Scientific and UtiUitarian Value of
FLUORINE CHEMISTRY J. H. S'imons FLUORINE LABORATORIES, T H E PENNSYLVAKIA STATE COLLEGE, STATE COLLEGE, P A .
THE abundance of fluorine on the earth is comparable to
reactions. The fluorocarbons show promise for many uses. They are important tools for studies of molecular forces and structure of carbon compounds. The structure of hydrogen fluoride is still challenging, and studies of liquid hydrogen fluoride may help clarify the problems of associated liquids. Organic fluorine compounds have been made in limited number. New methods of synthesis or adaptation of older techniques may provide a greater variety. Both the mechanism of reaction and the structure of organic compounds may be thus aided. A great number and variety of inorganic compounds are known. Those formulas not duplicable with other elements offer possibilities for important scientific studies as well as specific uses not available with other compounds.
carbon, nitrogen, and chlorine. It is the most electronegative element, and its compounds exhibit extreme properties important in the study of many fundamental scientific problems and utilitarian use. Comparisons of the chemical and physical properties of certain fluorine compounds with analogous compounds of the other halogens or hydrogen show significant facts which are the basis for the present uses of fluorine compounds and open up many scientific problems. The catalytic properties of some fluorine compounds have already found extensive use. The theory of this property is important, but in addition the employment of these substances enables important studies to be made of the mechanism of organic
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HIS paper will deal chiefly with the scientific value and significance of fluorine chemistry but will indicate the present and potential uses as these stem from the fundamental groundwork. The commercial applications of fluorine compounds are excellent examples of the necessity of fundamental studies and show how these are based upon and grow from academic research. Recent press releases from industrial companies and the papers of this symposium indicate many of the future promising uses of fluorine compounds. These uses have been known, predicted, and discussed by the academic scientific investigators for many years. Since the isolation of elementary fluorine by Moissan (14) a t the end of the last century, attention has been focused on the extreme properties of fluorine and its compounds. Because of the special techniques required and the unavailability of presentday laboratory devices, the work of the investigation of fluorine chemistry was carried on by only a few courageous individuals. Chief among these were Moissan, collaborators, and followers in France, Ruff, collaborators, and followers in Germany, and Swarts in Belgium. Interest in this field began to increase more rapidly after about 1920 when new workers, particularly in this country, began research. Their activities and predictions that the properties of fluorine compounds would make them valuable for a large number of uses stimulated interest and led to the consideration of the uses of these substances for many purposes a t the beginning of the recent war.
liable, and assumptions of reactions based upon analogy of reactions of similar nonfluorine containing compounds are likely to be incorrect. As a member of the top row of the Periodic Table, fluorine (and its compounds) can be expected to have unusual properties and also to have many uses, as is the case with the other elements in the first row. Fluorine forms compounds with all elements except the inert gases, and many of these compounds have properties which would hardly be anticipated from a casual consideration of the other compounds of these elements. Many fluorine compounds are extremely stable; nevertheless, some of them, like hydrogen fluoride, are extremely reactive. The fluorine molecule is very stable but under proper conditions is reactive in the extreme. This is the reason why reactions of elementary fluorine are difficult to control and why catalysts are important in reactions of the element. Although the reaction between fluorine and hydrogen is very energetic, it is difficult to control. In determining the heat of this reaction (as), an electric discharge a t the site of the flame was found necessary to prevent flame extinction and subsequent explosion. This is an example of a reaction with a high energy of activation in addition to a high energy of reaction. The same situation exists in the reaction of fluorine with hydrocarbons, and headway \vas not made with gas-phase fluorination of hydrocarbons, as a result of explosions and complete fragmentation of the hydrocarbon, until the reaction was performed in restricted volumes with conducting walls such as were provided by copper gauze (2, 8) or in the presence of selected catalysts. Fluorine is not a scarce element on the earth's surface; it is about as abundant as either nitrogen or carbon and more plentiful than chlorine on the land masses. From the standpoint of abundance and availability alone it could be expected that fluorine compounds would have extensive uses, even disregarding their extreme properties which make it impossible in many cases to duplicate them with other combinations of elements. All ultimate uses of fluorine compounds will depend upon their unique physical and chemical properties. Fluorides are frequently lower melting and lower boiling than other compounds of the same elements. The higher fluorides of many of the heavy elements, such as uranium, are either gases or readily volatilized
FLUORINE
Fluorine, the first member of the halogen family of elements,
is the most electronegative element, exceeding in this respect its nearest competitors, chlorine and oxygen. Quantitatively, the potential of the normal fluorine electrode is given as 2.85 volts (11); those of chlorine and oxygen are, respectively, 1.36 and 1.22. These figures are sufficient to show not only that fluorine is more electronegative than any other element but that the difference between it and its nearest neighbors is very large. From this we should expect to find the properties of fluorine compounds and their reactions not slightly but greatly different from those of other electronegative elements. Extrapolation of the properties of the other halogens to those of fluorine is likely to be unre-
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liquids a t room temperature. The fluorides of many of the more metallic elements are relatively low melting and find use in ceramic materials for that reason. Much of the interest in fluorine compounds, however, lies in their chemical properties. Most of them are very stable. Some are extremely unreactive, some react rapidly, and others occupy an intermediate position. Some fluorine compounds are of interest for purposes of ultimate use, whereas it can be expected that others will be employed as intermediates in making nonfluorine-containing products. Certain fluorine compounds have been widely used recently because of their catalytic properties. Boron trifluoride and hydrogen fluoride are well known as catalysts for certain organic reactions. Fortunately academic research had discovered the catalytic value of hydrogen fluoride (18) long enough before the start of the recent war so that it could be used for many important purposes; the production of aviation gasoline is one example.
this study were the value of basic promoters such as water, methanol, etc., and also the action of hydrogen chloride as an apparent retarding agent, Attempting to correlate the known facts about these substances and this reaction resulted in effectively discarding all mechanisms which involved an identifiable intermediate such as an ion, a free radical, or a molecular complex. This places a strain on previous theories of this type of reaction. It led to a theory of mutual attack by both acidic and basic molecules or, as named, “the amphoteric medium effect”. This study may open the way to an understanding of alkylation and related reactions. This particular reaction is an unusually good one for intensive study as its properties make for precise measurement. I t is homogeneous, proceeds a t a convenient rate a t ordinary thermostat temperature, can be followed for rate determinations by pressure measurements, and proceeds to completion in the production of one organic product.
HYDROGEN FLUORIDE
FLUOROC4RBONS AYD DERlVATIVES
One of the most important compounds of fluorine is hydrogen In the combinations of fluorine with carbon many compounds fluoride, just as one of the most important compounds of oxygen of unusual properties and considerable interest, have already been is water. Liquid hydrogen fluoride shows greater similarity to prepared. The fluorocarbons are characterized by great thermal water in many respects than it does t o hydrogen chloride. It is stability and resistance to chemical action. The possibility of an excellent ionizing solvent, has a relatively high density ($(I), this extensive field of new compounds occurring in many homolohas a boiling point which is high for its molecular weight, has a gous series was first seen when the products of the reaction of the high dielectric constant, etc. It has been generally assumed that elementary substance carbon and fluorine, toned down t o feasible these properties of water, hydrogen fluoride, and other related operating conditions with a little mercury catalyst, were first liquids are due to association caused by hydrogen bonding, collected, separated, and identified (19). The fluorocarbons Hydrogen fluoride, however, is very different from water in have nearly the same freezing and boiling points as hydrocarbons certain properties such as surface tension (20)and viscosity (21). of the same structure and number of carbon atoms. The boiling No adequate explanation is as yet available. Much valuable points are extremely low relative to the molecular weight, coming scientific work has been accomplished by the study of liquid closer in this respect to the inert gases than any other type of water and its solutions. Work on liquid ammonia, a related chemical compounds. The fluorocarbons have relatively high substance, has contributed to the knowledge of this type of solfluidities and low surface tensions. They appear to be subvent. Hydrogen fluoride will contribute still further for H3X, stances with a minimum of attractive force between molecules. H20, and HF are hydrogen compounds of the three neighboring Compared to hydrocarbons, they are completely resistant to oxielectronegative elements of the first row of the Periodic Table dation and do not burn. Their liquid densities are somewhere and constitute an important class of liquids. Although hydrogen between 50 and 100% greater than corresponding hydrocarbons. fluoride is an apparently weak electrolyte in water, water is a They resist the action of most chemical reagents, even the alkali strong electrolyte in hydrogen fluoride; another observation is metals, up to relatively high temperatures. It is fortunate for that hydrogen chloride is very soluble in water but quite insoluble the ultimate use of these valuable and unique compounds that in hydrogen fluoride. A naive but inadequate explanation for means of preparing them, other than the initial one of reaction these effects was previously given (17). between the elements, have been and are being found. It was The structure of chemical compounds is of great importance to the initial fundamental academic research, however, that esall concerned with chemistry. I n so far as theories correlate tablished their possibility and pointed out their potentially properties, they are acceptable, but a substance which is either valuable properties. difficult to correlate with published theories without undue strain The possibilities and properties of these compounds indicate or is in apparent contradiction t o them is an immediate challenge their eventual use as heat transfer and dielectric media, fire exto the theory and its development. Hydrogen fluoride is one of tinguishers, turbine impellents, high temperature lubricants, the few such substances. The combination of similarities and thermal and chemical resistant plastics, etc. These possibilities differences between the properties of liquid hydrogen fluoride were pointed out and the first sample of liquid fluorocarbon was and those of water are important in theories of the liquid state. supplied to agents of the Government prior to the beginning of The vapor density of hydrogen fluoride has been shown to be the war, and further experimental work in a number of other correlated on the basis of a n equilibrium between monomer and laboratories was undertaken. I n a field of this nature, however, polymers (23). The structures of the polymers is still uncertain where the products are new and different and where new methods although both electron diffraction (I) and x-ray diffraction (9) of synthesis need to be invented, progress is slow and an enormous studies have been made. amount of research and development is required. Some results The mechanism of chemical reactions is second in importance were obtained and some of these compounds were employed for to no other branch of chemistry. The employment of catalysts a number of purposes, chiefly experimental. depends upon mechanism. From considerations of the mechaThe fluorocarbons offer the possibility of studying many nism of acid catalysis in alkylation and related organic reactions important theoretical problems. Comparison of their properties we first arrived a t the idea of using hydrogen fluoride aa a catawith those of the corresponding hydrocarbons may shed light lyst for these reactions. We viewed hydrogen fluoride as a most upon many aspects of the nature of hydrocarbons and the carbonpowerful acid in the anhydrous state despite its apparent weakcarbon linkage. In the determination of structure by diffracness in aqueous solution. Subsequently i t has been shown that tion methods the fluorocarbons present an opportunity not availhydrogen chloride (88) and trifluoroacetic acid (16) are also able for the hydrocarbons. Because the mass of the fluorine catalysts under suitable conditions. atom is closer to the mass of the carbon atom than is that of An intensive study has been made of one alkylation reactionhydrogen, more complete patterns will be found for all the disthe alkylation of toluene by tertbutyl chloride with hydrogen tances and angles in the determination of structure. Electron fluoride a? catalyst (16, $5). Significant facts brought out by
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diffraction studies of chlorine-substituted methanes with progressively increasing number of fluorine atoms ( 5 ) seems t,o indicate that the carbon-fluorine distance is actually shortened with increasing number of fluorine atoms. Similarly, studies of hexafluoroethane (6) have indicated that the carbon-carbon distance is different in this compound from that in ethane. The theoretical implications of this phenomenon are considerable, and they begin to show the limitations of many of our widely used theories of additive properties. Further work along these lines should lead t o more information relative to the structure of carbon compounds and to the bonding conditions between the atoms in the molecule. For studies of the force relations, molecule t o molecule, we have long desired substances, liquid at room temperature or above, which are very stable and unreactive and which approach the inert gases in physical properties. These would be valuable to give a fundamental criteria for theories of the liquid state, solubility, etc. The fluorocarbons will provide compounds for these important st,udies. Precise studies of the Raman and infrared adsorption spectra of hexafluoroethane and also precise heat capacity measurements are a t present under way. The work so far indicates that new information is being uncovered and that studies along these lines on other fluorocarbons will be highly profitable and significant, particularly in connection with fundamental studies of force relations within the molecule, rotation of part of the molecule with respect t o the rest, etc. The fluorocarbons are so unreactive, except a t very high temperatures, that they are not usable for most' chemical processes as reactants. However, derivatives are slowly being found which will enable syntheses to be carried out. Fluorocarbons containing one or two atoms of chlorine, bromine, and hydrogen are gradually being prepared. Compounds such as trifluoroacetic acid and its derivatives have been known for a long time. Olefinic fluorocarbons such as tetrafluoroethylene and hexafluoropropylene are known, and from t,he former an interesting resin has been made. It was recently s h o m (4) that a hydrogencontaining fluorocarbon can be halogenated to replace the hydrogen with chlorine or bromine. The bromine compounds offer considerable promise, for preliminary work shows that they may serve in the Grignard reaction. This will certainly open up the possibility of preparing great numbers of nrw membcrs of this field of chemical compounds. As more and more .synthetic methods are discovered, i t can be espected that fluorocarbon compounds in great numbers and hvith a large variation of properties mill be prepared. The chlorine-containing fluorocarbons have been used for some time as refrigerants under the name of Freons. Large numbers of these chlorides were prepared initially by one group of investigators (3)in this country and subsequently by another group (12), and were described in long series of excellent papers. The methods of replacement of chlorine by fluorine were estensively studied by the-tl workers. IXORGANIC FLUORIZlE COllPOUSDS
A large variety of inorganic fluorine compounds is known, and some of them have established large scale uses. A few examples will suffice: fluorite in general metallurgy, particularly of iron, and in t,he ceramic industries, cryolite in the metallurgy of aluminum, and sodium fluoride and barium fluosilicate in insecticides. The possibilities of great numbers of inorganic fluorine compounds are large because of the great tendency of fluorides to form double salts, of which cryolite is an example. Many compounds of crystallization are also formed wit,h hydrogen fluoride, such as the hydrofluorates of sodium fluoride and potassium fluoride. The determination of the decomposit,ion pressure properties of potassium fluoride hydrofluorates (1)established the conditions under which elementary fluorine can be most conveniently prepared. Many complex ions in which fluorine is involved are known. The iron-fluorine complex is the basis for the use of
Vol. 39, No. 3
potassium difluoride to remove rust spots from clothing and similar materials and is thus used in the laundry industry. The formation of these complexes adds difficulties to analytical procedures, and much work remains to be done in the analytical chemistry of fluorine compounds. The compounds of fluorine with the othcr elect,ronegative elements are of interest. Sulfur hexafluoride is a surprisingly unreactive compound and is a t present being considered for use where high voltages are employed. Bromine fluoride and chlorine fluoride are very reactive. The latter was made by the Germans in a relatively small quantity for military purposes (incendiary). The idea of the use of these compounds for rnihary purposes n-as pointed out t o our Government long before it was known that the Germans were interested in them. As fluorine in combination can force all other elements to exhibit their highest valence states, combinations with more fluorine exist than with any other element; examples are SFB,ClF,, VF:, etc. ORGAIYlC FLUORINE COMPOUNDS
The organic fluorine compounds are not yet as imposing in number as, for esample, the organic chlorine compounds. One reason is that the usual methods of synthesis frequently do not function satisfactorily for the preparation of organic fluorine compounds. The Swarts reaction and the modified Sandmeyer (sometimes called Schiemann) reaction are responsible for most of the known compounds of this kind. Other significant reactions are the replacement of chlorine with fluorine by such metallic fluorides as silver and mercury and, in some cases, by hydrogen fluoride alone, and also the addit'ion of hydrogen fluoride to unsaturated compounds ( I O ) . Organic fluorides vary in chemical properties from the extreme of inertness to the extreme of reactivity. I n general, fluorine imparts inertness to the substance. Fluorine in t'he aromatic nucleus is quite unreactive, and fluorine in all organic compounds seems more resistant to hgdrolysis than is chlorine. The trifluoromethyl group imparts inertness to most compounds, and dyes containing this group have been made because of stability and light fastness. Much more research needs to be done on the investigation of methods of synthesis and on the reasons underlying the properties of organic fluorine compounds. Trifluoromethpl organic compounds are beginning to form an interesting class of substances. It was recently found (24) that benzotrifluoride, when brominated in the ring, undergoes the usual organic reactions such as the Grignard. I t was also found that trifluoroacetyl chloride can be used in the Friedel-Crafts acylation, despite the fact that aluminum chloritlc sometimes causes a replacement of fluorine by chlorine, and that the rcsulting ketone of the reaction with benzene (trifluoroacetophenone) can be used with a Grignard reagent. Thus a beginning is being made o n the adaptation of known methods of synthesis of organic compounds to those containing fluorine. PHYSIOLOGICAL PROPERTIES
A property xhich is not well understood is the toxicity of organic fluorine compounds. The fluorocarbons are probably the most nontoxic organic compounds possible, but the new rat poison knorvn as 1080 (monofluoroacetic acid) is ari extremely toxic substance. Fluoride ion is well known for its tosicity, and this has been the basis of the use of inorganic fluorides as insecticides. The toxicity of monofluoroacetic acid, however, can hardly be blamed upon fluoride ion, because it is much more toxic. The physiological action of fluoride ion needs further investigation. The function of fluoride ion in the health of teeth has been studied a t some length. -1symposium report (13) deals with this subject. It appears that there are a t least two factors. The incidence of dental caries decreases with increasing fluoride concentration in domestic water but the incidence of mottled teeth increases.
March 1947
INDUSTRIAL AND ENGINEERING CHEMISTRY LITERATURE CITED
COSCLUSIONS
All present indications point to the conclusion that fluorine chemistry is destined to become a large and important division of the science, both from its theoretical importance and its commercial application. It is just beginning t’o broaden out to show its individual significance, its bearing on the rest of science, and its industrial applications. It has been slow in starting, because its techniques are different and difficult, but chiefly because its foundation depended primarily upon basic academic research which has been carried out by only a fen- investigators due to technical difficulties. Fluorine and its compounds have unique properties, due t,o the fact that fluorine not only occupies the extreme upper right-hand corner of the Periodic Table but that it, is much more electronegative than its nearest neighbors. Having extreme properties, these compounds not only test and extend our theories of chemical combination, struct,ure, and reaction, but provide large numbers of nerv substances of unique and extreme properties for many important utilitarian purposes. Luckily fluorine is not a rare element. Much more fundamental academic research is required and an increasing amount of development to exploit this field. Our theories will need to be tested and renovated, and new ones formulated. New methods of synthesis will have to tie discovered, old ones modified, and new catalysts found. Fluorine compounds are of chief interest in the estrenies of properties of chemical substance.. On the one side there re compounds of great. inertness and stability, on t’he ot,her, compounds of great reactivity. Certain compounds of fluorine are among the most nontoxic of substances, others are among the most poisonous substances known. Both inorganic and organic compounds in enormous numbers will be prepared with all conceivable gradation of properties. Many fluorine compounds are and will be utilized as end products, others are finding use in small quantities in mixtures. Some are useful as powerful catalysts and others serve or will serve as intermediates. Fluorine compounds are certain to find application or eventual use in all ramifications of chemical production, theory, and utilization.
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In all references only the beginning papers on the phaae of the subject are given. Bauer, S.H., Beach, J. I-., and Simons, J. H., J . Am. Chem. Soc., 61,19 (1939). Bigelow and Calfee, Ibid., 59, 2073 (1937). Booth, H. S.,Mong, IT. L., and Burchfield. P. E . , ISD. ENG. C H E U . . 24. ,~328. 637 (1932). Brice, T: J., Pearlson, IT, H., and Simons, J . H . , J . Am. Chem. Soc., 68, 968 (1946). Brockway, L. O., J . Phys. Chem., 41,185, 746 (1937). Brockway, L. 0.. Secrist, J. H., and Lucht. C. 11..Div. of Phys. Br Inorg. Chem., .1.C.S, Buffalo, 1942. C a d y , G . H., J . ilm. Chem. Soc., 56, 1431 (1934). Fredenliagen and Cadenhach. Ber., 67B,928 (19341. Geunther, P., Holm, I analogous to the hydrocarbon system poses a problem of nomenclature that has been a source of much inconvenience in the field. The present Chemical Abstracts system of nomenclature is completely adequate in that it is an explicit description which follon-s established internat’ional rules \vorlted out to fit into the whole picture of chemical nomenclature. However, some of the rcsulting names are so lengthy and involved that there has been a strong and understandable desire to develop a simpler terminology, at least for everyday use. Twice while the articles in this issue were under review, concert,ed efforts lvere made t,o effect such a simplification. The first was made by t,he panel reviewing the submitted manuscripts, which contained several proposals for simplified terminology. After discussing the various proposals, the members of the panc.1 who were present made the recommendation that the prefix “perfluoro” be used to denote complete substitution by fluorine of all positions attached to the carbon skeleton. Correspoiiclencc with the editor of Chemical Abstracts, viho is also the chairman of the official A.C.S. committee on nomenclature, and with the chairman of the nomenclature committee of the A.C.S. Division of Organic Chemistry indicated their general agreement: this recommendation was transmitted to all authors a t the time the papers were returned with review comments. h s a result, most of the authors adopted the “per” nomenclature in Tvhole or in part. Bcceptance of this usage was not’ universal, however, and some of the objections raised appeared to just,ify additional consideration of the problem. At a very late &age a second meeting was held in the offices of Chemical Abstracts. Present at, this meet.ing were persons representing t,he various viewpoints concerning the
fluorine nomenclature. .Also present were the editor of Chemical Abstracfs, the chairman of the Organic Chemistry Division’s AND committee on nomenclature, a representative of INDUSTRIAL ENGISEERING CHEMISTRY,and several nomenclature experts from the Chemical Abstracts staff. Extended discussion failed to produce the hoped-for general agreement’. The point was developed, however, that the “per” usage proposed n-as recognized by Chemical -4bstracts as permissible, although it had not been adopted for use in the indexes as outlined in the ext>cnsiveintroduction on nomenclature to the 1946 subject indrls for that journal. Because of the lack of general agreement (luring the time available for discussion, it n’as the sentiment of the group that thv existing Chemical Abstracis usage be adhi~rt~tl to in publication until Iatrr discussions shed furt,her ligh: on the subject. Plans are now lieing developed for such R meetin:., open to all interested persons, to be held at the .ipril 1947 Atlantic C’itj- A.C.3. meeting under Chemical Abstracts sponsorship. .I h.;t CJf typical compounds, giving formula and Chernical A bsiracts name. t h e Internat,ional Union of Chemistry usage, and thtx “per” terminology. is presented on page 212 for tht, informztion of tho.;e interested. The nomenclature usage in this issue largely follo~~vs the preference of the individual authors after they had received the original recommendation by t,he review panel for the “per” usage. Time did not permit rephrasing of the text to a desirable uniformity in terminology after t,he last meeting. However, pains have been taken t,o ensure that no ambiguity remains regarding t,he actual compounds described. Perhaps those variations that exist will even serve some useful purpose in stimulating further thought and an early solution to the prohlem.
