Fluorocarbons are
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...
s with
carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commercial
polymers
A polymer (; Greek ''poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic an ...
,
refrigerants
A refrigerant is a working fluid used in the refrigeration cycle of air conditioning systems and heat pumps where in most cases they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are heavily regulated ...
,
drug
A drug is any chemical substance that causes a change in an organism's physiology or psychology when consumed. Drugs are typically distinguished from food and substances that provide nutritional support. Consumption of drugs can be via inhal ...
s, and
anesthetic
An anesthetic (American English) or anaesthetic (British English; see spelling differences) is a drug used to induce anesthesia — in other words, to result in a temporary loss of sensation or awareness. They may be divided into tw ...
s.
[
]
Nomenclature
Perfluorocarbons or PFCs, are
organofluorine compounds with the formula C
xF
y, i.e., they contain only
carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...
and
fluorine.
The terminology is not strictly followed and many fluorine-containing organic compounds are called fluorocarbons.
[
]
Compounds with the prefix perfluoro- are hydrocarbons, including those with heteroatoms, wherein all C-H bonds have been replaced by C-F bonds.
[
] Fluorocarbons includes perfluoroalkanes, fluoroalkenes, fluoroalkynes, and perfluoroaromatic compounds.
Perfluoroalkanes
Chemical properties
Perfluoroalkanes are very stable because of the strength of the
carbon–fluorine bond
The carbon–fluorine bond is a polar covalent bond between carbon and fluorine that is a component of all organofluorine compounds. It is one of the strongest single bonds in chemistry (after the B–F single bond, Si–F single bond, and H– ...
, one of the strongest in organic chemistry.
[
]
Its strength is a result of the electronegativity of fluorine imparting partial ionic character through
partial charge A partial charge is a non-integer charge value when measured in elementary charge units. Partial charge is more commonly called net atomic charge. It is represented by the Greek lowercase letter 𝛿, namely 𝛿− or 𝛿+.
Partial charges are cr ...
s on the carbon and fluorine atoms, which shorten and strengthen the bond through favorable
covalent
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
interactions. Additionally, multiple carbon–fluorine bonds increase the strength and stability of other nearby carbon–fluorine bonds on the same
geminal carbon, as the carbon has a higher positive partial charge.
[ Furthermore, multiple carbon–fluorine bonds also strengthen the "skeletal" carbon–carbon bonds from the inductive effect.][ 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
The Birch reduction is an organic reaction that is used to convert arenes to cyclohexadienes. The reaction is named after the Australian chemist Arthur Birch and involves the organic reduction of aromatic rings in an amine solvent (traditional ...
and very specialized organometallic complexes.[
]
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 low refractive indices.
As the high electronegativity
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
of fluorine reduces the polarizability
Polarizability usually refers to the tendency of matter, when subjected to an electric field, to acquire an electric dipole moment in proportion to that applied field. It is a property of all matter, considering that matter is made up of elementar ...
of the atom,[ fluorocarbons are only weakly susceptible to the fleeting dipoles that form the basis of the ]London dispersion force
London dispersion forces (LDF, also known as dispersion forces, London forces, instantaneous dipole–induced dipole forces, fluctuating induced dipole bonds or loosely as van der Waals forces) are a type of intermolecular force acting between at ...
. As a result, fluorocarbons have low intermolecular attractive forces and are lipophobic in addition to being hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, ...
and non-polar
In chemistry, polarity is a separation of electric charge leading to a molecule or its chemical groups having an electric dipole moment, with a negatively charged end and a positively charged end.
Polar molecules must contain one or more pol ...
. Reflecting the weak intermolecular force
An intermolecular force (IMF) (or secondary force) is the force that mediates interaction between molecules, including the electromagnetic forces of attraction
or repulsion which act between atoms and other types of neighbouring particles, e.g. a ...
s these compounds exhibit low viscosities
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the int ...
when compared to liquids of similar boiling points, low surface tension and low heats of vaporization. The low attractive forces in fluorocarbon liquids make them compressible (low bulk modulus
The bulk modulus (K or B) of a substance is a measure of how resistant to compression the substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume.
Other moduli describ ...
) and able to dissolve gas relatively well. Smaller fluorocarbons are extremely volatile.[ There are five perfluoroalkane gases: ]tetrafluoromethane
Tetrafluoromethane, also known as carbon tetrafluoride or R-14, is the simplest perfluorocarbon ( C F4). As its IUPAC name indicates, tetrafluoromethane is the perfluorinated counterpart to the hydrocarbon methane. It can also be classified as a h ...
(bp −128 °C), hexafluoroethane
Hexafluoroethane is the perfluorocarbon counterpart to the hydrocarbon ethane. It is a non-flammable gas negligibly soluble in water and slightly soluble in alcohol. It is an extremely potent and long-lived greenhouse gas.
Physical properties
...
(bp −78.2 °C), octafluoropropane
Octafluoropropane (C3F8) is the perfluorocarbon counterpart to the hydrocarbon propane. This non-flammable synthetic material has applications in semiconductor production and medicine. It is also an extremely potent greenhouse gas.
Manufacture
Oc ...
(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 is perfluorocyclohexane, which sublimes
Sublimation is the transition of a substance directly from the solid to the gas state, without passing through the liquid state. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point i ...
at 51 °C. Fluorocarbons also have low surface energies and high dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the m ...
strengths.[
File:Tetrafluormethan.svg, ]Carbon tetrafluoride
Tetrafluoromethane, also known as carbon tetrafluoride or R-14, is the simplest perfluorocarbon ( C F4). As its IUPAC name indicates, tetrafluoromethane is the perfluorinated counterpart to the hydrocarbon methane. It can also be classified as a h ...
, the simplest perfluoroalkane
File:Perfluorooctane.png, Perfluorooctane, a linear perfluoroalkane
File:Perfluoro(2-methylpentane).svg, Perfluoro-2-methylpentane, a branched perfluoroalkane
File:Perfluoro-1,3-dimethylcyclohexane.svg, Perfluoro-1,3-dimethylcyclohexane, a cyclic perfluoroalkane
File:Perfluorodecaline.svg, Perfluorodecalin, a polycyclic perfluoroalkane
Flammability
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 pure oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...
or pure nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...
(gases of importance in anesthesiology).
In 1993, 3M considered fluorocarbons as fire extinguishants to replace CFCs. This extinguishing effect has been attributed to their high heat capacity
Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Heat 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.
When combustion does occur, toxic fumes result, including carbonyl fluoride
Carbonyl fluoride is a chemical compound with the formula COF2. It is a carbon oxohalide. This gas, like its analog phosgene, is colourless and highly toxic. The molecule is planar with ''C''2v symmetry, bond lengths of 1.174 Å (C=O) and 1.31 ...
, carbon monoxide
Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
, and hydrogen fluoride.
Gas dissolving properties
Perfluorocarbons dissolve relatively high volumes of gases. The high solubility of gases is attributed to the weak intermolecular interactions in these fluorocarbon fluids.
The table shows values for the mole fraction, , of nitrogen dissolved, calculated from the Blood–gas partition coefficient, at 298.15 K (25 °C), 0.101325 MPa.
Manufacture
The development of the fluorocarbon industry coincided with World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the World War II by country, vast majority of the world's countries—including all of the great power ...
. 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.[Siegemund G, Schwertfeger W, Feiring A, Smart B, Behr F, Vogel H, McKusick B "Fluorine Compounds, Organic" in "Ullmann's Encyclopedia of Industrial Chemistry" 2005, Wiley-VCH, Weinheim. ]
Fowler process
A major breakthrough that allowed the large scale manufacture of fluorocarbons was the Fowler process The Fowler process is an industry and laboratory route to fluorocarbons, by fluorinating hydrocarbons or their partially fluorinated derivatives in the vapor phase over cobalt(III) fluoride.
Background
The Manhattan Project required the producti ...
. In this process, cobalt trifluoride is used as the source of fluorine. Illustrative is the synthesis of perfluorohexane:
:C6H14 + 28 CoF3 → C6F14 + 14 HF + 28 CoF2
The resulting cobalt difluoride is then regenerated, sometimes in a separate reactor:
:2 CoF2 + F2 → 2 CoF3
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
Electrochemical fluorination (ECF) (also known as the Simons' process) involves electrolysis of a substrate dissolved in hydrogen 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:
:N(C6H13)3 + 45 HF → 3 C6F14 + NF3 + 42 H2
The perfluorinated amine will also be produced:
:N(C6H13)3 + 39 HF → N(C6F13)3 + 39H2
Environmental and health concerns
Fluoroalkanes are generally inert and non-toxic.
Fluoroalkanes are not ozone depleting, as they contain no chlorine or bromine atoms, and they are sometimes used as replacements for ozone-depleting chemicals.
The term fluorocarbon is used rather loosely to include any chemical containing fluorine and carbon, including chlorofluorocarbon
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and pro ...
s, which are ozone depleting. Fluoroalkanes are sometimes confused with fluorosurfactant
Per- and polyfluoroalkyl substances (PFASs) are synthetic organofluorine chemical compounds that have multiple fluorine atoms attached to an alkyl chain. An early definition, from 2011, required that they contain at least one perfluoroalkyl ...
s, which significantly bioaccumulate.
Perfluoroalkanes do not bioaccumulate; those 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 for octafluoropropane
Octafluoropropane (C3F8) is the perfluorocarbon counterpart to the hydrocarbon propane. This non-flammable synthetic material has applications in semiconductor production and medicine. It is also an extremely potent greenhouse gas.
Manufacture
Oc ...
is less than 2 minutes, compared to about a week for perfluorodecalin.
Low-boiling perfluoroalkanes are potent greenhouse gases
A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), meth ...
, in part due to their very long atmospheric lifetime, and their use is covered by the Kyoto Protocol
The Kyoto Protocol was an international treaty which extended the 1992 United Nations Framework Convention on Climate Change (UNFCCC) that commits state parties to reduce greenhouse gas emissions, based on the scientific consensus that (par ...
. The global warming potential
Global warming potential (GWP) is the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide (). GWP is 1 for . For other gases it depends on the gas and the time ...
(compared to that of carbon dioxide) of many gases can be found in the IPCC 5th assessment report,[Myhre, G., D. Shindell, F.-M. Bréon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch, J.-F. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang (2013]
"Anthropogenic and Natural Radiative Forcing"
(see Table 8.A.1). In: ''Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change''. Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. with an extract below for a few perfluoroalkanes.
The aluminium smelting industry has been a major source of atmospheric perfluorocarbons (tetrafluoromethane
Tetrafluoromethane, also known as carbon tetrafluoride or R-14, is the simplest perfluorocarbon ( C F4). As its IUPAC name indicates, tetrafluoromethane is the perfluorinated counterpart to the hydrocarbon methane. It can also be classified as a h ...
and hexafluoroethane
Hexafluoroethane is the perfluorocarbon counterpart to the hydrocarbon ethane. It is a non-flammable gas negligibly soluble in water and slightly soluble in alcohol. It is an extremely potent and long-lived greenhouse gas.
Physical properties
...
especially), produced as by-product of the electrolysis process. However, the industry has been actively involved in reducing emissions in recent years.
Applications
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:
*Perfluorocarbon tracer
Perfluorocarbon tracers (PFTs) are a range of perfluorocarbons used in tracer applications. They are used by releasing the PFT at a certain point, and determining the concentration of that PFT at another set of points, allowing the flow from the s ...
* Liquid dielectric
*Chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (subst ...
*Organic Rankine cycle
In thermal engineering, the Organic Rankine Cycle (ORC) is a type of thermodynamic cycle. It is a variation of the Rankine cycle named for its use of an organic, high- molecular-mass fluid whose vaporization temperature is lower than that of w ...
*Fluorous biphasic catalysis
*Cosmetics
*Ski waxes
As well as several medical uses:
* Contrast-enhanced ultrasound
* Oxygen Therapeutics
*Blood substitute
A blood substitute (also called artificial blood or blood surrogate) is a substance used to mimic and fulfill some functions of biological blood. It aims to provide an alternative to blood transfusion, which is transferring blood or blood-based ...
* Liquid breathing
*Eye surgery
*Tattoo removal
Fluoroalkenes and fluoroalkynes
Unsaturated fluorocarbons are far more reactive than fluoroalkanes. Although difluoroacetylene
Difluoroacetylene is a compound of carbon and fluorine having molecular formula C2F2. A linear molecule, its two carbons are joined by a triple bond and have terminal fluorines: F-C≡C-F. The molecule is the perfluorocarbon analog of acetyle ...
is unstable (as is typical for related alkynes, see dichloroacetylene),[ ]hexafluoro-2-butyne
Hexafluoro-2-butyne (HFB) is a fluorocarbon with the chemical structure CF3C≡CCF3. HFB is a particularly electrophilic acetylene derivative, and hence a potent dienophile for Diels–Alder reactions.
HFB is prepared by the action of sulfu ...
and related fluorinated alkynes are well known.
File:Perfluoroisobutene.svg, Perfluoroisobutene, a reactive and highly toxic fluoroalkene gas
File:Tetrafluoroethylene.svg, Tetrafluoroethylene
Tetrafluoroethylene (TFE) is a fluorocarbon with the chemical formula C2 F4. It is the simplest perfluorinated alkene. This gaseous species is used primarily in the industrial preparation of fluoropolymers.
Properties
Tetrafluoroethylene is a ...
, an important perfluorinated monomer.
File:Hexafluoropropylene.svg, Hexafluoropropylene, another important perfluoroalkene.
File:Hexafluorobutyne.png, Hexafluoro-2-butyne
Hexafluoro-2-butyne (HFB) is a fluorocarbon with the chemical structure CF3C≡CCF3. HFB is a particularly electrophilic acetylene derivative, and hence a potent dienophile for Diels–Alder reactions.
HFB is prepared by the action of sulfu ...
, a perfluoroalkyne.
Polymerization
Fluoroalkenes polymerize more exothermically than normal alkenes.[ 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.][ The most famous member of this class is ]tetrafluoroethylene
Tetrafluoroethylene (TFE) is a fluorocarbon with the chemical formula C2 F4. It is the simplest perfluorinated alkene. This gaseous species is used primarily in the industrial preparation of fluoropolymers.
Properties
Tetrafluoroethylene is a ...
, which is used to manufacture polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Che ...
(PTFE), better known under the trade name Teflon
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chem ...
.
Environmental and health concerns
Fluoroalkenes and fluorinated alkynes are reactive and many are toxic for example perfluoroisobutene. To produce polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Che ...
various fluorinated surfactants are used, in the process known as Emulsion polymerization, and the surfactant included in the polymer can bioaccumulate.
Perfluoroaromatic compounds
Perfluoroaromatic compounds contain only carbon and fluorine, like other fluorocarbons, but also contain an aromatic ring. The three most important examples are hexafluorobenzene
Hexafluorobenzene, HFB, , or perfluorobenzene is an organic, aromatic compound. In this derivative of benzene all hydrogen atoms have been replaced by fluorine atoms. The technical uses of the compound are limited, although it is recommended a ...
, octafluorotoluene, and octafluoronaphthalene.
File:hexafluorobenzene.svg, Hexafluorobenzene
Hexafluorobenzene, HFB, , or perfluorobenzene is an organic, aromatic compound. In this derivative of benzene all hydrogen atoms have been replaced by fluorine atoms. The technical uses of the compound are limited, although it is recommended a ...
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 from perfluoromethylcyclohexane by heating to 500 °C with a nickel or iron catalyst.
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 be miscible
Miscibility () is the property of two substances to mix in all proportions (that is, to fully dissolve in each other at any concentration), forming a homogeneous mixture (a solution). The term is most often applied to liquids but also appli ...
with common solvents.
See also
* :Fluorocarbons
*Fluorochemical industry
The global market for chemicals from fluorine was about US$16 billion per year as of 2006. The industry was predicted to reach 2.6 million metric tons per year by 2015. The largest market is the United States. Western Europe is the ...
* Hydrofluorocarbon
* Fluorographene
* Perfluorocycloalkene (PFCA)
References
External links
Fluorocarbons and Sulphur Hexafluoride, proposed by the European Fluorocarbons Technical Committee
CFCs and Ozone Depletion
Freeview video provided by the Vega Science Trust.
Introduction to fluoropolymers
Organofluorine chemistry by Graham Sandford
{{Authority control
Fluorocarbons
Greenhouse gases