PRODUCTION OF AROMATIC POLYFLUORO COMPOUNDS FROM CHLORO= FLU0 ROCYCLOH EXA N ES PETER JOHNCOCK,
R. H . MOBBS, AND W .
K .
R. MUSGRAVE
Chemistry Department, The Uniuersity, South Road, Durham, England
Chlorofluorocyclohexanes, containing one to six chlorine atoms, can b e prepared from hexachlorobenzene and CoF3. Some of these are also obtained b y the action of ClFB on benzene, but in this case the product also contains some chlorofluorohydrocyclohexanes. The chlorofluorocyclohexanes are dehalogenated easily b y passing over hot iron gauze a t 430" C. and give good yields of hexafluorobenzene. However, when they are mixed with chlorofluorohydrocyclohexanes, a higher temperature (500" C.) must be used, and the yield of aromatic material i s lower. The mixtures used gave hexa-, penta-, tetra-, and trifluorobenzenes, but by suitable choice of starting material the product could be mainly o mixture of easily separated hexaand pentafluorobenzenes.
ARLY ATTEMPTS
to prepare highly fluorinated arcmatic
E compounds depended on the Schiemann reacrion;
ho\vever. no more than four fluorine atoms can be introduced into the benzene nucleus in this way ( 9 ) . There are now several methods available for preparing hexafluorobenzene. I n very low yield it may be made by treating hexachlorobenzene with BrF3 and the product with SbFj to obtain a mixture of average composition C6BrCl1F,. Dehalogenation of this with zinc dust and ethyl alcohol gives a little hexafluorobenzene ( 75- 77). Pyrolysis of tribromofluoromethane in a platinum tube gives a 55% yield. but this method can be used only on a small scale ( 3 , 7, 72). T h e pyrolysis of chloro1,2-difluoroethylene will give hexafluorobenzene in 147, yield, but the preparation of the starting material is tedious ( 4 ) .
The only method so far capable of application on a larger scale (6, 70, 77) involves the direct fluorination of benzene to give polyfluorocyclohexanes. follo\\.ed by their defluorination by passing over heated iron gauze. In this process dodecafluorocyclohexane gives hesafluorobenzene, while undecafluorocyclohexane gives a mixture of hexafluorobenzene and pentafluorobenzene: dehydrofluorination occurring as \vel1 as defluorination. The weakness in this last method lies in the wastage of fluorine which has to be put into the molecule only to be taken out again. The authors felt rhat hexachlorobenzene should be the starting ma.teria1, provided that it could be fluorinated simply by addition to the double bonds and then dechlorinated to leave hexafluorobenzene. This simple addi-
TRAP COOLED BY
SOLID CQAND
Nz FROM FLDWMETER
DUST PREClPlTAT
PR E H €ATE R
COPPER VESSEL CONTAINING C,Cl,
Figure 1.
Apparatus used for vapor-phase fluorinaiicn of hexachlorobenzene b y CoF3 VOL
1
NO. 4
OCTOBER
1962
267
Table 1.
Typical Experiments Showing Composition of Product from Action of CoF3 on Hexachlorobenzene Nitrogen flow rate varied from 1 to 6 liters per hour according to temperature of CGCIGreservoir
Run No.
8 a
Reaction Time, Min. 120 120 100 105
C6Cl6, Grams 106 97 91 93
Product Weight, Grams 115.5 104.0 97.0 101 . o
C 8 i iC1 6.0 2.6 2.7 4.5
CsFioClz 20,5 21.2 23.2 26.8
Percentage Compositiona C6F9ClS CsFsCla 40.3 30.2 41 .O 27.5 27.7 40.0 40.1 22,9
c87cla 3.0 7.6 6.3 5.6
Calculated from gas-liquid chromatografhy peak areas.
tion of fluorine does not occur in the reaction between hexachlorobenzene and BrFa-SbFs described by McBee and others (75-77)as> after dehalogenation, a large part of the product contains chlorine. Possibly, this liquid-phase halogenation has proceeded by the same mechanism as that described by Leffler ( 7 4 ) for the action of SbFS alone on hexachlorobenzene, with the formation of gem-dichloro groups in the molecule and their retention during the fluorination process. F2
c1
F?
F?
Dehalogenation of the products with zinc dust must then leave some chlorine in the molecule, presumably because the conditions of reaction are so mild that the intermediates cannot isomerize in such a \yay as to encourage the maximum elimination of chlorine. Reaction between CoF3 and Hexachlorobenzene
Apparatus a n d Procedure. I t seemed possible that the mixture of chlorofluorocyclohexanes obtained by the vaporphase fluorination of hexachlorobenzene by COF3 a t 350" C., in a n apparatus (Figure 1) similar to that described by Barbour and others (2) might satisfy the requirement that all of the chlorine atoms left in any of the molecules should be on different carbon atoms. The mixture was obtained by heating hexachlorobenzene to 350' C . in a copper vessel and distilling it into the reactor on a slow stream of nitrogen (2 to 6 liters per hour). The reactor, which contained 1 kilogram of COF3, was maintained a t a temperature of 350' C. as far as possible by a series of electrical heaters. As the reaction proceeded, a localized hot zone passed along the reactor, its position readily detectable
by a thermocouple which could be moved along the hollow axis of the stirrer. Its arrival a t the end of the reactor signified that the available fluorine had been used u p ; the supply of hexachlorobenzene was stopped, and the residual products were s l e p t from the reactor by a stream of nitrogen. Regeneration of the COF3 was carried out with ClFB (78) until the emergent gas spontaneously ignited a drop of acetone. The temperature of the hot zone \vas normally kept a t 30" C. above that of the rest of the reactor by regulating the nitrogen stream as well as the input of hexachlorobenzene, which, when excessive. led to a much higher. hot zone temperature and greater decomposition. \Vhen the temperature of the hot zone was kept at 20" to 50' C. above that of the reactor. the composition of the product remained more or less the same (Table I ) and c'ici not var)- appreciably e;.en \\.hen nitrogen \vas added to increase the dilution of the reaction mixture. Separation of Product. The product. a yellow liquid containing crystals of a solid with a very low melting point, smelled of chlorine; it was washed with alkaline sulfite ( 0 . 1 5 in both N a O H and Na*S03), warm water, and dried over MgSOI to give a waxlike solid melting in the range 20' to 25" C. Each run was monitored by gas-liquid chromatography (Table I), using silicone high-vacuum grease at 190" C. as the srationary phase. The products were then combined and fractionally distilled through a 20-plate concentric-tube column which gave a complete separation of all of the compounds of formula C6F12--Cln where n = 1 to 5 (see Table 11). The small residue in the distillation flask was analyzed for CsFsCls. Dehalogenation of Chlorofluorocyclohexanes
Apparatus a n d Procedure. The dehalogenator was similar to that described by Gething and others (10); being a mild steel tube 44 inches long and 2 inches in diameter but differing slightly in being packed with 1260 grams of iron gauze in a tightly Jvound roll. Electrical heating maintained the
Table II.
Composition of Total Product (897 Grams) from Action of CoF3 on Hexachlorobenzene after Distillation through Concentric Tube Column Analysis B.P., O C. Wt., Found Calcd. Fraction at M m . HZ no at O C. Grams Comjound Cl F Mol. wt. Cl F M o l . wt.
1 2 3 4 5 Residue
79.5-79.7(761) 109.1-110.1(767) 140.8-141.2(758) 173-175.3(767) 208.7-209.2(758)
1,297(45) 1.330(45) 1.362(45) 1.387(60)
23.2 165.0 310.9 223.0
...
49.0
16 78
Intermediate fractions a
268
After single distillation from flask with side arm.
I&EC
PROCESS DESIGN A N D DEVELOPMENT
C,Fi,C1 CoFiUCl? CGFgCI, CgFaC14 CsF,Clj CsFGC16
11.3 21.2 30.6 38.8 46.3 51.0Q
65.7 57.1 48.6 41.2 35.0 29.2a
. ..
341 357 376 397
11.2 21.3 30.5 38.8 46.4 53.3
66.0 57.1 48.9 41.6 34.8 28.6
316.5 333 349.5 366 382.5
~~
Table 111.
Products Obtained by Dehalogenation of Fully Halogenated Cyclohexanes
Products (Gas-Liquid Chromatography, Separation), Grams Starting Time of Wt. Starting material Others iV2 Reactor Introduc- Starting Wt. Material, Cyclic tion, Material, Product, Cyclic Cyclic Cyclic Cyclic rematerials Cyclic Ttmp., Flow, Min. C. Grams Grams csF10 1,4-C~F8 C6F7Cl 7,3-CBF8 C6Fs covered present C6F12-,CI7, M l . / M i n . 25 625 120 9.70 ca.0.02 ... ... ... ... 0.02 c6F 1 i c l 25 350 150 3.78 2.60 ... ... ... ... ... 2.60 0.93 30 430 195 10.40 3.70 1.29 1.65 0.05a 25 430 120 9.80 3.81 1.21 0 , 33b8C 1 , 7BbtC 0 . 86b8c 2.31 30 375 65 22.70 16.50 4.59 125 19.00 7.08 5.66 0.16a 30 430 25 12.30 6.19 25 430 0.72 2.3767' 0 . 36brd 0.72 35 30.00 21.80 0.07 30 340 0.77 11 . 1 4 V 0 . 93btc 0.47 6.10 2.02 70 11.04 4.10 25 420 3.90 0.05a 2.11 0.11a 90 10.00 2.22 30 420 Under conditions uspd for the preparaa Same matprial in each case; its retention time on tricresyl phosphate at 94" C. slightly greater than that for C6Fs. Components and amounts of each present estimated by analytical gas-liquid tive gas-liquid chromatography separation these materials collected together. Amounts of two components present calculated from halogen analysis figures after infrared spectroscopy had chromatography carried out at lower temperature. s h o u x presence of both.
central 20-inch portion of the tube a t a temperature sensibly constant throughout a run (Table 111). The material was introduced on a stream of nitrogen, by distillation into the stream if a solid or by flash vaporization if a liquid, and the product was condensed out in two traps cooled by liquid air. The nitrogen flow rate, measured by a n oil-filled U-tube flowmeter, was maintained constant throughout a run. Regeneration of the clean metal suiface after a dehalogenation was carried out by passage of hydrogen a t 400' to 600" C. until H F was no longer evolved. Separation of Product. The products were analyzed by analytical gas-liquid chromatography and separated by preparative gas-liquid chromatography (using dinonyl phthalate or tricresyl phosphate as stationary phases). Their identity was established by halogen analysis, infrared spectroscopy, and coincident gas-liquid chromatography retention times with authentic samples where possible. The dehalogenation conditions and the products obtained, with analytical data, are given in Table 111. Results and Discussion. At reaction temperatures of 430 C..provided that the material was distilled into the dehalogenator sufficiently slo\vly> decafluorocyclohexene and hexafluorobenzene were the only significant products from the monochloride and dichloride. Hexafluorobenzene alone was obtained from the tri-, tetra-. and pentachlorides. The weaker C-C1 bond (compared with the C-F bond) permits aromatization of these compounds to occur at lower temperatures than the saturated perfluorocyclohexanes. For example, cyclic C S F Irequires ~ reaction temperatures of about 550' C. or more for its defluorination to hexafluorobenzene (6). lntroduction of double bonds does cause more ready defluorination and a consequent decrease in the temperature required-e.g., cyclic CsFlo will defluorinate to hexafluorobenzene at about 5 10 ' C.?while the two octafluorocyclohexadienes defluorinate readily to give hexafluorobenzene in good yield at temperatures between 400' and 450' C. (6). T h e monochloroundecafluorocyclohexane? by loss of chlorine and a n adiacent fluorine atom, forms decafluorocyclohexene which is relatively stable to further defluorination at 430' C., and hence appears in the product. Probably most of the perfluorocyclohexene present in the product from the dichloride comes from adjacent chlorine elimination from the 1,2-isomer, since the octafluorocyclohexadienes formed by chlorine and fluorine elimination should be easily defluorinated to give hexafluorobenzene at 430' C.
The exclusive formation of perfluorobenzene in these dehalogenations indicates that there may be no gem-dichloro groups in the chlorofluorocyclohexanes concerned but does not form proof that this is so. Tatlow and others (6, 7 7 ) have shown can isomerize under that 1-H-heptafluorocyc1ohexa-1~3-diene the conditions used and then eliminate HF to give perfluorobenzene.
F
H
H
F
F
F
F
H
F
Therefore, the following and other similar reactions involving gem-dichloro- groups can no doubt occur. c1
Clr
CIF
F
No chloropentafluorobenzene was detected in any of the products, but small amounts of a chlorine-containing material with a retention time (on tricresyl phosphate) just greater than that of hexafluorobenzene were found in all products except that from the monochloride. I t was not present in large enough amounts for isolation and characterization. O n this basis, those compounds containing two or more gem-dichloro groups would almost certainly give aromatic products containing chlorine, although with both in the 1,2position all of the chlorine could be eliminated :
c1
Cl?
CIS F2@
+ cl:@
+
Fn
c1
\
VO1. 1
+ Fz
F2
c1
F
c1
NO. 4
OCTOBER 1 9 6 2
269
Table IV.
Products from Dehalogenating Fractions from Chlorofluorination of Benzene
Starting Material Fraction B.p., a C.
(11)
100 O - 1 1 0
(111)
110 O-120 O 120°-1300 130 "150' 150°-165' 165 "(760 mm. Hg)60 O ( 3 mm. Hg)
