Fluorocarbons arechemical compounds withcarbon-fluorine bonds. Compounds that contain many C-F bonds often have distinctive properties, e.g., enhanced[clarification needed] stability, volatility, and hydrophobicity. Several fluorocarbons and their derivatives are commercialpolymers,refrigerants,drugs, andanesthetics.[1]
Perfluorocarbons orPFCs, areorganofluorine compounds with the formula CxFy, meaning they contain onlycarbon andfluorine.[2] The terminology is not strictly followed and many fluorine-containing organic compounds are also called fluorocarbons.[1] Compounds with the prefixperfluoro- are hydrocarbons, including those withheteroatoms, wherein all C-H bonds have been replaced by C-F bonds.[3] Fluorocarbons includes perfluoroalkanes, fluoroalkenes, fluoroalkynes, and perfluoroaromatic compounds.
Perfluoroalkanes are very stable because of the strength of thecarbon–fluorine bond, one of the strongest in organic chemistry.[4]Its strength is a result of the electronegativity of fluorine imparting partial ionic character throughpartial charges on the carbon and fluorine atoms, which shorten and strengthen the bond (compared to carbon-hydrogen bonds) through favorablecovalent interactions. Additionally, multiple carbon–fluorine bonds increase the strength and stability of other nearby carbon–fluorine bonds on the samegeminal carbon, as the carbon has a higher positive partial charge.[1] Furthermore, multiple carbon–fluorine bonds also strengthen the "skeletal" carbon–carbon bonds from theinductive effect.[1] Therefore,saturated fluorocarbons are more chemically and thermally stable than their corresponding hydrocarbon counterparts, and indeed any other organic compound. They are susceptible to attack by very strong reductants, e.g.Birch reduction and very specialized organometallic complexes.[5]
Fluorocarbons are colorless and have high density, up to over twice that of water. They are not miscible with most organic solvents (e.g., ethanol, acetone, ethyl acetate, and chloroform), but are miscible with some hydrocarbons (e.g., hexane in some cases). They have very low solubility in water, and water has a very low solubility in them (on the order of 10 ppm). They have lowrefractive indices.
As the highelectronegativity of fluorine reduces thepolarizability of the atom,[1] fluorocarbons are only weakly susceptible to the fleeting dipoles that form the basis of theLondon dispersion force. As a result, fluorocarbons have low intermolecular attractive forces and arelipophobic in addition to beinghydrophobic andnon-polar. Reflecting the weakintermolecular forces these compounds exhibit lowviscosities when compared to liquids of similarboiling points, lowsurface tension and lowheats of vaporization. The lowattractive forces in fluorocarbon liquids make themcompressible (lowbulk modulus) and able to dissolve gas relatively well. Smaller fluorocarbons are extremelyvolatile.[1] There are five perfluoroalkane gases:tetrafluoromethane (bp −128 °C),hexafluoroethane (bp −78.2 °C),octafluoropropane (bp −36.5 °C),perfluoro-n-butane (bp −2.2 °C) and perfluoro-iso-butane (bp −1 °C). Nearly all other fluoroalkanes are liquids; the most notable exception isperfluorocyclohexane, whichsublimes at 51 °C.[6] Fluorocarbons also have lowsurface energies and highdielectric strengths.[1]
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In the 1960s there was a lot of interest in fluorocarbons as anesthetics. The research did not produce any anesthetics, but the research included tests on the issue of flammability, and showed that the tested fluorocarbons were not flammable in air in any proportion, though most of the tests were in pureoxygen or purenitrous oxide (gases of importance in anesthesiology).[7][8]
| Compound | Test conditions | Result |
|---|---|---|
| Hexafluoroethane | Lower flammability limit in oxygen | None |
| Perfluoropentane | Flash point in air | None |
| Flash point in oxygen | −6 °C | |
| Flash point nitrous oxide | −32 °C | |
| Perfluoromethylcyclohexane | Lower flammability limit in air | None |
| Lower flammability limit in oxygen | 8.3% | |
| Lower flammability limit in oxygen (50 °C) | 7.4% | |
| Lower flammability limit in nitrous oxide | 7.7% | |
| Perfluoro-1,3-dimethylcyclohexane | Lower flammability limit in oxygen (50 °C) | 5.2% |
| Perfluoromethyldecalin | Spontaneous ignition test in oxygen at 127 bar | No ignition at 500 °C |
| Spontaneous ignition in adiabatic shock wave in oxygen, 0.98 to 186 bar | No ignition | |
| Spontaneous ignition in adiabatic shock wave in oxygen, 0.98 to 196 bar | Ignition |
In 1993, 3M considered fluorocarbons as fire extinguishants to replace CFCs.[9] This extinguishing effect has been attributed to their highheat capacity, which takes heat away from the fire. It has been suggested that an atmosphere containing a significant percentage of perfluorocarbons on a space station or similar would prevent fires altogether.[10][11]When combustion does occur, toxic fumes result, includingcarbonyl fluoride,carbon monoxide, andhydrogen fluoride.
Perfluorocarbons dissolve relatively high volumes of gases. The high solubility of gases is attributed to the weak intermolecular interactions in these fluorocarbon fluids.[12]
The table shows values for the mole fraction,x1, of nitrogen dissolved, calculated from theBlood–gas partition coefficient, at 298.15 K (25 °C), 0.101325 MPa.[13]
| Liquid | 104x1 | Concentration ( mM ) |
|---|---|---|
| Water | 0.118 | 0.65 |
| Ethanol | 3.57 | 6.12 |
| Tetrahydrofuran | 5.21 | 6.42 |
| Acetone | 5.42 | 7.32 |
| Cyclohexane | 7.73 | 7.16 |
| Perfluoro-1,3-dimethylcyclohexane | 31.9 | 14.6 |
| Perfluoromethylcyclohexane | 33.1 | 16.9 |
The development of the fluorocarbon industry coincided withWorld War II.[14] Prior to that, fluorocarbons were prepared by reaction of fluorine with the hydrocarbon, i.e., direct fluorination. Because C-C bonds are readily cleaved by fluorine, direct fluorination mainly affords smaller perfluorocarbons, such as tetrafluoromethane, hexafluoroethane, and octafluoropropane.[15]
A major breakthrough that allowed the large scale manufacture of fluorocarbons was theFowler process. In this process,cobalt trifluoride is used as the source of fluorine. Illustrative is the synthesis ofperfluorohexane:
The resulting cobalt difluoride is then regenerated, sometimes in a separate reactor:
Industrially, both steps are combined, for example in the manufacture of the Flutec range of fluorocarbons by F2 chemicals Ltd, using a vertical stirred bed reactor, with hydrocarbon introduced at the bottom, and fluorine introduced halfway up the reactor. The fluorocarbon vapor is recovered from the top.
Electrochemical fluorination (ECF) (also known as the Simons' process) involveselectrolysis of a substrate dissolved inhydrogen fluoride. As fluorine is itself manufactured by the electrolysis of hydrogen fluoride, ECF is a rather more direct route to fluorocarbons. The process proceeds at low voltage (5 – 6 V) so that free fluorine is not liberated. The choice of substrate is restricted as ideally it should be soluble in hydrogen fluoride. Ethers and tertiary amines are typically employed. To make perfluorohexane, trihexylamine is used, for example:
The perfluorinated amine will also be produced:
Fluoroalkanes are generally inert and non-toxic.[16][17][18]
Fluoroalkanes are notozone depleting, as they contain no chlorine or bromine atoms, and they are sometimes used as replacements for ozone-depleting chemicals.[19]The term fluorocarbon is used rather loosely to include any chemical containing fluorine and carbon, includingchlorofluorocarbons, which are ozone depleting.
Perfluoroalkanes used in medical procedures are rapidly excreted from the body, primarily via expiration with the rate of excretion as a function of the vapour pressure; the half-life foroctafluoropropane is less than 2 minutes,[20] compared to about a week for perfluorodecalin.[21]
Low-boiling perfluoroalkanes are potentgreenhouse gases, in part due to their very long atmospheric lifetime, and their use is covered by theKyoto Protocol.[citation needed][22] Theglobal warming potential (compared to that of carbon dioxide) of many gases can be found in the IPCC 5th assessment report,[23] with an extract below for a few perfluoroalkanes.
| Name | Chemical formula | Lifetime (y) | GWP (100 years) |
|---|---|---|---|
| PFC-14 | CF4 | 50000 | 6630 |
| PFC-116 | C2F6 | 10000 | 11100 |
| PFC-c216 | c-C3F6 | 3000 | 9200 |
| PFC-218 | C3F6 | 2600 | 8900 |
| PFC-318 | c-C4F8 | 3200 | 9540 |
The aluminium smelting industry has been a major source of atmospheric perfluorocarbons (tetrafluoromethane andhexafluoroethane especially), produced as by-product of the electrolysis process.[24] However, the industry has been actively involved in reducing emissions in recent years.[25]
As they are inert, perfluoroalkanes have essentially no chemical uses, but their physical properties have led to their use in many diverse applications. These include:
As well as several medical uses:
Unsaturated fluorocarbons are far more reactive than fluoroalkanes. Althoughdifluoroacetylene is unstable (as is typical for related alkynes, seedichloroacetylene),[1]hexafluoro-2-butyne and related fluorinated alkynes are well known.
Fluoroalkenes polymerize more exothermically than normal alkenes.[1] Unsaturated fluorocarbons have a driving force towards sp3 hybridization due to the electronegative fluorine atoms seeking a greater share of bonding electrons with reduced s character in orbitals.[1] The most famous member of this class istetrafluoroethylene, which is used to manufacturepolytetrafluoroethylene (PTFE), better known under the trade nameTeflon.
Fluoroalkenes and fluorinated alkynes are reactive and many are toxic for exampleperfluoroisobutene.[29][30] To producepolytetrafluoroethylene variousfluorinated surfactants are used, in the process known asEmulsion polymerization, and the surfactant included in the polymer can bioaccumulate.
Perfluoroaromatic compounds contain only carbon and fluorine, like other fluorocarbons, but also contain an aromatic ring. The three most important examples arehexafluorobenzene,octafluorotoluene, and octafluoronaphthalene.
Perfluoroaromatic compounds can be manufactured via the Fowler process, like fluoroalkanes, but the conditions must be adjusted to prevent full fluorination. They can also be made by heating the corresponding perchloroaromatic compound with potassium fluoride at high temperature (typically 500 °C), during which the chlorine atoms are replaced by fluorine atoms. A third route is defluorination of the fluoroalkane; for example, octafluorotoluene can be made fromperfluoromethylcyclohexane by heating to 500 °C with a nickel or iron catalyst.[31]
Perfluoroaromatic compounds are relatively volatile for their molecular weight, with melting and boiling points similar to the corresponding aromatic compound, as the table below shows. They have high density and are non-flammable. For the most part, they are colorless liquids. Unlike the perfluoralkanes, they tend to bemiscible with common solvents.[citation needed]
| Compound | Melting point (°C) | Boiling point (°C) |
|---|---|---|
| Hexafluorobenzene | 5.3 | 80.5 |
| Benzene | 5.5 | 80.1 |
| Octafluorotoluene | <−70 | 102–103 |
| Toluene | −95 | 110.6 |
| Perfluoro(ethylbenzene) | 114–115 | |
| Ethylbenzene | −93.9 | 136.2 |
| Octafluoronaphthalene | 86–87 | 209[32] |
| Naphthalene | 80.2 | 217.9 |
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