Background and overview[1-2]
Perfluorobenzene is often used as an anesthetic and an excellent solvent for a variety of organic compounds. It is also an important intermediate in the synthesis of perfluorinated aromatic compounds. It has broad application prospects in chemical industry, medicine, and liquid crystal materials. So far, there are two main methods for synthesizing perfluorobenzene, namely direct fluorination of aromatic halogenated compounds and high-temperature cracking of fluorohaloalkane. However, since the reaction substrate of the direct fluorination method does not have strong electron-withdrawing groups, it is not conducive to the halogen exchange reaction. Therefore, this method usually requires a high temperature to proceed, and the reaction rate is very slow. Prolonged high temperature conditions will cause the solvent and reactants to denature and produce tar. This will seriously affect the yield of the reaction and increase the difficulty of post-processing. Considering the reaction process and atom utilization, the cost is very high. The method of using high-temperature cracking of fluorohaloalkane to produce perfluorobenzene is simple, fast, and convenient for continuous production. However, the complexity of the free radical reaction makes the composition of the product more complex, which brings difficulties to post-processing and the conversion of the reaction substrate. The rate is lower and therefore the production efficiency is lower.
Apply[2]
Perfluorobenzene can be used as an important intermediate in the synthesis of perfluorinated aromatic compounds. Like benzene, perfluorobenzene can be quantitatively converted to its Dewar isomer 2 under ultraviolet irradiation. Various types of fluorocarbons can be prepared through cycloaddition and other reactions using 2 as raw material. In recent years, research on the development of liquid crystal materials has found that liquid crystal compounds derived from perfluorobenzene have better properties.
Preparation[2]
1. Direct fluorination method
The earliest reports on the preparation of perfluorobenzene came from Mcbee, Lindgren and Ligett. They used BrF3, SbF5 and zinc powder to treat hexachlorobenzene. A small amount of perfluorobenzene could be obtained. Other by-products include perfluoro and fluorochlorine. of cyclohexene and cyclohexadiene compounds. Also using hexachlorobenzene as a raw material, anhydrous KF was used to replace the chlorine atoms to prepare fluorobenzene. The reaction is carried out in an autoclave at a temperature of 450 to 500°C, and finally C6F6 (yield 21%) and incompletely fluorinated fluorochlorobenzene (C6F5Cl: 20%; C6F4Cl2: 14%; C6F3Cl3: 12%) are separated.
2.Dehydrofluorination method
Perfluorobenzene was obtained by dehydrofluorination in nonafluorocyclohexane alkali solution. However, nonafluorocyclohexane needs to be prepared by reacting benzene vapor with CoF3 at 150°C, and its low conversion rate limits its application.
3. High temperature cracking method
CFBr3 is cracked at high temperature in a platinum tube at 630-640°C to prepare perfluorobenzene. The yield is 45% based on the CFBr3 participating in the reaction. In addition to perfluorobenzene, the author also isolated pentafluorobromobenzene (6%) and difluorotetrabromoethane (2%).
Using Desirant’s method, the effects of reaction tube material, reaction temperature, pressure and other conditions were investigated (Table 1). It was found that using a platinum tube at 540°C and a reaction pressure of 0.456MPa, the highest yield was 55%. When the reaction is carried out in a graphite tube with carbon as filler at 642-654°C, the reaction pressure is 0.101MPa, and the maximum yield is 30%. When the reaction tube was replaced with borosilicate glass and platinum was used as the filler, the maximum yield was only 11%. When the cracking temperature of the three reaction tubes was continued to increase, although the cracking rate of bromine increased, the yield of perfluorobenzene gradually decreased.
Use a nickel tube lined with platinum foil filler to crack CFBr3 at 640°C to prepare C6F6, with the highest yield of 48%. It was found that increasing the feed rate of CFBr3 will reduce the conversion rate of raw materials, while the feed rate that is too slow will lead to an increase in by-products. When using a nickel tube, the yield was 25%. When nickel wire was added as a filler in the nickel tube, the yield was significantly reduced. The author also speculates that CFBr3 may use C2F2 as an intermediate to obtain C6F6 (CFBr3→CFBr2·CFBr2→C2F2→C6F6) based on the trimerization of C2Cl2 to obtain C6Cl6. In order to verify the above reasoning, the cracking of 1,2-dichlorodifluoroethylene and 1-chloro-1,2-difluoroethylene were studied respectively. The former was passed through a platinum tube at 600°C with a contact time of 2.3 minutes, and the highest yield of perfluorobenzene was 14%. On the surface of the nickel tube, at 650°C, the yield of perfluorobenzene is only 1% to 2%. The latter was cracked on the surface of nickel and iron fillers at 450 to 1000°C, and no generation of perfluorobenzene was observed. But when 1,2-dichlorodifluoroethylene decomposes on the surface of the platinum tube at 625°C,��When the purge gas flow was changed from nitrogen to hydrogen, very trace amounts of perfluorobenzene were observed to be formed. Research on the use of nickel tube platinum packing to crack dichlorofluoromethane and dibromofluoromethane to prepare perfluorobenzene. When the former is used as the raw material, the raw material reacts completely at 750°C and the contact time is 3.3s. The yield is 9%. Products include hydrogen chloride, trichlorofluoromethane, 1,2-dichlorodifluoroethylene, and tetrachloro-1,2-difluoroethane. When dibromofluoromethane is cracked at 555-765°C and the contact time is 2-10s, a yield greater than 10% is obtained. Among them, at 715°C and the contact time is 9s, the highest yield is 33%.
Main reference materials
[1] CN201711114677.8 Preparation method of perfluorobenzene
[2] Research progress in the preparation of perfluorobenzene
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