this reagent will be most welcome in view of the belief that complex nickel fluorides play a major role in Simons' electrochemical fluorination (see Vol. 2, p. 5).

Preliminary studies on the mechanism of electrochemical fluorinat ion have been reported in the form of la note dealing with the voltammetric behaviour of arenes in anhydrous hydrogen fluoride, and preparative Simons' electrochemical fluorination of benzene [FORMULA NOT REPRODUCIBLE IN ASCII] (referred to below as C.P., common products), n-C6F14, cycloC5F9•CF3], fluorobenzene (-> C.P., n-C6F14, cyclo-C5F9 • CF3), chlorobenzene (->C.P., nC6F14, CF3Cl, cyclo-C5F9•CF3, cycloC6F11Cl), m-dichlorobenzene (->C.P., n-C6F14, CF3Cl, cyclo-C6F11Cl, cyclo-C6F10Cl2), anisole ([FORMULA NOT REPRODUCIBLE IN ASCII]) o-chloroanisole ([FORMULA NOT REPRODUCIBLE IN ASCII]) (sodium fluoride was used as a conductivity aid in each of the foregoing cases), thiophenol (->C.P., n-C6F14, cyclo-C5F9 • CF3, SF6), p-chlorothiophenol ([FORMULA NOT REPRODUCIBLE IN ASCII]), m-thiocresol (->C.P., cyclo-C6F11 • CF3, SF6), 2-chloropyridine [->C.P. except cyclo-C6F12, perfluoro- (Nfluoropiperidine), C5F11Cl, CF3Cl, NF3], 3-chloropyridine [->C.P. except cyclo-C6F12, cyclo-C5F10NF, C5F11Cl, perfluoro(N-fluoro-3-chloropiperidine), NF3], branched perfuoroalkenes [e.g. (CF3)2C:CF•CF2•CF3 -> (CF3)2CF•CF2•CF2.CF3 (78 % yield)], gem-difluorocycloalkanes [e.g. 1,1-difluorocyclohexane (in the presence of CoF2 and NaF)-> cyclo-C6F13, cyclo-C5F9•CF3], and (trifluoromethy1) benzenes [e.g. 4-ClC6H4•CF3 -> cycloC6F11•CF3, cyclo-C5F8(CF3)2-1,2, and perfluoro-(1-chloro-4-methylcyclohexane) has been described. Phillips direct electrofluorination (see vol. 1, p. 14, and Vol. 2, pp. 11 and 21) has been used to prepare perfluoropropane from propane and propyl chloride.

The cage compounds perfluoro-( 1,3,5-triineihvlietrac:lclo [2,2,0,0,0]hexane) (3 ) (see p. 92),42 perfluoro-octamethylcubane (2) (n1.p. 253-254 °C), and perfluorooctamethylcuneane (3) (m.p. 186 — 187 °C) (see p. 96) have been synthesized from unsaturated precursors. Defluorination of a commercial sample of perfluoro-(l,3,5trimethylcyclohexane) with tri-iron tetraoxide at 475 — 490 °C was used to procure the perfluoro(trimethy1benzenes) (see p. 92) needed to prepare the prismane (1), which, despite its moderate thermal stability [half-life in solution towards reversion to perfluoro-(1,3,5- and 1,2,5-trimethylbicyclo[2,2,0]hexa-2,5-diene) (4 and 5), 19 h at 35 °C] has avoided isolation, owing, it appears, to catalysis of its isomerization to the Dewarbenzenes (4) and (5) by glass and other surfaces. Interestingly, defluorination of each of the thermal head-to-head dimers of αβ β-trifluorostyrene (6) (see p. 86) to 3,3,4,4-tetrafluoro-1,2-diphenylcyclobutene (7) has been effected with chromous ion [Cr2(SO4)3-Zn dust-H2O-DMF at 95 °C], a reagent that was used previously by the same group during work on the cycloaddition of cis- and a Obtained by hydrogenation of the thermal 1,4-cycloadducts of cis-/trans-CFCl: CFCl and trans-l,2-dichloro-l,2-difluoroethylene (e.g. see Scheme 5) but otherwise does not appear to have seen much service in organofluorine chemistry. Also noteworthy is that treatment of each geometrical isomer of (6) with 96 % sulphuric acid at 96 °C yields the ketone (8), presumably via a cationic sequence initiated by proton-catalysed loss of fluoride from a [??] CFPh group.

Full details of the investigation by i.c.r. (ion cyclotron resonance) spectroscopy of the gas-phase ion chemistry of the fluoromethanes CH4-nFn (n = 1 - 4 ) have become available (the order of carbocation stability established is CHF2+ > CH2F+ > CF3+ > CH3+), and the results of pulsed i.c.r. spectrometric investigation of attack by the carbocations CF3+, CCl3+, and C2F5+ [generated from CF4, CCl4, and C2F6 (CF3+ is also formed), respectively, with 40 eV electrons] on aldehydes and ketones have been briefly presented; the major reaction between CF3+ or CCl3+ and most aldehydes and ketones is cleavage of the carbonyl link and formation of a monohalogenated product cation, e.g. CF3+ + Me2CO -> [C3H6F]+ + CF2O. Photoionization mass spectrometric investigation of reactions of fragment ions formed in the photoionization of C2F6 and Xe-C2F6 and KrC2F6 mixtures has revealed that trifluoromethyl cations having no internal excitation energy undergo the fluoride-transfer reaction CF3+ + C2F6 -> CF4 + C2F5+, and the new value of ≤ 3.8 kJ mol-1 (0.9 kcal mol-1) has been computed for ΔHf(C2F5+).