Reaction with tetrafluoromethane

The reaction of arc-generated carbon atoms with CF in the presence of dioxygen resulted in the formation of carbonyl difluoride [1670]. COFj is also formed when a 4 1 molar ratio of CO and CF, (at a pressure of 0.7 kPa) is passed through an electric arc produced between graphite electrodes at 7500 V [1884]. 

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In a dichlorodifluoromethane system, frictional wear exposed fresh metal surfaces on an aluminium compressor impellor, causing an exothermic reaction which melted much of the impellor. Later tests showed similar results, decreasing in order of intensity, with tetrafluoromethane chlorodifluoromethane bromotrifluoromethane dichlorodifluoromethane 1,2-difluorotetrafluoroethane ... 

In a dichlorodifluoromethane system, frictional wear exposed fresh metal surfaces on an aluminium compressor impellor, causing an exothermic reaction which melted much of the impellor. Later tests showed similar results, decreasing in order of intensity, with tetrafluoromethane chlorodifluoromethane bromotrifluoromethane dichlorodifluoromethane 1,2-difluorotetrafluoroethane 1,1,2-trichlorotrifluoroethane [6], In similar tests, molten aluminium dropped into liquid dichlorodifluoromethane burned incandes-cently below the liquid [7]. Aluminium bearing surfaces under load react explosively with polytrifluoroethylene greases or oils. The inactive oxide film will be removed from the metal by friction, and hot spots will initiate reaction [8]. An attempt to scale up the methylation of 2-methylpropane with chloromethane in presence of aluminium chloride and aluminium went out of control and detonated, destroying the autoclave. The preparation had been done on a smaller scale on 20 previous occasions without incident [9]. 

The distributions of concentrations F and HF are shown on Fig. 1, a and Fig. 1, b correspondingly at 25 % fraction H2 mCF4/H2- The main part of fluorine atoms obtained from the dissociation of tetrafluoromethane are consumed in the reactions with hydrogen atoms to form an abundant component HF. As a result the fluorine concentration monotonous decreases along the flow direction whereas the concentration HF rises and reaches a maximum at the outlet of reactor. 

In 1930, Otto RufF(1871-1939) and his coworker Otto Bretschneider synthesized CF4 by a reaction of F2 with charcoal and reported about violent combustion reactions of tetrafluoromethane with both calcium and magnesium at elevated temperatures [28]. Ruff and Brettschneider were also the first to obtain the important monomer tetrafluoroethylene, C2F4 (TFE), from tetrafluoromethane by arc discharge between graphite electrodes (Scheme 2.1) in 1933. They described its fierce combustion reaction with sodium [29]. 

The reactivity of crystalline silicon with tetrafluoromethane, CF4, has been investigated by Holcombe et al. [36]. They found a mild exothermic reaction with onset at about 500 °C (peak at 530 °C) with no significant mass change and the onset of a strongly exothermic reaction at 990 °C (peak at 1020°C) with a significant mass loss indicative of volatile SiFx formation. Samples held under CF4 atmosphere for 75 min at 1030 °C exhibited both S-SiC and metastable carbon formation on the sttrface of Si. The exotherm at 500°C is believed to occur from highly reactive amorphous silicon present on the surface of crystalline Si. The formation of metastable carbon, particularly diamond phase on both Si and SiC substrates by thermal treatment in fluorocarbon atmosphere has been the subject of a disclosure by Holcombe et al. [37]. 

Scheme 1.15 Reactions of electrons with tetrafluoromethane. The asterisk denotes an electronically excited state.

The decomposition of trifluoroacetyl fluoride is interesting in that it involves the rupture of the O-F bond. The products are tetrafluoromethane and carbon dioxide. Stewart and Cady found the reaction to be an essentially homogeneous, second-order process in a glass vessel coated with aluminium phosphate. For decomposition at 30-80 °C in the pressure range 1.5 to 3 torr, the rate coefficient was expressible as, A = 8.6x10 exp ( —11,200/lir) l.mole. sec. The following mechanism was proposed... 

By-products. The presence in the products of small quantities of compounds which would arise from combination of free radical intermediates can provide evidence for a free radical process. For example, the explosive reaction of methane with fluorine gives mainly hydrogen fluoride and a mixture of mono-, di-, tri- and tetrafluoromethanes, but small quantities of fluorinated ethanes, including C2F6, are also produced. These two-carbon products cannot be readily explained on the basis of possible molecular reactions (see reaction 6.16), but would arise naturally as combination products of the fluorinated methyl radicals produced in a radical chain reaction sequence (reaction 6.17). 

The reaction between carbon monoxide and oxygen atoms produced by the thermal decomposition of ozone was studied in the range 80-160 C The chemiluminescence from 002( 82) was used to follow the course of the reaction. The effect of added carbon dioxide, tetrafluoromethane and oxygen on the kinetics and chemiluminescence was investigated. It is concluded that there are simultaneous bimolecular and third body channels for the reaction of CO with 0-atoms to produce electronically excited C02 ... 

The hrst rehable report of the preparation of tetrafluoroethene (TFE) was given in 1933 by Rulf and Bretschneider [643], who decomposed tetrafluoromethane in an electric arc. Other syntheses are based on the dechlorination of yn-chlorodifluoromethane [644], the pyrolysis of chlorodifluoromethane [645], and the decarboxylation of sodium perfluoroproprionate [646]. Since then, a number of synthetic routes have been developed [644-646]. On a laboratory scale, depolymerization of PTFE by heating at about 600 °C is probably the preferred method to obtain a smah amount of pure monomer (97%) [647], along with the highly toxic perfluoroisobutene as well as octafluorocyclobutane as a side product formed by the thermal (2ji -I- 2n) cyclodimerization of TFE [648,649]. The most common commercial approach for the preparation of TFE is the pyrolysis of chlorodifluoromethane [645,650]. The noncatalytic gas-phase reaction is carried out in a flow reactor at atmospheric pressure, yields over 95% TFE at 590 to 900 °C. The synthesis of TFE involves the following steps ... 

In the course of the electrolytic preparation of beryllium from molten KHF2/ BeF2, in 1926, Paul Lebeau (1858-1959) and his collaborator Damien were able to purify and isolate tetrafluoromethane, CF4, by fractional distillation with a liquid air-cooled condenser. They observed vigorous combustion reactions of CF4 with both sodium and calcium [27]. 

CF = CF, CAS number 116-14-3) from decomposition of tetrafluoromethane in an electric arc. TFE was obtained by bromination and separation of the dibromide (CF Br-CF Br) from the other reactive products. Dehiogenation with zinc was the next step for obtaining pure TFE. Commercially significant techniques for TEE preparation list fluorspar (CaF ), hydrofluoric acid, and chloroform as the starting ingredients [7-13]. The reaction sequence is shown in Fig. 3.2. 

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