Chlorofluorocarbons (CFCs) are fluorocarbons that also contain chlorine atoms. They were formerly used widely in industry as refrigerants, propellants, and cleaning solvents (dichlorodifluoromethane and chlorodifluoromethane were among the most widely used refrigerants). However, CFCs generally have potent ozone-depleting potential primarily due to homolytic cleavage of the carbon-chlorine bonds. Their use has now been mostly prohibited by the Montreal Protocol.
Hydrofluorocarbons (HFCs) are hydrocarbons in which some, but not all, of the hydrogen atoms have been replaced with fluorine. The fluorine atoms in these compounds do not catalyse ozone destruction, therefore HFCs do not damage the ozone layer. Consequently, HFCs such as tetrafluoroethane have become favored replacements for CFCs.
Fluorocarbon polymers are also well-known. These polymers are tough, chemical inert, and electrically insulating. The most famous example is PTFE (polytetrafluoroethylene), a polymer of the monomer tetrafluoroethylene. Other important polymers include polyvinylidene fluoride ([CH2CF2]n) and polychlorotrifluoroethylene ([CFClCF2]n or PCTFE, or Kel-F).
Perfluorocarbons (PFCs) are fluorocarbons that contain only carbon and fluorine atoms, such as octafluoropropane, perfluorohexane and perfluorodecalin. PFCs are inert to a wide range of chemicals, and are generally stable to about 400degC, and have been used for cooling and heating in aggressive environments. They also have very low toxicity and a relatively high ability to dissolve gases, and this has led to medical applications including liquid breathing and blood substitutes. PFCs have no effect on atmospheric ozone, but are notable greenhouse gases.
Many volatile anesthetics used to render surgical patients unconscious are fluorocarbons, such as methoxyflurane, enflurane, isoflurane, sevoflurane and desflurane. The fluorine atoms reduce their flammability compared to the non-fluorinated anesthetics originally used, such as diethyl ether and cyclopropane, which are dangerously flammable.
Some fluorocarbons (e.g. Freon) have been used as refrigerants. These fluorocarbons combine good thermodynamic properties (they have boiling points somewhat below typical target temperatures, a high heat of vaporization, a moderate density in liquid form and a high density in the gas phase) with a safe (low toxicity and flammability) and noncorrosive nature. Because of their negative effect on the ozone layer, many fluorocarbons have been banned as refrigerant after the Montreal Protocol.
Compounds that have a boiling point just around room temperature, with a high vapour pressure can be used as propellant gas. Some fluorocarbons have these properties, and, before the Montreal Protocol, many of these low boiling fluorocarbons were used as propellants, but now recognized as endangering the ozone layer in the earth's atmosphere.
Fluorocarbons are used as industrial solvents due to their specific properties, including: non-flammability, stability, excellent dielectric properties, low surface tension and viscosity, very low toxicity and a favourable environmental profile.
Prior to the Montreal Protocol, CFCs, such as Freon and chlorodifluoromethane were used as cleaning solvents. Also HFCs were developed with similar properties. Quite often these HFC's are blended with other fluids to obtain tailored properties for specific application.
Main applications are:
HFCs, particularly 1,1,1,2-tetrafluoroethane, are used for specialist extraction of extremely important natural products; such as Taxol for cancer treatment from yew needles, evening primrose oil food supplement, and vanilla. The use of 1,1,1,2-tetrafluroethane compliments other methods of extraction, in being highly selective and allowing high quality and high yield extractions.
Fluorocarbon based greases are sometimes used in demanding applications. Advantages include low reactivity and very high temperature ranges. Examples include Fomblin by Solvay Solexis and Krytox by DuPont.
Also used in certain firearm lubricants such as "Tetra Gun"
In general, highly fluorinated organic compounds are hydrophobic and have water-repellant and stain-repellant properties. The original formulations of products such as Scotchgard contained fluorocarbons including perfluorobutane sulfonate and perfluorooctane sulfonate (PFOS). But many of these uses have been phased out due to environmental concerns, such as those associated with perfluorooctanoic acid, an intermediate in the manufacture of PFOS. Similarly, products containing Gore-Tex and Teflon are made from fluoropolymers.
Fluorocarbons are also used in fishing line, in myriad precision plastics applications, and in highly precise lubrication applications.
Triflic acid (CF3SO3H) and trifluoroacetic acid (CF3CO2H) are important reagents in organic synthesis. They are valuable for their properties as very strong acids that are soluble in organic solvents. The electronegative nature of the fluorine atoms stabilizes the dissociated anions of triflic acid and trifluoroacetic acid, leading to stronger acidity compared to their unfluorinated analogs, methanesulfonic acid and acetic acid, respectively. The fluorine atoms also enhance the thermal and chemical stabilities of the conjugate bases. In fact, the polymeric analogue of triflic acid, nafion is used as a proton-exchange material in fuel cells.
The triflate-group (the conjugate base of the triflic acid) is a good leaving group in organic chemistry.
Carbon-fluorine bonds have found application in non-coordinating anions. In these anions (e.g. BF4-, PF6-, B(C6H3(CF3)2)4-, and B(C6F5)4- the charge is 'smeared' out over many electronegative atoms.
As mentioned above, chlorofluorocarbons have been criticized for their harm to the ozone layer. It is estimated that a single CFC molecule has the ability to decompose approximately 100,000 ozone molecules. However, because fluorocarbons lack a chlorine atom, they cannot participate in the ozone-destroying reactions that are such a problem with CFCs. Fluorocarbons are considered ozone safe.
Although there are thousands of known naturally-occurring organic compounds containing chlorine and bromine, there are only a handful of natural fluorocarbons. They have been found in microorganisms and plants, but not animals. The most common natural fluorocarbon is fluoroacetic acid, a potent toxin found in a few species of plants. Others included ω-fluoro fatty acids, fluoroacetone, and 2-fluorocitrate which are all believed to be biosynthesized from fluoroacetic acid.
Since the C-F bond is generally metabolically stable and fluorine is considered a bioisostere of the hydrogen atom, many pharmaceuticals contain C-F bonds. An example of this is fluorinated uracil. When elemental fluorine is reacted with uracil, 5-fluorouracil is produced. The resulting compound is an anticancer drug (antimetabolite) used to masquerade as uracil during the nucleic acid replication process. This can lead to the incorporation of 5-fluorouracil into DNA and RNA as well as inhibition of the enzymes that are responsible for the synthesis of the normal components of DNA. These factors can be toxic to cancer cells that need to rapidly produce normal nucleic acids in order to continue growing.
The carbon-fluorine bond length is typically about 1.35 Å (1.39 Å in fluoromethane). This is shorter than any other carbon-halogen bond, and shorter than C-N and C-O bonds. Since fluorine is a very electronegative atom (much more so than carbon), the carbon-fluorine bond has a significant dipole moment. The carbon-fluorine bond is stronger than other carbon-halogen bonds. The bond dissociation energy in CH3X is 115 kcal/mol for the carbon-fluorine bond compared to 83.7, 72.1, and 57.6 kcal/mol for bonds between carbon and chlorine, bromine, and iodine, respectively. The strength of the carbon-fluorine bond is also stronger than the carbon-hydrogen bond, which is 104.9 kcal/mol in methane.
As a result of these unique features of the carbon-fluorine bond, an overarching theme in fluorocarbon chemistry is the contrasting set of physical and chemical properties in comparison to the corresponding hydrocarbons. Case studies follow.
Pentakis(trifluoromethyl)cyclopentadiene (C5(CF3)5H) is a strong acid, with a pKa = −2. Its high acidity and robustness is indicated by the fact that this compound is typically purified by distillation from H2SO4. In contrast, C5(CH3)5H requires a strong base such as butyllithium for deprotonation, as is typical for a hydrocarbon. This compound is prepared in a multistep, one-pot reaction of potassium fluoride (KF) with 1,1,2,3,4,4-hexachlorobutadiene.
The molecule hexafluoroacetone ((CF3)2CO), the fluoro-analogue of acetone, has a boiling point of −27 °C compared to +55 °C for acetone itself. This difference illustrates one of the remarkable effects of replacing C-H bonds with C-F bonds. Normally, the replacement of H atoms with heavier halogens results in elevated boiling points due to increased van der Waals interactions between molecules. Further demonstrating the remarkable effects of fluorination, (CF3)2CO forms a stable, distillable hydrate, (CF3)2C(OH)2. Ketones rarely form stable hydrates. Continuing this trend, (CF3)2CO adds ammonia to give (CF3)2C(OH)(NH2) which can be dehydrated with POCl3 to give (CF3)2CNH. Compounds of the type R2C=NH are otherwise quite rare.
Aliphatic fluorocarbons tend to segregate from aliphatic hydrocarbons while aromatic fluorocarbons tend to mix with aromatic hydrocarbons. This is evidenced by the following crystal structures.
Since fluorocarbons very rarely occur naturally, they must be prepared using synthetic chemistry. Some methods include:
Joint Release with the Ministry of the Environmentresults of the Amount of Destroyed Fluorocarbons Pursuant to the Law for Ensuring the Implementation, Recovery and Destruction of Fluorocarbons concerning Specified Products of Fy2011
Aug 03, 2012; TOKYO, Japan -- The following information was released by the Ministry of Economy, Trade and Industry of Japan (METI): Pursuant...