Pyrolysis fluorides

Cyanogen fluoride, FCN. Colourless gas (b.p. — 46 C) prepared by pyrolysis of cyanuric fluoride. Polymerizes to (FCN), cyanuric fluoride, at room temperature. 

B.p. — 29X. Monomer used to form polymers (only under rather drastic conditions) or copolymers with C2F4 and vinylidene fluoride, CH2 = CF2. Hexafluoropropene may be prepared by thermal decomposition of CF3CF2CF2C02Na or is prepared commercially by low pressure pyrolysis of C2F4. 

As a result of the development of electronic applications for NF, higher purities of NF have been required, and considerable work has been done to improve the existing manufacturing and purification processes (29). N2F2 is removed by pyrolysis over heated metal (30) or metal fluoride (31). This purification step is carried out at temperatures between 200—300°C which is below the temperature at which NF is converted to N2F4. Moisture, N2O, and CO2 are removed by adsorption on 2eohtes (29,32). The removal of CF from NF, a particularly difficult separation owing to the similar physical and chemical properties of these two compounds, has been described (33,34). 

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. 

The Du Pont HaskeU Laboratory for Toxicology and Industrial Medicine has conducted a study to determine the acute inhalation toxicity of fumes evolved from Tefzel fluoropolymers when heated at elevated temperatures. Rats were exposed to decomposition products of Tefzel for 4 h at various temperatures. The approximate lethal temperature (ALT) for Tefzel resins was deterrnined to be 335—350°C. AH rats survived exposure to pyrolysis products from Tefzel heated to 300°C for this time period. At the ALT level, death was from pulmonary edema carbon monoxide poisoning was probably a contributing factor. Hydrolyzable fluoride was present in the pyrolysis products, with concentration dependent on temperature. 

Vlayl fluoride [75-02-5] (VF) (fluoroethene) is a colorless gas at ambient conditions. It was first prepared by reaction of l,l-difluoro-2-bromoethane [359-07-9] with ziac (1). Most approaches to vinyl fluoride synthesis have employed reactions of acetylene [74-86-2] with hydrogen fluoride (HF) either directly (2—5) or utilizing catalysts (3,6—10). Other routes have iavolved ethylene [74-85-1] and HF (11), pyrolysis of 1,1-difluoroethane [624-72-6] (12,13) and fluorochloroethanes (14—18), reaction of 1,1-difluoroethane with acetylene (19,20), and halogen exchange of vinyl chloride [75-01-4] with HF (21—23). Physical properties of vinyl fluoride are given ia Table 1. 

It may also be prepared by pyrolysis of 1,1-difluoroethane at 725°C over a chromium fluoride catalyst in a platinum tube or by the action of zinc dust on bromodifluoroethane at 50°C. 

The catalytic pyrolysis of R22 over metal fluoride catalysts was studied at 923K. The catalytic activities over the prepared catalysts were compared with those of a non-catalytic reaction and the changes of product distribution with time-on-stream (TOS) were investigated. The physical mixture catalysts showed the highest selectivity and yield for TFE. It was found that the specific patterns of selectivity with TOS are probably due to the modification of catalyst surface. Product profiles suggest that the secondary reaction of intermediate CF2 with HF leads to the formation of R23. 

In our previous work [4, 5], results on the catalytic pyrolysis of R22 over Cu-promoted catalysts were reported. In this work, various metal fluoride catalysts were introduced to improve the relatively poor yield of TFE. 

In the Antek Fluoride Analyzer, a pyrolysis furnace is combined with an ion-specific electrode cell (ISE). Table 8.9 compares this specific analyser to a conventional combustion bomb. 

Thus, heptafulvalene (522) was isolated in 33 and 65% yield after thermolysis of 517 in diglyme and its photolysis in THF, respectively [193]. An almost quantitative yield of 522 resulted when a mixture of 1-, 2- and 3-chloro-l,3,5-cycloheptatriene (518a) was treated with KOtBu in THF [206]. Even on variation ofthe concentration of the starting material and the temperature of the reaction, 522 turned out to be the exclusive product [207]. Also, the treatment of (trimethylsilyl)tropylium tetrafluoro-borate (519) with tetrabutylammonium fluoride [208] and the gas-phase pyrolysis of 7-acetoxynorbornadiene and 7-acetoxy-l,3,5-cycloheptatriene [209] afforded high yields of 522. Further, 522 was observed on FVT of N-nitroso-N-(7-norbornadienyl)-urea at 350 °C, which is believed to be converted into 7-diazonorbornadiene initially. Its decomposition should proceed via 7-norbornadienylidene to bicyclo[3.2.0]hepta-l(2),3,6,-triene (514) (Scheme 6.103) and then on to 5 [210]. The intermediacy of 514 is also suspected in the formation of 522 from 7-acetoxynorbornadiene. 

The decomposition products, up to a temperature of 500°C, are principally the monomer, tetrafluoroethylene, but also include perfluoropropene, other perfluro compounds containing four or five carbon atoms, and an unidentified particulate waxy fume. From 500°C to 800°C, the pyrolysis product is carbonyl fluoride, which can hydrolyze to form HE and CO2. 

Several high-temperature methods leading to fluoridated apatites can be found in the literature they involve solid-gas reaction, pyrolysis or crystal growth processes. 

Reaction of acetone with hydrogen sulfide in the presence of acidified ZnCl2 at 25° C gives a good yield of a product composed of60-70% hexamethyltrithiane and 30-40% of 2,2-propanedithiol. Thioacetone can be obtained by pyrolysis of either of these compounds. The trithiane is pyrolyzed either on quartz rings heated to 500-650° C at 5-20 mm (30) or by means of a hot wire (32). The dithiol is pyrolyzed on sodium fluoride pellets heated to 150° C at 11 mm (50). In both cases the pyrolysate is immediately collected in a trap cooled to — 78° C. 

As mentioned earlier, pyrolysis of 2,2-propanedithiol over sodium fluoride at about 200° C also leads to thioacetone. The product obtained at 150° C is more than 90% the thioketo form. At 250° C, the thioenol amounts to over 50% of the product. 

Polyvinyl fluoride and PVDF are more stable to elevated temperatures than the corresponding chloride polymers. The decomposition temperatures of polytrifluoroethylene and polytetrafluoroethylene (ptfe) are progressively higher than polymers of vinyl fluoride or vinylidene fluoride. The pyrolysis of PAN and polymethacrylonitrile yields polycyclic ladder polymers. 

Thietanes are formed by intramolecular cyclization of thiocarbenes (110) (75BCJ1490), on pyrolysis of the copolymer of sulfur, tetrafluoroethylene and thiocarbonyl fluoride (111) (69CC1274), and upon electrolysis or base treatment of dithione (112) (67JOC1562,78JOC1980, 79JCR(S)320>. 

The successful synthesis of furanophane (27) by pyrolysis of the quaternary ammonium hydroxide (28) (60JA1428) has prompted the adaptation of this 1,6-Hofmann elimination procedure to the synthesis of numerous heterophanes (77CRV513,77H(7)81,78T1641). Fluoride anion-induced 1,6-elimination of (29) or (30) gives (27) or (31) and (32), respectively, in good yields (81JOC1043). 

Sulfur tetrafluoride undergoes addition to perfluoroalkenes, e.g. 1 and 4, in the presence of cesium fluoride to give bis(perfluoroalkyl)sulfur difluorides and perfluoroalkylsulfur trifluorides.188,198 The ratio of the products is controlled by the reactant ratios. The role of cesium fluoride is ascribed to the formation of perfluorocarbanions which then subsequently attack sulfur tetrafluoride.188 Perfluoroalkyl disulfides, which are formed as side products, arise from the pyrolysis and disproportionation of bis(perfluoroalkyl)sulfur difluoridcs.189... 

The pyrolysis of 4-ethylphenylthallium(III) difluoride, for example, gives ethylbenzene (41 %) as the only identifiable product.10 Conversion to the aryl fluorides can be performed by using boron trifluoride. 

The syntheses of benzosilacyclopentenes have been tabulated (b-82MH2000). The pyrolysis of phenyl-substituted tris(trimethylsilyl)methylsilanes affords a novel route to 1,3-disilain-danes. The fluoride (101) decomposes with loss of trimethylfluorosilane, probably via a series of interconverting silaalkene intermediates, to give the three indanes (102)-(104)... 

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