Introduction

The preparation of hexafluorobenzene (Figure 1) has been described by several investigators. Of the methods used, the one of Desirant, pyrolysis of tribro-mofluoromethane, is the simplest and most direct procedure for preparing sizable quantities of hexafluorobenzene. When the procedure is carried out under pressure and with a platinum furnace,this method has produced yields of better than 60 percent. Yields of between 20 and 50 percent have been obtained with Pyrex, graphite, and nickel furnaces. In addition, other valuable materials for example, bromo-pentafluorobenzene and dibromotetrafluoro-benzene-are obtained as by-products.[1]

Reactions of Hexafluorobenzene

Aromatic hydrocarbons can undergo substitution or displacement reactions by attack of an electrophilic, nucleophilic, or free-radical species. The most common aromatic substitution reactions, however, involve the attack of an electrophilic reagent on the aromatic ring, e.g., nitration, sulfonation, halogenation, and the Friedel-Crafts reaction.In contradistinction to this behavior, the principal mode of aromatic substitution in hexafluorobenzene is by attack of a nucleophilic reagent. In fact, with few exceptions, all of the reactions of hexafluorobenzene that have been reported in the literature may be classed as bimolecular, nucleophilic, substitution reactions. The feature common to all these reactions is the displacement of one or more of the ring fluorines as the fluoride ion, by a nucleophile of sufficient strength.[2]

Pentafluoroanisole

Into a three necked flask were placed 25g (0.134M) of hexafluorobenzene and 25ml of dry pyridine. The solution was stirred and heated to 800℃, at which time a solution of 3.45 g of sodium in 40 ml of methanol was added dropwise over a 1/2-hour period and refluxed for an additional 1/2 hour. The reaction mixture was cooled and poured into 100 ml of cold 10 percent hydrochloric acid. The bottom organic layer was separated, dried(Na2SO4), and distilled. There was obtained 18.66 g (70 percent) of pentafluoroanisole (boiling point 154℃to 156℃), reported boiling point 155℃ to157℃, and 1.04 g (5 percent) of unreacted hexafluorobenzene.

Alcoholic potassium hydroxide and hexafluorobenzene

The procedure was the same as that stated above for the pentafluoroanisole, except the following quantities were used: 10 g (0.054M) hexafluorobenzene, 25ml pyridine, and 6.16g (0.11M) of potassium hydroxide in 40 ml of 95 percent ethanol. The usual workup gave 9.21 g of a colorless liquid having the following mass spectrometer analysis: 40 percent pentafluorophenol, 40 percent tetrafluorodihydroxybenzene, 4 percent pentafluorophenetole, and 4 percent tetrafluorodiethoxybenzene.

Sodium hydride and hexafluorobenzene

Twenty grams (0.107M) of hexafluorobenzene, 2.64 g (0.106M) of sodium hydride, and 0.56 g of pyridine were refluxed for 5 hours. The reaction mixture was cooled, and water was slowly added until the sodium hydride was destroyed. The organic layer was separated and distilled to give 16 g of unchanged hexafluorobenzene. No other product was evident from the mass spectrometer analysis.[1]

High-Temperature Reactions of Hexafluorobenzene

The direct replacement of aromatic fluorine in ·hexafluorobenzene has hitherto been possible only by the use of nucleophilic reagents. In following investigation, the replacement of nuclear fluorine by nonnucieophilic, or weakly nucleophilic, reagents was achieved by reaction at relatively high temperatures, 300 to 850℃. [2]

Reaction With Bromine

In order to test the possibility that hexafluorobenzene can undergo a free-radical substitution reaction with bromine at elevated temperatures, both these reagents were simultaneously passed through a heated reactor tube under various conditions. The reactor tube was usually made of high-silicate glass and packed with borosilicate glass helices. Other reactor tubes employed were platinum, steel, and quartz. In addition to borosilicate glass helices, other packing material included glass wool, carbon pellets, platinum gauze, and nickel turnings. A few experiments involving unpacked reactors were also tried. The results in all these cases were similar. The examination of the pyrolyzates from several runs revealed the presence of significant amounts of bromopentafluorobenzene. In some cases, smaller amounts of polybromo derivatives of hexafluorobenzene were also noted. The conversion of hexafluorobenzene into products ranged from a few percent to about 25 percent. The yield of bromopentafluorobenzene, based on unrecovered hexafluorobenzene, was 85 to 95 percent. The yield of bromopentafluorobenzene, as expected, is dependent on such variables as (1) the temperature of the reactor, (2) the molar ratio of the reactants, (3) the contact time in the heated zone, and (4) the packing material and reactor material. The nature of the products is also dependent on the conditions of the reaction. For example, hexafluorobenzene and bromine, in the molar ratio of 1 to 3, when passed through a reactor packed with borosilicate glass helices, gave at 650℃an 8 percent conversion into bromopentafluorobenzene (95% yield).However, at 740℃ and with the other variables approxmately the same, the conversion into products was increased to 25 percent. Although bromopentafluorobenzene was still the major product (85% yield), other more highly brominated products began to appear.The yield of these products was about 10 percent,and the main constituent appeared to be, from an examination of its retention time on the vapor-phas echromatogram, one of the isomers of dibromotetrafluorobenzene.

Reaction With Perfluoroalkenes

The copyrolysis of hexafluorobenzene with perfluoroolefins, such as tetrafluoroethylene and hexafluoropropylene, resulted in excellent yields of octafluorotoluene. The conversion into products was of the order of 15 to 20 percent at temperatures of 800 to 850℃.

References

[1]Pummer WJ, Wall LA. Reactions of Hexafluorobenzene. Science. 1958;127(3299):643-644. doi:10.1126/science.127.3299.643

[2]Antonucci JM, Wall LA. High-Temperature Reactions of Hexafluorobenzene. J Res Natl Bur Stand A Phys Chem. 1966;70A(6):473-480. doi:10.6028/jres.070A.041