A. K. Barbour
P. Thomas
HIGHLY FLUORINATED AROMATIC AND ALICYCLIC COMPOUNDS of practicable synthetic routes a vast Development new range of highly fluorinated cyclic compounds to
has turned industrial attention to their potential application. Current studies d m t e d by development of new knowledge concerning synthetic mutes to the basic fluorocarbon building blocks, the substitution chemistry of fluoroaromatic ring systems, and the end-use patterns for these compounds raise hopes for their industrial use. Application Potential
The relatively high cost of the basic building blocks such as hexa- and pentafluorobenzene (in the range of $2.00 to $3.00 per pound) is at present the l i i t i n g factor which may regulate the industrial destiny of highly
fluorinated hydrocarbons. The apparent potential for these compounds puts them in the relatively high cost specialty chemicals category. Possible applications include formulation of specialty high polymers, functional fluids such-as lubricants and hydraulic fluids, surfactants, pharmaceuticals, agricultural chemicals, dyestuffs, and rubber additives. Table I compares some of the typical physical properties of hexafluorobenzene and two typical alicyclic fluorocarbonswith their hydrogenic analogs. The fully fluorinated compounds have two characteristics which set them apart from the hydrocarbons: low boiling points for compounds of such high molecular weight and their high densities. In addition, they possess
CAI. HIGHLY FLUORINATED HYDROCARBONS
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%
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Increased industrial use of high& Juorinated aromatic and bdrocarbons draws attention to practicable Droduction methods presented here excellent electrical characteristics. On the other hand, the fully fluorinated alicyclics are distinct from both the fully fluorinated aromatics and the hydrogenic compounds in exhibiting very low surface tensions and being relatively poor solvents. Table I1 summarizes the chemical properties of these compounds and shows that fully fluorinated compounds are essentially nonflammable, possess excellent thermal and radiation stability, and are inert except for some pyrolytic reactions mentioned later. Hexafluorobenzene is reactive toward nucleophiles but inert to electrophiles whereas benzene is inert to nucleophiles but attacked by electrophiles. The relationship of these chemical and physical properties to the potential end-uses leads to the following principal categories of application : High Polymers. I t should be chemically possible to make high molecular weight polymers with good thermal and radiation stability from fluoroaromatic building blocks. It is perhaps unlikely that such polymers would possess outstanding solvent resistance or surface properties. Polymers containing completely fluorinated cyclohexyl groups might be good from the stability, solvent, and surface viewpoints, but cannot be made from fully fluorinated alicyclic starting materials. Low Polymers. Similarly, there is potential for manufacture of low polymers based on fluoroaromatic building blocks; these might be suitable for use as lubricants, hydraulic and gyro fluids possessing good thermal and oxidative stability, and reasonable viscosity/temperature characteristics. Structures containing perfluorocyclohexyl rings might be dubious for these applications because of a poor viscosity/temperature relationship. Heat Engine Fluids. Excellent chemical stability, combined with high molecular weight, suggests the application of both fluoroaromatics and fluorinated alicyclics of moderate boiling point as working fluids in small heat engines. Dielectric Fluids. Good electrical characteristics suggest the use of both fluoroaromatics and fluorinated alicyclics as dielectric fluids ; as evaporative or convec-
tive coolants, low surface tension, low viscosity, and high volume expansion coefficients are particularly useful. Essentially all other potential end uses for these compounds depend on elaborating molecules containing fluorinated rings through classical synthetic reactions and most work to date has been done in the fluoroaromatic field. This synthetic work is detailed in this article. Synthesis of Fluorinafed Aromatics and Heferocyclicr
Processes of three general types have been used : pyrolysis of simple fluorohalogenomethanes, ethanes and ethylenes; fluorination/dehalogenation methods, in which the starting material is usually a hydrocarbon aromatic; and exchange fluorination of perhalogenoaromatics. Pyrolytic Methods. Despite a basically unfavorable molecular weight situation, such processes would clearly be the methods of choice if based on cheap starting materials and if developed to the point where competitive efficiencies are obtained. The economics are obviously damaged if expensive starting materials are used, or if low efficiencies are obtained; this is particularly so where the second factor makes it difficult to isolate pure product by fractional distillation. Equations l (9, 21, 35), 2 (2, 47, 67), and 3 ( 1 7 ) illustrate typical pyrolysis processes. Fluorination/Dehalogenation Methods. Such methods are illustrated in Equation 4 which results from the work of Professor Tatlow’s school at the University of Birmingham. $ince the simultaneous discovery at Birmingham and at Avonmouth of the aromatization stage in route 4B, considerable development work has been done at Avonmouth on processes of this type. For example, the use of magnetite, FeeOd, for the aromatization stage gives appreciable operating and economic benefits (49). This is an entirely practicable process for the commercial scale production of hexaand pentafluorobenzene, although it suffers from the basic economic defect that about half of the input fluorine is degraded to hydrogen fluoride. A modification of this approach by Musgrave involves the chlorofluorination of benzene using chlorine VOL. 5 8
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49
Direct halogen exchange in
I
I
41 ,
fluoride, followed by dechlorofluorination(6, 54,58). In this process, the final aromatization stage is mostly effected by dechlorination, and hence the economics could ultimately be better than the fluorination/defluorination approach shown in Equation 4. Apart from the economic aspects, the fluorination/ defluorination approach is also a general route to the synthesis of fluoroakyl-substituted fluorobenzenes, fusedand multiring fluoroaromatics and highly fluorinated heterocyclics, as shown in Equations 5 through 8. Specific examples of basic ring systems now include toluene (ZS), xylenes (28),naphthalene (28),biphenyl (ZS),indane (34),acenaphthylene (a), anthracene (34), pyrene (34),and pyridine (4, 75). Direct Exchange Method. A basic defect of the fluorination/defluorinationapproach is that the aromaticity of the organic starting material is destroyed in the initial fluorination stage and then has to be re-created by methods which degrade the fluorine to hydrogen fluoride. Methods based on the direct exchange of halogen atoms in a perhalogenoaromatic avoiding this difficulty have been studied. In 1956, Finger, at the Illiiois Geological Survey, showed that in cases where aromatic chlorine atoms were activated (e.g., by the nitro groups in 2,4dinitrochlorobenzene), exchange by fluorine could be effected successfully by potassium fluoride in an aDrotic solvent such as dimethvl fonnamide at 100’ C. (%,24). See Equation 9. Sdbsequently, Wallenfels in Germany showed that KF could be used to convect tetrachloroanil to tetrafluoroanil using KF in the absence of solvent (36, 74). Researchers at Du Pont announced at the Estes Park Fluorine Symposium in 1962 (57) that the exchange fluorination of hexachlorobenzene proceeded succe8& fully using potassium fluoride in a wide range of aprotic solvents, but at that time they reported that the main product from such reactions was trichlorotrihorobenzene with smaller proportions of dichlorotetrafluorobenzene. Prior to this announcement, work in the Imperial Smelting laboratories (SO),reported in detail at the Munich Symposium in September 1965, demonstrated that, using sulfolane as the reaction medium, excellent organic recoveries could be obtained from the fluorination of hexachlorobenzene with potassium fluoride (Equation 1OA) and the major products were 1,3dichlorotetrafluorobenzene and chloropentahorobenzene and only traces of hexafluorobenzene. Workers at Imperial Chemical Industries have also reported essentially the same results (47). Significant quantities of hexafluorobenzene could not be made by this method, although cesium fluoride in sulfolane provides an effective system for convecting chloropeatafluorobenzeneinto hexafluorobenzene (50).
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