Carbon tetrafluoride production

Initial attempts at reactions between fluorine and hydrocarbons were described as similar to combustion and the reaction products contained mostly carbon tetrafluoride and hydrogen fluoride ... 

Uses. The chemical inertness, thermal stability, low toxicity, and nonflammability of PFCs coupled with their unusual physical properties suggest many useflil applications. However, the high cost of raw materials and manufacture has limited commercial production to a few, small-volume products. Carbon tetrafluoride and hexafluoroethane are used for plasma, ion-beam, or sputter etching of semiconductor devices (17) (see loN implantation). Hexafluoroethane and octafluoropropane have some applications as dielectric gases, and perfluorocyclobutane is used in minor amounts as a dielectric fluid. Perfluoro-1,3-dimethyl cyclohexane is used as an inert, immersion coolant for electronic equipment, and perfluoro-2-methyldecatin is used for... 

Because PTFE resins decompose slowly, they may be heated to a high temperature. The toxicity of the pyrolysis products warrants care where exposure of personnel is likely to occur (120). Above 230°C decomposition rates become measurable (0.0001% per hour). Small amounts of toxic perfiuoroisobutylene have been isolated at 400°C and above free fluorine has never been found. Above 690°C the decomposition products bum but do not support combustion if the heat is removed. Combustion products consist primarily of carbon dioxide, carbon tetrafluoride, and small quantities of toxic and corrosive hydrogen fluoride. The PTFE resins are nonflammable and do not propagate flame. 

Lebeau and Damiens obtained the simplest fluorocarbon carbon tetrafluoride (CF4), separating it from fluorine formed at a carbon anode during electrolysis of beryllium difluoride (the first organic product from electrochemical fluorination). 

Lebeau and Damiens, and Ruff and Keim, both reported isolation of carbon tetrafluoride as the major product of the reaction of fluorine with carbon at high temperatures. 

As shown in Table 1, with stable fluorine flow rate, at ideal partial pressure, and with control of the fluorination time and temperature, the desired product composition with carbon yield up to 96% can be obtained. Fluorination of carbon above 500 C typically gives low yields due to further fluorination to carbon tetrafluoride as shown by the reaction mechanism below.12... 

The active carbon (C ), produced by the disproportionation reaction, reacts immediately with available fluorine producing more carbon tetrafluoride, which dilutes the fluorine concentration, thereby decreasing the rate of reaction.12 Formation of carbon tetrafluoride releases a large amount of heat (436 kJ moF ), thereby providing more energy to the product reactor to create more carbon tetrafluoride. This process can become uncontrolled when the entire reactor contents deflagrate and an explosive reaction can occur. 

The anodic oxidation of alkanes on a platinum anode in anhydrous hydrogen fluoride19 at low potentials is accompanied by exhaustive fluorination, with all of the hydrogen atoms being replaced by fluorine. Thus, the major fluorination products of methane and propane are carbon tetrafluoride and octafluoropropane, respectively. 

The products of electrochemical fluorination of nitrosodiethylamine are perfluoro(jV,A-dimethylcthylamine), perfluoro(Ar,Ar-diethylmethylamine), and perfluoro(triethylamine). Byproducts are gaseous fragmentation products, e.g. carbon tetrafluoride, nitrogen trifluoride and nitrogen.57... 

The photolysis of trifluoroacetone with light of wavelength 3130 A. has been studied by Sieger and Calvert.48 The products of decomposition were shown to be carbon monoxide, methane, ethane, 1,1,1-trifluoro-ethane, and hexafluoroethane. There was no indication of the presence of carbon tetrafluoride or methyl fluoride. The quantum yield of all products was low at low temperature and it is assumed that the excited molecule of trifluoroacetone has an appreciable lifetime and may be deactivated by collision before decomposition can occur. This contention is supported by the decrease in the quantum yields observed when foreign gases such as carbon dioxide are added, and by the fall in quantum yields with increase in trifluoroacetone concentration. 

Since the time of the earliest work concerned with the reaction of hydrocarbons and fluorine in 1890 by Moissan (who isolated fluorine in 1886), numerous difficulties have been reported. According to Lovelace et al the action of fluorine on a carbon compound can be likened to a combustion process where the products are carbon tetrafluoride and hydrogen fluoride (1). 

The interaction of fluorine with an organic compound liberates a quantity of energy which is frequently of the order of magnitude of, or greater than, the energy which binds the carbon atoms in chains. It is estimated 63 that the addition of fluorine to a double bond liberates ldS calories per mole, whereas chlorine liberates only 30 calories. Careful control qf the temperature throughout the reacting masses is therefore essential. Even in the most favorable cases, much decomposition occurs and carbon tetrafluoride is frequently the main reaction product.62-6 ... 

Carbonyl fluoride can be prepared by any of several methods, including the conversion of carbonyl chloride to the fluoride by such reagents as hydrogen fluoride1 and antimony (III) fluoride.2 The direct combination of carbon monoxide and fluorine is another route to this fluoride, but carbon tetrafluoride is a by-product of the reaction.8 A particularly suitable laboratory preparation of carbonyl fluoride is the fluorination of carbon monoxide by silver (II) fluoride.4 This method, described below, gives directly carbonyl fluoride of rather high purity without recourse to a low-temperature distillation. 

Carbon tetrafluoride, CF4, mp — 185 °C, bp — 128 °C, which is the end product of the fluorination of carbon compounds, is a very stable gas. It can also be made by the fluorination of silicon carbide (equation 3). The SiF4 is removed from the CF4 by passing the product gases through 20% aqueous NaOH, which removes the SiF4 as soluble sodium silicate but leaves the CF4 unaffected. This major difference in hydrolytic reactivity of CF4 and SiF4 is a consequence of accessible d orbitals on silicon but not on carbon. 

Oxygen dilluoride reacts rapidly with selected alkenes at low temperatures 40 however, per-fluoroalkenes are less reactive than the corresponding alkylalkenes and require either thermal or ultraviolet activation.41 The oxidation, which is a homogeneous chain reaction,42 of hexa-fluoropropene, tetrafluorocyclopropene, and perfluorobut-2-ene all give a variety of products such as carbon tetrafluoride. carbonyl difluoride, hexafluoroethane, oxiranes, and other oxidation products.41 This reaction is not useful for preparative organic work. 

Molybdenum Carbides.—The carbide, MojC, may be obtained by heating molybdenum dioxide with carbon or with calcium carbide in an electric furnace. The product shows a brilliant, white, crystalline fracture, cleaves readily, and has a density of 8-9. If a mixture of fused molybdenum, carbon, and excess of aluminium is heated in the electric furnace, the carbide, MoC, is obtained as a grey crystalline powder, of density 8-40 at 20° C., and of hardness 7 to 8. It burns readily in fluorine, forming carbon tetrafluoride and molybdenum... 

Carbon tetrafluoride is extremely inert and may be prepared by the reaction of SiC and F2, with the second product, SiF4, being removed by passage through aqueous NaOH. Equation 13.36 shows a convenient laboratory-scale synthesis of CF4 from graphite-free calcium cyanamide (see structure 13.7) trace amounts of CsF are added to prevent the formation of NF3. 

Carbon tetrafluoride is an extraordinarily stable compound. It is the end product in the fluorination of any carbon-containing compound. A useful laboratory preparation, for example, involves the fluorination of silicon carbide. The SiF4 also formed is removed easily by passing the mixture through 20% NaOH solution. The CF4 is unaffected, whereas the SiF4 is immediately hydrolyzed the difference is due to the fact that, in CF4, carbon is coordinately saturated whereas silicon in SiF4 has 3d orbitals available for coordination of OH- ions in the first step of the hydrolysis reaction. 

Tetrafluoroethylene boils at -76.3 C. It is not the only product from the above pyrolytic reaction of difluorochloromethane. Other fluorine byproducts form as well and the monomer must be isolated. The monomer polymerizes in water at moderate pressures by a free-radical mechanism. Various initiators appear effective. Redox initiation is preferred. The polymerization reaction is strongly exothermic and water helps dissipate the high heat of the reaction. A runaway, uncontrolled polymerization can lead to explosive decomposition of the monomer to carbon and carbon tetrafluoride ... 

Carbon tetrafluoride is an extremely stable gas at room temperature and hence is suitable for applications calling for a stable gas of higher molecular weight than can be obtained by use of the inexpensive inert gases of the atmosphere. The method of preparation described here gives a product of high purity without attendant explosion hazards such as present themselves in the direct fluorination of carbon. 

The flow of fluorine is started and as soon as the reaction has commenced, as indicated by heating of the reactor, the fluorine is diluted with an equal volume of dry nitrogen. When a good flow of product is obtained, as evidenced by copious fumes at the exit, the trap is cooled in liquid nitrogen to condense the silicon tetrafluoride and carbon tetrafluoride. The reaction is allowed to continue imtil the... 

The gaseous products of the "Co y-radiolysis (in Pyrex ampoules) of liquid trifluoroacetic acid at 23 C and of the polycrystalline acid at - 196°C have been identified (CgF, CHFg, COg, CO, Ha, and CgF, CHF3, COg, respectively) and their C-values determined. Failure to detect the formation of carbon tetrafluoride, tetrafluoroethylene, or silicon tetrafluoride was taken as circumstantial evidence for the non-occurrence of reactions involving fluorine-atom abstraction by trifluoromethyl radical or the generation of... 

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