US2927138A - Method for the preparation of hexafluorobenzene - Google Patents

Description

NIETHOD FOR THE PREPARATION OF HEXAFLUOROBENZENE '5 Claims. c1. ido -650) a This invention relates to animproved methodfor the production of hexafluorobenzene and particularly contemplates the production of such compound from the pyrolysis of tribromofiuoromethane. I

In accordance with the principles of, this invention tribromofluoromethane is pyrolyzed in graphite-platinum, and nickel tubesat temperatures. ranging from 500 to 750 C. The resulting yield of hexafluorobenzene is superior compared to known methods of preparation.

Hexafluorobenzene is becoming increasingly important as a precursor for the preparation of thermally stable liquids and polymers and accordingly considerable research is being directed to the preparation of such compound. Previously hexafluorobenzene was synthesized by the fluorination of hexachlorobenzene with bromine trifluoride followed by treatment of the intermediate with antimony pentafiuoride and finally by dehalogenation with the zinc. Relatively small amounts of hexafluorobenzene were obtained in accordance with such process. I A more direct method gives higher yields was described by Y. Desirant in the Bulletin of the Academy of the Royal Belgian Classe Sci, In accordance with such described process tribromofluoromethane was pyrolyzed in a platinum tubeat temperatures ranging from 630640 C. The reported yield of hexafluorobenzene was 45 percent on the basis of the reactant tribromofiuoromethane. Our yields under these conditions were about 30 percent.

The present invention provides a convenient method for synthesizing larger amounts of hexafluorobenzene by the pyrolysis of tribromofluoromethane .by varying the temperature and pressure conditions of reaction.

It isaccordingly an immediate object of the'present inventionto provide an improved method 'for 'the'preparation of hexafluorobenzene. f

nited States meato "ice The pyrolysis reaction can be represented by the stoichiometric equation p a small amount of bromine.

, cocoanut charcoal was used as packing.

RESULTS Table I summarizes the results of all runs performed in graphite, Vycor, and platinum tubes. The data pre- 1 sented are yields of hexafluorobenzene (gross and net,

In accordance with such objective it is the purpose of this invention to provide relatively large yields of hexafluorobenzene from the pyrolysis of tribromofluoromethans;

suresabove atmospheric toabout 20 atmospheres. I

' It is a still further object of the present invention to columns 7 and 9), the recovery of unreacted tribromofluoromethane (column 8), and the amount of bromine liberated in the pyrolysis (column 6). In the graphite tubes with charcoal packing the best yield of hexafluorobenzene was obtained at the lowest temperature, about 650. This yieldwas 24 percent or 30 percent based on reacted tribromofluoromethane. Increasing temperatures lowered the yield of hexafiuorobenzene, although the amount of bromine liberated was increased. The two runs performed with platinum gauze as packing in graphite tubes showed a reverse trend inasmuch as the better yield was obtained at a higher temperature (about 750). A possible explanation of this may be that the tighter charcoal packing allowed for more contact time, which favored better reactions at lower temperatures, whileat higher temperatures this may .have given rise to :more side reactions.

The results of the pyrolyses in Vycor glass tubes show considerably lower yields of hexafluorobenzene, as well as poor recovery of tribromofluoromethane. Increasing temperatures again produce lower yields. ,As one might expect, the interaction of fluorine-containing materials with glass results in the removal of fluorine atoms and consequently poorer yields of fiuoro compounds.

Finally, a number of runs were performed in platinum tubes. The first three runs listed in Table I were done under nitrogen pressure of 1 atmosphere. The optimum yield obtained 'was 30 percent gross, or 32 percent net, at 600. However, a noticeable improvement in the yield was achieved by performing the pyrolyses under nitrogen pressure of 2-20 atmospheres. Under these conditions the efficiency of the reaction was improved as is evidenced by a lowered optimum temperature. .The highest yield of hexafluorobenzene was obtained at 540 to 550" with the net yield reaching about 55 percent. I

Table I PYROLYSIS OF TRIBRO MOFLUOROMETHANE Pres- Feed Total Bromine Gross Recov- Net Tube material Temp., sure, rate, recovliber- Yield, ery, Yield, 0. atm. gJmln. ery, ated, 05F5, CFBI's, CflFfl Percent Percent Percent Percent Percent Graphite I 642-654 1. 3. 90 63 24 18 30 D 1. 0 3. 5 96 85 23 8 25 Do 1. 0 4.1 93 91 19 1 19 Do a 96 a a a a 97 1.0 7.3 95 91 24 8 2e 1. 0 3. 4 88 85 11 1 11 Vycor 1. 0 4. 5 93 3 10 1.0 5. 1 89 4 1 4 1. 0 8. 3 97 30 14 55 31 1. 0 8.2 98 74 30 8 32 1. 0 8.9 93 89 18 0 l8 4. 5 0. 5 04 24 12 68 36 4. 5 5. 1 99 50 29 43 50 4. 5 7. 0 95 62 38 30 55 Platinum 650 4. 5 3. 5 96 67 40 52 580 4. 5 8.8 96 88 34 4 35 620 4. 5 7. 8 95 88 31 0 31 540-550 8. 3 l1. 7 98. 8 79. 6 41. 6 16. 5 49. 5 510-550 8. 3 11.7 97. 8 79. 0 41. 7 18. 3 50. 9 550-560 13. 3 l1. 7 95. 8 75. 6 37 10. 8 41. 4 550-550 17. 7 17.0 95. 2 83. 8 41. 1 8. 7 44. 8 540-550 17. 7 8. 5 95. 0 38. 35 25 50. 7 52. 3 Nickel 570-580 17. 7 8. 1 90.0 46.5 23. 6 40. 7 41. 2 590-600 17. 7 9. 2 90. 5 74. 8 29. 7 10. 7 33. 8

. These four tubes were packed with charcoal. All others listed were packed with platinum.

Tentative identification of some of the by-products was made by combining the high boiling fractions from all pyrolyses performed in graphite tubes and refractionating' them under reduced pressure. Samples of individual cuts were analyzed by a mass spectrometer. A number of compounds were tentatively identified. Some of these compounds, in order of their decreasing abundance, are listed below:

It appears that most of the by-products were bromofluoro derivatives of ethane and propane as well as benzene and toluene. The presence of the latter seems to indicate that occasionally fluorine was removed instead of bromine.

In the light of the foregoing evidence, the mechanism of the reaction is postulated, which consists of the following four steps:

Steps 1 and 2 should proceed rapidly at the temperature of the reaction. Increased pressure should have no effect on Step 1 and a relatively minor efiect' on Step 2. Step 3 would be the rate-determining step which should be Strongly pressure dependent. The postulated intermediate C F Br however, would be expected to be very unstable and decompose rapidly with the loss of bromine. One of the possible products of this decomposition should be hexafluorobenzene. The effect of presure would be to drive the reaction to the left but again the rapidity of the bromine removal could be expected to overcome the opposing effect of pressure.

The residues (B.P. 115) of all the pressure runs in the platinum tube were combined and subjected to a steam distillation. The volatile fraction (79 percent) was dried and pyrolyzed again under the optimum conditions listed above. The material recovery was essentially quantitative. The weight of bromine removed amounted to 34 percent by weight of the recovered products; A partial analysis of the remaining fraction. was as follows: 4.6, percent byweight hexafiuorobenzene. 15.5 percent bromopentafluorobenzene, and small amounts of some c F Br product.

From these results it is quite evident, then, that the serted between the. traps.

residues obtained from the initial pyrolysis of CFBr still contain intermediates, suchas CFBr=CFBr and CFBm-CFBr which are capable of producing more hexafluorobcnzene by further concentration and pyrolysis. This, however, improves the net yield of'C F by only a few percent.

Experimental procedure T ribr0m0flu0r0methane.-This compound was prepared essentially by using stoichiometric amounts of tetrabromomethane and antimony trifluoride in the pres- 'cnce of a small amount of bromine. Standard glass equipment was used. By refluxing for 4-6 hours from a bath kept at to and subsequent distillation, yields of 60 to 70 percent were obtained. The product was purified by washing with sodium bisulfite and sodium hydroxide. After drying over calcium chloride the product was fractionated in a four-foot column.

- EXAMPLE I H exaflubrabenzene.--Pyrolysis of tribromofluoromethane was'performed in a platinum tube 80 cm. long, 1 cm. wide and about 1 mm. thick. An electrically heated furnace was wound around this tube and-the temperature of the furnace was controlled by an automatic relay. The entrance and exit of the tube were. cooled by watercooled brass condensers. Tribromofluoromethane was introduced from a reservoir by means of a solenoid pump. The outlet of the pyrolysis tube was connected to an icecooled trap followed by Dry Ice traps. For the runs performed at higher pressures a capillary tube was in This proved to be adequate for keeping the system under pressure and ensuring a slow and even flow rate. In a typical run tribromofluoromethane was introduced into the reservoir, and a slow stream of dry; nitrogen was passed through the system while the furnace was brought to the desired temperature- The relay controlling the addition of tribromofluoromethane was then activated and the reagent added at a constant rate. After completion of the addition the product in the receiver and traps was carefully'washed with sodium bisulfite and dilute sodium hydroxide. The loss of bromine was determined by weighingthe product before and after. washing. After: drying, the washed product was fractionated. tov recover hexafluorobenzene (13.1. 79 to 81) and unreacted tribromofluoromethane (B.P. 106 to. 108.. The. residues were saved for further purificadon, V

nxmm 1'1 The experiments in graphite and Vycor tubes were performed in similar apparatus. The tubes were inserted vertically into a ceramic furnace with the inlet and outlet cooled by brass condensers. The dimensions of the tubes were 86 cm. long, 1.3 cm. wide and 2 mm. thick. The

heated length of the tubes was 60 cm. The graphite tube was inserted into a tight fitting iron tube to ensure even heating and to protect the tube from breaking. At the top of the tube a dropping funnel with a bypass tube was inserted by means of a rubber stopper. The outlet of the pyrolysis tube was connected to two traps, one cooled in ice, the other in Dry Ice. Again dry nitrogen was passed through the system during the run. The rate of addition was controlled by setting the stopcock of the dropping funnel. The product was worked up as described above, with the additional step of combining all residues and fractionating them under reduced pressure to obtain samples for mass spectrometer analysis.

It is to be understood that the invention is not limited to the specific examples herein,-except as defined in the appended claims.

What is claimed is:

1. The method of preparing hexafluorobenzene comprising pyrolyzing tribromofluoromethane in a reaction chamber at a temperature range of from 530 C. to 550 C. under flowing nitrogengas at pressures from 4.5 to 17.7 atmospheres for a time suflicient to convert from 60 to 90 percent of the tribromofluoromethane.

2. The invention of claim 1 in which the pyrolyzation is conducted in a platinum reaction chamber at a tem perature range of 530 to 550 C. and in which the pressure of said flowing nitrogen lies within the range of from 4.5 to 10 atmospheres.

3. The invention of claim 2 in which said pyrolyzation is conducted at a temperature of about 540 C. and at a pressure of about 4.5 atmospheres.

4. The invention of claim 2 in which said pyrolyzation is conducted in a temperature range of from 540 to 550 C. and at a pressure of about 10 atmospheres.

5. The invention of claim 1 in which the pyrolyzation is conducted in a nickel reaction chamber at a'temperature range of from 540 to 550 C. and the pressure of said flowing nitrogen is about 17.7 atmospheres.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Desirant: Bull. Classe Sci. Acad. Roy. Belg, vol. 41,

Claims (1)

1. THE METHOD OF PREPARING HEXAFLUOROBENZENE COMPRISING PYROLYZING TRIBROMOFLUOROMETHANE IN A REACTION CHAMBER AT A TEMPERATURE RANGE OF FROM 530*C. TO 550* C. UNDER FLOWING NITROGEN GAS AT PRESSURES FROM 4.5 TO 17.7 ATMOSPHERES FOR A TIME SUFFICIENT TO CONVERT FROM 60 TO 90 PERCENT OF THE TRIBROMOFLUOROMETHANE.