Fluorocarbon

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Fluorocarbons are organofluorines that contain only carbon and fluorine. The carbon-fluorine bond distinguishes the chemical properties of fluorocarbons from hydrocarbons. As such, fluorocarbons find unique applications as polymers, solvents, refrigerants, oil-repellants, and water-repellants. Some fluorocarbons are considered harmful because of contributions to global warming and toxicity.

[edit] Usage of term

The formal IUPAC definition of a fluorocarbon is a molecule consisting wholly of fluorine and carbon.^[1] However, other poly-fluorinated molecules such as anesthetics or perfluorooctyl bromide are commonly referred to as fluorocarbons, because of identical properties.^[2] However, compounds with atoms other than carbon and fluorine are not true fluorocarbons.

[edit] General properties

Fluorocarbons are more chemically and thermally stable than the corresponding hydrocarbons, with high dielectric strengths.^[2] The high electronegativity of fluorine reduces the polarizability of fluorocarbons.^[2] Therefore, fluorocarbons are only weakly susceptible to the fleeting dipoles that form the basis of the van der Waals force. As a result, fluorocarbons have low intramolecular attractive forces and are lipophobic in addition to being hydrophobic/non-polar. Thus fluorocarbons find applications as oil-, water-, and stain-repellants in products such as Gore-Tex, and fluoropolymer carpet coatings. The reduced participation in the van der Walls interactions make the fluorocarbon solid polytetrafluoroethylene (PTFE) have the second lowest coefficient of friction known, while fluorocarbon liquids are compressible and gas soluble, and smaller fluorocarbons are extremely volatile.^[2] Gas soluble fluorocarbon liquids have medical applications. Fluorocarbons have low surface energies.^[2]

[edit] Types of fluorocarbons and derivatives

[edit] Fluoropolymers

Fluoropolymers can be high molecular weight fluorocarbons or fluorocarbon derivatives. PTFE is the most common. Other examples include polyvinylidene fluoride ([CH₂CF₂]_n) and polychlorotrifluoroethylene ([CFClCF₂]_n. Fluoropolymers are processed using fluorosurfactants such as perfluorooctanoic acid (PFOA) and perfluorononanoic acid.

[edit] Perfluorocarbons

Perfluorocarbons are true fluorocarbons. Examples include perfluorodecalin and perfluoroisobutylene. They can be very low (perfluorodecalin) or very high (perfluoroisobutylene) in toxicity. Perfluorocarbon liquids readily solubilize oxygen, allowing some such as to find medical applications in liquid breathing and blood substitution. Perfluorocarbons are notable greenhouse gases.

[edit] Chlorofluorocarbons and hydrofluorocarbons

Chlorofluorocarbons (CFCs) are fluorocarbon derivatives and haloalkanes. They were formerly used widely in industry as refrigerants, propellants, and cleaning solvents. Dichlorodifluoromethane and chlorodifluoromethane were widely used refrigerants. CFCs have potent ozone depletion potential due to the homolytic cleavage of the carbon-chlorine bonds; their use is mostly prohibited by the Montreal Protocol. Hydrofluorocarbons (HFCs), such as tetrafluoroethane, contain only hydrogen and fluorine atoms on the carbon chain. They serve as CFC replacements because they do not catalyze ozone depletion.

[edit] Fluorosurfactants

Fluorosurfactants have a polyfluorinated "tail" and a hydrophilic "head" and they are potent surfactants because they concentrate at the liquid-air interface due to their lipophobicity.^[3] Fluorosurfactants have low surface energies^[2] and dramatically lower surface tension.^[4] The fluorosurfactants perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are two of the most studied because of their ubiquity, toxicity, and long residence times in humans and wildlife.

[edit] Anesthetics

Many volatile anesthetics are fluorocarbon-based ethers, such as methoxyflurane, enflurane, isoflurane, sevoflurane and desflurane. Fluorinated anesthetics reduce the hazard of flammability with diethyl ether and cyclopropane.

[edit] Natural Occurrence of Fluorocarbons and derivatives

CFCs and tetrafluoromethane have been reported in igneous and metamorphic rock.^[5]

[edit] Environmental and Health Concerns

CFC's deplete the ozone layer while fluoroalkanes are potent greenhouse gases. The fluorosurfactants PFOS and PFOA, and other related chemicals, are persistent global contaminants. PFOS is a proposed persistent organic pollutant and may be immunocompromising wildlife.

[edit] Chemical properties

The carbon-fluorine bond length is typically about 1.35 Å (1.39 Å in fluoromethane).^[6] 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.^[7] The strength of the carbon-fluorine bond is also stronger than the carbon-hydrogen bond, which is 104.9 kcal/mol in methane.^[7]

As a result of these unique features of the carbon-fluorine bond, an overarching theme in organofluorine chemistry is the contrasting set of physical and chemical properties in comparison to the corresponding hydrocarbons. Case studies follow.

[edit] Pentakis(trifluoromethyl)cyclopentadiene

Pentakis(trifluoromethyl)cyclopentadiene (C₅(CF₃)₅H) is a strong acid, with a pK_a = −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.^[8] This compound is prepared in a multistep, one-pot reaction of potassium fluoride (KF) with 1,1,2,3,4,4-hexachlorobutadiene.

[edit] Hexafluoroacetone and its imine

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,^[9] (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.^[10] Compounds of the type R₂C=NH are otherwise quite rare.

[edit] Aliphatic vs. Aromatic Organofluorines

Aliphatic organofluorines tend to segregate from aliphatic hydrocarbons while aromatic organofluorines tend to mix with aromatic hydrocarbons. Aliphatic systems self-segregate due to hydrocarbons experiencing greater intermolecular attractive forces (van der Waals forces) over organfluorines.^[2] This behavior is evidenced by the following crystal structures.^[11][12]

Aliphatic Fluorocarbon-Hydrocarbon Packing (Fluorine atoms are green)

Aromatic Fluorocarbon-Hydrocarbon Packing (Fluorine atoms are green)

[edit] Methods for preparation of organofluorines

Since organofluorines very rarely occur naturally, they must be synthesezed. Some methods include:

• Direct fluorination of hydrocarbons with F₂, often highly diluted with N₂.

R₃CH + F₂ → R₃CF + HF

Such reactions are important preparatively but require care because hydrocarbons can uncontrollably "burn" in F₂, analogous to the combustion of hydrocarbon in O₂. For example, butane burns in an atmosphere of fluorine.

C₄H₉ + 12.5 F₂ → 4 CF₄ + 9 HF

• Metathesis reactions employing alkali metal fluorides ^[13].

R₃CCl + MF → R₃CF + MCl (M = Na, K, Cs)

• Metathesis with antimony trifluoride, sometimes with antimony pentachloride as catalyst – the Swarts reaction
• From preformed fluorinated reagents. Many fluorinated building blocks are available: CF₃X (X = Br, I), C₆F₅Br, and C₃F₇I. These species form Grignard reagents that then can be treated with a variety of electrophiles.^[14]
• Decomposition of aryldiazonium tetrafluoroborates in the Sandmeyer reaction^[15] or Schiemann reaction:

ArN₂BF₄ → ArF + N₂ + BF₃

• 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:

RCO₂H + SF₄ → RCF₃ + SO₂ + HF

Alternately, organic reagents such as diethylaminosulfur trifluoride (DAST, NEt₂SF₃) and bis(2-methoxyethyl)aminosulfur trifluoride (deoxo-fluor) are easier to handle and more selective:^[16]

• Electrophilic fluorination reagents also exist, for example F-TEDA-BF₄.

[edit] See also

• Trifluoromethyl

[edit] 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

[edit] References

1. ^ IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A.Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook.F02459
2. ^ ^{a b c d e f g} Lemal DM. "Perspective on Fluorocarbon Chemistry" J Org Chem. 2004, volume 69, p 1-11. doi:10.1021/jo0302556
3. ^ Mason Chemical Company: "Fluorosurfactant - Structure / Function" Accessed November 1, 2008.
4. ^ Jean-Louis Salager: "FIRP Booklet # 300-A: Surfactants-Types and Uses" Universidad de los Andes Laboratory of Formulation, Interfaces Rheology, and Processes. 2002. (Retrieved September 7, 2008).
5. ^ Murphy CD, Schaffrath C, O'Hagan D.: "Fluorinated natural products: the biosynthesis of fluoroacetate and 4-fluorothreonine in Streptomyces cattleya" Chemosphere. 2003 Jul;52(2):455-61.
6. ^ O'Hagan D (February 2008). "Understanding organofluorine chemistry. An introduction to the C-F bond". Chem Soc Rev 37 (2): 308–19. doi:10.1039/b711844a. PMID 18197347. 
7. ^ ^{a b} Blanksby SJ, Ellison GB (April 2003). "Bond dissociation energies of organic molecules". Acc. Chem. Res. 36 (4): 255–63. doi:10.1021/ar020230d. PMID 12693923. 
8. ^ 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).
9. ^ Van Der Puy, M. ; Anello, L. G.. "Hexafluoroacetone". Org. Synth.: 251; Coll. Vol. 7. 
10. ^ Middleton, W. J.; Carlson, H. D.. "Hexafluoroacetoneimine". Org. Synth.; Coll. Vol. 6: 664. 
11. ^ J. Lapasset, J. Moret, M. Melas, A. Collet, M. Viguier, H. Blancou, Z. Kristallogr. 1996, 211, 945. CSD entry TULQOG.
12. ^ 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. CSD entry ASIJIV.
13. ^ See: Gryszkiewicz-Trochimowski and McCombie method
14. ^ Crombie, A.; Kim, S.-Y.; Hadida, S; Curran, and D. P. (2004). "Synthesis of Tris(2-Perfluorohexylethyl)tin Hydride: A Highly Fluorinated Tin Hydride with Advantageous Features of Easy Purification". Org. Synth.; Coll. Vol. 10: 712. 
15. ^ Flood, D. T.. "Fluorobenzene". Org. Synth.; Coll. Vol. 2: 295. 
16. ^ 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