Fluorocarbon 78517 212759641 2008-05-16T03:16:49Z Mayfare 3934517 /* Perfluorocarbons */ [[Image:Fluorocarbon-montage.png|thumb|85px|Some important fluorocarbons.<br/> <small> A: [[fluoromethane]]<br/> B: [[isoflurane]]<br/> C: [[dichlorodifluoromethane|a CFC]]<br/> D: [[1,1,1,2-Tetrafluoroethane|an HFC]]<br/> E: [[trifluoromethanesulfonic acid|triflic acid]]<br/> F: [[polytetrafluoroethylene|Teflon]]<br/> G: [[perfluorooctane sulfonate|PFOS]]<br/> H: [[fluorouracil]]<br/> I: [[fluoxetine|Prozac]] </small>]] '''Fluorocarbons''' are chemical compounds that contain [[carbon]]-[[fluorine]] [[Chemical bond|bond]]s. The relatively low reactivity and high polarity of the carbon-fluorine bond imparts unique characteristics to fluorocarbons. Fluorocarbons tend to be only slowly broken down in the environment and therefore many are considered [[persistent organic pollutant]]s. Many commercially useful fluorocarbons also contain [[hydrogen]], [[chlorine]], or [[bromine]]. ==Classes of fluorocarbons== ===Chlorofluorocarbons and hydrofluorocarbons=== {{main|haloalkane}} Chlorofluorocarbons (CFCs) are fluorocarbons that also contain [[chlorine]] atoms. They were formerly used widely in industry as [[refrigerant]]s, [[propellant]]s, and cleaning solvents ([[dichlorodifluoromethane]] and [[chlorodifluoromethane]] were among the most widely used refrigerants). However, CFCs generally have potent [[Ozone layer|ozone]]-depleting potential primarily due to [[homolysis|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. {{main|fluoropolymer}} Fluorocarbon [[polymer]]s are also well-known. These polymers are tough, chemical inert, and electrically insulating. The most famous example is [[polytetrafluoroethylene|PTFE (polytetrafluoroethylene)]], a polymer of the [[monomer]] [[tetrafluoroethylene]]. Other important polymers include [[polyvinylidene fluoride]] ([CH<sub>2</sub>CF<sub>2</sub>]<sub>n</sub>) and [[polychlorotrifluoroethylene]] ([CFClCF<sub>2</sub>]<sub>n</sub> or PCTFE, or Kel-F). ===Perfluorocarbons=== {{main|perfluorocarbon}} ===Fluoropolymers=== 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. ==Uses== ===Anesthetics=== {{main|anesthetic}} Many [[volatile anesthetic]]s 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. ===Refrigerants=== {{main|refrigerant}} Some fluorocarbons (e.g. [[Freon]]) have been used as [[refrigerant]]s. 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]]. ===Propellants=== {{main|aerosol spray}} 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 [[Earth's atmosphere|atmosphere]]. ===Solvents=== Fluorocarbons are used as industrial solvents due to their specific properties, including: [[flammability|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: * Precision Cleaning (Degreasing) * Electronic Assemblies Defluxing * Particulate Removal * Drying after Aqueous Cleaning * as a Carrier Fluid * as a Dielectric Coolant HFCs, particularly [[1,1,1,2-tetrafluoroethane]], are used for specialist [[Liquid-liquid extraction|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.<ref>[http://www.fluorocarbons.org/en/applications/other_app/solvents.html Fluorocarbons and Sulphur Hexafluoride], maintained by European Fluorocarbons Technical Committee (EFCTC)</ref> ===Lubrication=== Fluorocarbons are unreactive and are often used for demanding applications. Also, solid fluoropolymers have a low [[coefficient of friction]], while fluid fluoropolymers can act as lubricants. [[Polytetrafluoroethylene|Teflon]] and other similar fluoropolymers are applied as layers to help reduce [[friction]]. Small, self-lubricated parts such as [[stopcock]]s for [[laboratory glassware]] may be entirely made of Teflon. 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" ===Water repellant and stain repellant products=== 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. ===Chemical reagents=== [[Triflic acid]] (CF<sub>3</sub>SO<sub>3</sub>H) and [[trifluoroacetic acid]] (CF<sub>3</sub>CO<sub>2</sub>H) are important reagents in [[organic synthesis]]. They are valuable for their properties as very strong acids that are soluble in organic [[solvent]]s. 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 base]]s. In fact, the polymeric analogue of triflic acid, [[nafion]] is used as a proton-exchange material in [[fuel cell]]s. 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 anion]]s. In these anions (e.g. BF<sub>4</sub><sup>-</sup>, PF<sub>6</sub><sup>-</sup>, B(C<sub>6</sub>H<sub>3</sub>(CF<sub>3</sub>)<sub>2</sub>)<sub>4</sub><sup>-</sup>, and B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub><sup>-</sup> the charge is 'smeared' out over many electronegative atoms. ==Pollution effects== {{main|pollution}} {{main|Ozone depletion}} 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.<ref>[http://www.bom.gov.au/lam/Students_Teachers/ozanim/ozoanim.shtml BOM-ozone-lesson<!-- Bot generated title -->]</ref> 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. ==Biological role== Although there are thousands of known naturally-occurring organic compounds containing chlorine and bromine, there are only a handful of natural fluorocarbons.<ref>D.B. Harper and D. O'Hagan. The Fluorinated Natural Products. ''Natural Product Reports'', 1994, 123-133.</ref> 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 acid]]s, [[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.<ref name="Garrett1">Garrett, Reginald H.; Grisham, Charles M. Principles of Biochemistry with a Human Focus. United States: Brooks/Cole Thomson Learning, 1997.</ref> 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.<ref name=Soong_Diasio">Soong, Richiea and Diasio, Robert B. "Advances and challenges in fluoropyrimidine pharmacogenomics and pharmacogenetics." ''Pharmacogenomics'' 6(8): 835-847, December 2005.</ref> Well known pharmaceutical drugs incorporating fluorine include [[fluoxetine]] (Prozac), [[paroxetine]] (Paxil), [[ciprofloxacin]] (Cipro), [[mefloquine]], [[fluconazole]], and many more. ==Chemical properties== The carbon-fluorine bond length is typically about 1.4 Å (1.39 Å in [[fluoromethane]]). This is shorter than any other carbon-halogen bond, and comparable in length to a carbon-hydrogen bond. 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]] is 552 kJ/mol for carbon-fluorine compared to 397, 288, 209 kJ/mol for bonds between carbon and chlorine, bromine and [[iodine]], respectively.<ref>[http://www.webelements.com/webelements/elements/text/C/enth.html Webelements]</ref> The strength of the carbon-fluorine bond is also significantly stronger than the carbon-hydrogen bond, which is only 338 kJ/mol. 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=== Pentakis(trifluoromethyl)cyclopentadiene (C<sub>5</sub>(CF<sub>3</sub>)<sub>5</sub>H) is a strong [[acid]], with a pK<sub>a</sub> = −2. Its high acidity and robustness is indicated by the fact that this compound is typically purified by [[distillation]] from [[sulfuric acid|H<sub>2</sub>SO<sub>4</sub>]]. In contrast, [[Cp*|C<sub>5</sub>(CH<sub>3</sub>)<sub>5</sub>H]] requires a strong [[Base (chemistry)|base]] such as [[butyllithium]] for deprotonation, as is typical for a hydrocarbon.<ref>R. D. Chambers, A. J. Roche, J. F.S. Vaughan "Direct syntheses of Pentakis(trifluoromethyl)cyclopentadienide Salts and Related Systems" Canadian Journal of Chemistry volume 74, pages 1925-1929 (1996).</ref> This compound is prepared in a multistep, one-pot reaction of [[potassium fluoride]] (KF) with 1,1,2,3,4,4-hexachlorobutadiene. ===Hexafluoroacetone and its imine=== The molecule [[hexafluoroacetone]] ((CF<sub>3</sub>)<sub>2</sub>CO), 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 force|van der Waals]] interactions between molecules. Further demonstrating the remarkable effects of fluorination, (CF<sub>3</sub>)<sub>2</sub>CO forms a stable, distillable hydrate,<ref>{{OrgSynth | author = Van Der Puy, M. ; Anello, L. G. | title = Hexafluoroacetone | collvol = 7 | pages = 251 | prep = cv7p0251}}</ref> (CF<sub>3</sub>)<sub>2</sub>C(OH)<sub>2</sub>. [[Ketone]]s rarely form stable hydrates. Continuing this trend, (CF<sub>3</sub>)<sub>2</sub>CO adds [[ammonia]] to give (CF<sub>3</sub>)<sub>2</sub>C(OH)(NH<sub>2</sub>) which can be dehydrated with POCl<sub>3</sub> to give (CF<sub>3</sub>)<sub>2</sub>CNH.<ref>{{OrgSynth | author = Middleton, W. J.; Carlson, H. D. | title = Hexafluoroacetoneimine | collvol = 6 | collvolpages = 664 | prep = cv6p0664}}</ref> Compounds of the type R<sub>2</sub>C=NH are otherwise quite rare. ===Aliphatic vs. Aromatic Fluorocarbons=== 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.<!-- illustrated here --><ref>J. Lapasset, J. Moret, M. Melas, A. Collet, M. Viguier, H. Blancou, ''Z. Kristallogr.'' '''1996''', ''211'', 945. [[Cambridge Structural Database|CSD]] entry TULQOG. </ref><ref>C.E. Smith, P.S. Smith, R.Ll. Thomas, E.G. Robins, J.C. Collings, Chaoyang Dai, A.J. Scott, S. Borwick, A.S. Batsanov, S.W. Watt, S.J. Clark, C. Viney, J.A.K. Howard, W. Clegg, T.B. Marder, ''J. Mater. Chem.'' '''2004''', ''14'', 413. [[Cambridge Structural Database|CSD]] entry ASIJIV.</ref> [[Image:Aliphatic Fluorocarbon.jpg|left|thumb|325px|Aliphatic Fluorocarbon-Hydrocarbon Packing (Fluorine atoms are green)]] [[Image:Aromatic Fluorocarbon.jpg|thumb|Aromatic Fluorocarbon-Hydrocarbon Packing (Fluorine atoms are green)]] <br clear="all" /> ==Methods for preparation of fluorocarbons== Since fluorocarbons very rarely occur naturally, they must be prepared using synthetic chemistry. Some methods include: *Direct fluorination of hydrocarbons with F<sub>2</sub>, often highly diluted with N<sub>2</sub>. :R<sub>3</sub>CH + F<sub>2</sub> → R<sub>3</sub>CF + HF :Such reactions are important preparatively but require care because hydrocarbons can uncontrollably "burn" in F<sub>2</sub>, analogous to the [[combustion]] of hydrocarbon in O<sub>2</sub>. For example, butane burns in an atmosphere of fluorine. :C<sub>4</sub>H<sub>9</sub> + 12.5 F<sub>2</sub> → 4 CF<sub>4</sub> + 9 HF *[[Metathesis]] reactions employing [[alkali metal]] fluorides <ref>See: [http://www.chempensoftware.com/reactions/RXN202.htm Gryszkiewicz-Trochimowski and McCombie method]</ref>. :R<sub>3</sub>CCl + MF → R<sub>3</sub>CF + MCl (M = Na, K, Cs) *From preformed fluorinated reagents. Many fluorinated building blocks are available: CF<sub>3</sub>X (X = Br, I), C<sub>6</sub>F<sub>5</sub>Br, and C<sub>3</sub>F<sub>7</sub>I. These species form [[Grignard reagents]] that then can be treated with a variety of [[electrophile]]s.<ref>{{OrgSynth | author = Crombie, A.; Kim, S.-Y.; Hadida, S; Curran, and D. P. | title = Synthesis of Tris(2-Perfluorohexylethyl)tin Hydride: A Highly Fluorinated Tin Hydride with Advantageous Features of Easy Purification | collvol = 10 | collvolpages = 712 | year = 2004 | prep = v79p0001}}</ref> *Decomposition of aryldiazonium tetrafluoroborates in the [[Sandmeyer reaction]]<ref>{{OrgSynth | author = Flood, D. T. | title = Fluorobenzene | collvol = 2 | collvolpages = 295 | prep = cv2p0295}}</ref> or [[Schiemann reaction]]: :ArN<sub>2</sub>BF<sub>4</sub> → ArF + N<sub>2</sub> + BF<sub>3</sub> * [[Nucleophilic displacement]] of [[hydroxyl]] and [[carbonyl]] groups by so-called '''deoxofluorination agents'''. One method of fluoride for oxide exchange in carbonyl compounds is with [[sulfur tetrafluoride]]: :RCO<sub>2</sub>H + [[Sulfur tetrafluoride|SF<sub>4</sub>]] → RCF<sub>3</sub> + [[Sulfur dioxide|SO<sub>2</sub>]] + HF :Alternately, organic reagents such as [[diethylaminosulfur trifluoride]] (DAST, NEt<sub>2</sub>SF<sub>3</sub>) and bis(2-methoxyethyl)aminosulfur trifluoride (deoxo-fluor) are easier to handle and more selective:<ref>''Bis(2-methoxyethyl)aminosulfur trifluoride: a new broad-spectrum deoxofluorinating agent with enhanced thermal stability'' Gauri S. Lal, Guido P. Pez, Reno J. Pesaresi and Frank M. Prozonic [[Chem. Commun.]], '''1999''', 215 - 216, {{DOI|10.1039/a808517j}}</ref> :[[Image:Deoxo-Fluor application.png|400px|bis(2-methoxyethyl)aminosulfur trifluoride reaction]] * Electrophilic fluorination reagents also exist, for example [[F-TEDA-BF4|F-TEDA-BF<sub>4</sub>]]. ==References== <references/> ==External links== *[http://www.fluorocarbons.org]Fluorocarbons and Sulphur Hexafluoride, proposed by the European Fluorocarbons Technical Committee (EFCTC) *[http://www.vega.org.uk/video/programme/119 CFCs and Ozone Depletion] Freeview video provided by the Vega Science Trust. *[http://fluoroconsultants.com/db4/00322/fluoroconsultants.com/_download/INTRODUCTIONTOFLUOROPOLYMERS.pdf Introduction to fluoropolymers] *[http://www.journals.royalsoc.ac.uk/content/9jcptv5dak4nafm2/fulltext.pdf Organofluorine chemistry by Graham Sandford] [[Category:Organofluorides]] [[de:Fluorcarbone]] [[es:Fluorocarbono]] [[ja:フルオロカーボン]]