Fluorine Compounds, Organic
Fluorine Compounds, Organic
organic compounds, the molecules of which contain one or more F—C bonds. The chemistry of organic fluorine compounds began developing rapidly only in the second half of the 20th century and since then has grown into a large specialized field of organic chemistry. Its development was governed by the needs of the young atomic industry for materials resistant to the fluorinating action of UF6, which is used in uranium isotope separation. Fluorine derivatives of all types of organic compounds are known.
Nomenclature. The position of the fluorine atom in organic fluorine compounds is designated in accordance with the rules of nomenclature for organic compounds. It is more convenient to use the prefix “per” when naming polyfluorinated compounds. Thus, completely fluorinated hydrocarbons are called perfluorohydrocarbons (or fluorocarbons); for example, CF3(CF2)5CF3 is called perfluoroheptane. Partially fluorinated compounds may be regarded as derivatives of perfluoro-hydrocarbons; for example, CF3CFH(CF2)4CF2H is called 1,6-dihydroperfluoroheptane. In organic fluorine compounds, the prefix “perfluoro” is often replaced by the Greek letter ϕ thus, perfluoroethane is ϕ-ethane. The designation of completely fluorinated hydrocarbons also involves the use of the element “fluoro,” which is included in the designation of the corresponding hydrocarbon; for example, CF4 is tetrafluoromethane and C2F6 is hexafluoroethane.
Methods of synthesis. DIRECT FLUORINATION AND ADDITION OF F2 ACROSS DOUBLE BOND. Direct fluorination and the addition of F2 across the double bond are both highly exothermic radical reactions:
Since the thermal effect of fluorination is greater than that of the splitting of C—C bonds (80–85 kcal/mole), the destruction of the fluorinated compounds is possible. To avoid this, it is necessary to draw off the heat effectively and to dilute the reagent mixture with nitrogen. A copper mesh (or copper rods) coated with, for example, Ag, Co, or Ni, is (are) introduced to draw off heat into a reaction space (tube). Higher metal fluorides are formed along the surface of the mesh (or rods); the fluorides also serve as fluorinating agents. The role of fluorine, in this case, apparently reduces to the regeneration of the fluorides.
In the metal fluoride process, the vapors of the substance undergoing fluorination, strongly diluted with nitrogen, are passed through a tube with CoF3:
½(—CH2—) + 2CoF3 → ½(—CF2—) + HF + 2CoF2 + 46 kcal/mole
The CoF2 formed upon the action of fluorine at 250°C is again converted into CoF3. The perfluorohydrocarbon yield is 80–85 percent.
Electrochemical fluorination, or electrofluorination, is an important method. A solution of the substance being fluorinated in anhydrous hydrogen fluoride serves as the electrolyte. KF is usually added in the case of nonconducting compounds. This method is used to obtain good yields of perfluoroethers, perfluorocarboxylic acids, perfluoroamines, and perfluorooxides. All the aforementioned processes are used in industry.
REPLACEMENT OF CHLORINE BY FLUORINE. The replacement of chlorine atoms by fluorine is an important commercial method of introducing fluorine (seeSWARTS REACTION) and may be carried out using anhydrous HF or various fluorides, such as NH4F, KF, SbF3Cl2, AgF2, and HgF2. The ease of the exchange depends on the structure of the chlorine-containing compound. Thus, the acid chlorides are often easily converted into acid fluorides by dissolving them in anhydrous HF. The Cl atoms in ethylene chlo-rohydrin and in chloroacetic acid and its derivatives are readily replaced by F on reaction with KF in polar solvents, such as ethylene glycol; the Cl atoms in monohalogenohydrocarbons are replaced by F only by the action of AgF2 or HgF2 at 150°C. Chlorine atoms are more readily replaced by fluorine in compounds containing the trichloromethyl group. Solutions of SbF3 or SbF3Cl2 in anhydrous HF are generally used in industry for this type of exchange. This method is used to obtain chlorodifluoro-methane (used in the preparation of tetrafluoroethylene) from chloroform CHCI3, dichlorodifluoromethane (one of the most important Freons) from CCI4, and trichlorotrifluoroethane (a source material for the preparation of chlorotrifluoroethylene) from C2CI6.
Chlorine atoms are relatively easily replaced by fluorine in hexachlorobenzene by the action of KF at 450°–530°C; good yields of C6F6 and C6F5CI are thus obtained. Other polychloroa-romatic and polychloroheterocyclic compounds also react in a similar manner.
DIAZO METHOD. The diazo method of preparing aromatic fluorine compounds is based on the formation of diazonium fluoborate (diazonium borofluoride), which is isolated in solid form and is decomposed on heating:
REPLACEMENT OF OXYGEN-CONTAINING GROUPS. The replacement of oxygen-containing groups in various organic compounds with fluorine is carried out with the help of SF4, for example, in alcohols, aldehydes, ketones, and acids:
where R is an organic residue.
ADDITION OF ANHYDROUS HYDROGEN FLUORIDE. Another method of preparing organic fluorine compounds is the addition of anhydrous hydrogen fluoride to olefins, haloolefins, oxides, isocyanates, and cycloparaffins; for example,
CONJUGATE ADDITION OF FLUORINE. The conjugate addition of fluorine and other atoms or groups to compounds containing multiple bonds occurs easily in an excess of anhydrous HF, as for example, in fluoronitration:
PREPARATION OF FLUOROOLEFINS. There are several methods of preparing fluoroolefins. One method is the dehalogenation of vicinal dihalopolyfluoroalkanes by metals, such as Zn and Mg; for example,
CF2Cl—CF2Cl + Zn → CF2 ═ CF2 + ZnCl2
Another method is the pyrolysis of polytetrafluoroethylene, which leads to the formation of perfluoropropylene and perfluoroisobutylene, along with tetrafluoroethylene, perfluorobutene, and perfluorocyclobutane:
This method is used in industry, as is the pyrolysis of tetrafluoroethylene, to obtain perfluoropropylene—an important monomer in the manufacture of fluorine rubbers.
Still another method is the pyrolysis of the salts of perfluorocarboxylic acid; for example,
PREPARATION OF FLUORINATED ALCOHOLS. Fluorinated alcohols are obtained by standard methods of alcohol synthesis, such as the reduction of esters of perfluorocarboxylic acids and fluorinated aldehydes and ketones. An important commercial method of obtaining such alcohols is the telomerization of tetrafluoroethylene with methanol:
nCF2═ CF2 + CH3OH→H[—CF2CF2—]n CH2OH
Properties. PHYSICAL PROPERTIES. The lower fluorocarbons of the paraffin series (general formula CnF2n+2) are gases, the fluorocarbons beginning with C5 are liquids, and the higher ones are solid waxlike compounds. Only the first four representatives of this series boil at temperatures slightly higher than the corresponding hydrocarbon analogues, whereas all the others boil at lower temperatures.
The replacement of one hydrogen atom in a hydrocarbon molecule with F causes an increase in the boiling point, but a lesser increase than during replacement with chlorine. The boiling points drop sharply during the full replacement of hydrogen atoms with fluorine in any hydrocarbon derivatives (see Table 1).
|Table 1. Comparison of boiling points of selected compounds|
|Formula||Boiling point (°C)|
Fluorocarbons are good dielectrics, with an electrical resistivity of about 1014 ohm·cm; the dielectric constant is considerably higher than in paraffins. The rate of ultrasonic propagation in fluorocarbons is usually low, less than 800 m/sec.
CHEMICAL PROPERTIES OF THE MOST IMPORTANT ORGANIC FLUORINE COMPOUNDS. Fluorocarbons of the paraffin and alicyclic series are characterized by unusually high chemical inertness and heat resistance. Only a small number of reactions are known for them, which occur only at high temperatures. For example, the pyrolysis of perfluoroethane begins at about 1000°C, and that of perfluoroheptane, at about 800°C. Fluorocarbons do not react under ordinary conditions or on moderate heating with concentrated acids, strong oxidizing agents, metals, and alkalies; their reaction with metallic sodium and with sodium peroxide begins at 400°C. Zn, Al, Fe, and Sn react very slowly under these conditions, whereas Cu, Ag, Hg, and certain other elements do not take part in the reaction.
Perfluorobenzene and certain other perfluoroaromatic compounds interact readily with nucleophilic reagents, such as ammonia, amines, alcoholates, and sodium sulfide. The replacement of one fluorine atom is followed by that of a second, located in the para-position relative to the first:
Chloropentafluorobenzene forms the organomagnesium compound C6F5MgCl, which is widely used in organic synthesis.
Unlike nucleophilic olefins, perfluoroolefins are strongly elec-trophilic. They react easily with various nucleophilic substances and, depending on the type of nucleophilic substance, form products of the addition or replacement of the F atom in the vinyl (a) position or allyl (b) position with a nucleophilic (Nu) residue:
Electrophilic compounds react far less readily with fluoroolefins than with their hydrocarbon analogues. However, fluoroolefins combine with halogens, mixed halogens, sulfuric anhydride, and other strong electrophilic reagents. Perfluoroolefins readily take part in free-radical reactions; for example,
and are also readily polymerized and copolymerized. The oxidation of perfluoroolefins in an alkaline medium yields perfluoro-oxides (see below).
Monofluoromethanol is an unstable liquid with a boiling point of 51°C. Difluoromethanol and trifluoromethanol have not been isolated, but derivatives of trifluoromethanol are known: trifluoromethylhypofluorite, CF3OF, a gas with a boiling point of –95°C, and the alcoholates CF3OK and CF3OCs. Fluoroalcohols (β- and γ- but not α-) are stable liquids that readily undergo distillation. The acidic properties of alcohols are increased in proportion to the accumulation of fluorine atoms.
The electrophilic properties of the carbonyl atom of the hydrocarbon are sharply intensified with increased fluorine content in aldehyde and ketone molecules. Perfluoroaldehydes and perfluoroketones, like chloral, form stable geminal diols, such as CF3—CH(OH)2, CF3—C(OH)2—CF3, and semiacetals; they attach NH3, HCN, NH2OH, and other nucleophilic reagents more readily than their hydrocarbon analogues and decompose easily to form trifluoromethane; for example,
CF3COCF3+ NaOH→CF3H + CF3COONa
Partially fluorinated ketones and aldehydes are characterized by a high content of enol forms, which tend to form chelates. Use is made of this property in the separation of rare and trace elements; for example, thenoyltrifluoroacetone is used to isolate and purify Be, Co, Hf, Zr, and Ac, as well as radioisotopes formed in nuclear reactors.
Fluorocarboxylic acids are stronger than the nonfluorinated acids and corresponding chlorinated acids. However, p-fluorobenzoic acid is weaker than chlorobenzoic acid because of the greater tendency of the F atom to conjugation.
The action of tertiary amines or fluorine ions results in the ready isomerization of perfluorooxides, as well as their polymerization, which yields oils that are extremely stable to the action of corrosive media.
Perfluoroalkyl primary and secondary amines of the type CF3NH2 and (CF3)2NH are poorly stable to the action of highly corrosive media, whereas the tertiary ones are extremely stable; they lack basic properties
owing to the strong reduction of electron density in the nitrogen atom.
Various organic compounds containing —NF2 groups are strong oxidizing agents.
Fluoronitroso compounds of the type RFN═O are stable. Unlike the hydrogen analogues, they are intense blue in color; for example, trifluoronitrosomethane is a blue gas, with a boiling point of –84°C. Its copolymerization with tetrafluoroethylene yields nitroso rubber, one of the most chemically stable fluorine rubbers.
The known organic fluorine compounds that contain sulfur include fluoromercaptans, fluorosulfides, difluorosulfides, polyfluorosulfides, fluorosulfoxides, fluorosulfones, and fluoro-sulfonic acids and their derivatives. Perfluorosulfonic acids, especially trifluoromethanesulfonic acid, are used in industry, as well as difluorothiophosgene, CF2S (in the synthesis of elastomers).
The most important fluoroalkyl compounds of metals and metalloids are the compounds with Li, Mg, Hg, and Si; compounds with P, As, and Sb have been studied relatively well. Mercuric perfluorodimethyl, (CF3)2Hg, is markedly different from the ordinary organomercury compounds. It is a colorless crystalline substance, with a melting point of 161°C; it is freely soluble in water and, unlike (CH3)2Hg, it is not an alkylating agent. Mercuric diperfluorovinyl is a good perfluorovinylating agent. The most important silicon compound is CF3CH2CH2SiCl2(CH3), which is used in the manufacture of heat-resistant fluorosiloxane elastomer.
Uses. Organic fluorine compounds are widely used in all fields of technology, where extreme conditions often prevail. They are used for the preparation of fluoroplastics, which are superior to the precious metals with respect to stability to the action of corrosive media. They are used in the production of fluoroelastomers and anticorrosive coatings. They are used as noncombustible, heat-resistant, and nonoxidizing lubricants and as hydraulic fluids, surfactants, fire-extinguishing agents, propellants, and refrigerants. Trifluoroacetic acid and its anhydride are used as esterification promoters; trifluoroperacetic acid is a specific and convenient oxidizing agent. In medicine, organic fluorine compounds are used as drugs and anesthetics and as materials for the manufacture of artificial blood vessels and heart valves. Organic fluorine compounds are also used in the study of fundamental problems in a number of theories, such as the theory of the nature of the hydrogen bond, the theory of Van der Waals forces, and the theory of reaction mechanisms.
REFERENCESKnuniants, I. L., and A. V. Fokin. Pokorenie nepristupnogo elementa. Moscow, 1963.
Knuniants, I. L., and G. A. Sokol’skii. “Elektrokhimicheskoe ftorirovanie.” Reaktsii i metody issledovaniia organicheskikh soedinenii, book 6. Moscow, 1957.
Sheppard, W., and C. Sharts. Organicheskaia khimiia flora. Moscow, 1972.(Translated from English.)
Uspekhi khimil flora, vols. 1–4. Leningrad, 1964–70. (Translated from English.)
Fluorine Chemistry Reviews, vols. 1–7. Edited by P. Tarrant. New York–[et al.] 1967–74.
Chambers, R. D. Fluorine in Organic Chemistry. New York–[et al.] 1973.
I. L. KNUNIANTS