Fluorocarbon

  Fluorocarbons are chemical compounds that contain carbon-fluorine bonds. 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 pollutants. Many commercially useful fluorocarbons also contain hydrogen, chlorine, or bromine.

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Classes of fluorocarbons

Chlorofluorocarbons and hydrofluorocarbons

Main article: haloalkane

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.

Fluoropolymers

Main article: fluoropolymer

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).

Uses

Anesthetics

Main article: anesthetic

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.

Refrigerants

Main article: refrigerant

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.

Propellants

Main article: 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.

Solvents

Flurocarbons 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:

• 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 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.^[1]

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.

Teflon and other similar fluoropolymers are applied as layers to help reduce friction. Small, self-lubricated parts such as stopcocks 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 (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.

Pollution effects

Main article: pollution

Main article: 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.^[2] 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.^[3] 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.^[4] 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.^[5]

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.^[6] 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 (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.^[7] 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 ((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,^[8] (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.^[9] Compounds of the type R₂C=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.^[10][11]

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 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 ^[12].

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

• 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.^[13]
• Decomposition of aryldiazonium tetrafluoroborates in the Sandmeyer reaction^[14] 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:^[15]

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

References

1. ^ Flurocarbons and Sulphur Hexafluoride, maintained by European Fluorocarbons Technical Committee (EFCTC)
2. ^ http://www.bom.gov.au/lam/Students_Teachers/ozanim/ozoanim.shtml
3. ^ D.B. Harper and D. O'Hagan. The Fluorinated Natural Products. Natural Product Reports, 1994, 123-133.
4. ^ Garrett, Reginald H.; Grisham, Charles M. Principles of Biochemistry with a Human Focus. United States: Brooks/Cole Thomson Learning, 1997.
5. ^ Soong, Richiea and Diasio, Robert B. "Advances and challenges in fluoropyrimidine pharmacogenomics and pharmacogenetics." Pharmacogenomics 6(8): 835-847, December 2005.
6. ^ Webelements
7. ^ 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).
8. ^ Van Der Puy, M. ; Anello, L. G.. "Hexafluoroacetone". Org. Synth.: 251; Coll. Vol. 7. 
9. ^ Middleton, W. J.; Carlson, H. D.. "Hexafluoroacetoneimine". Org. Synth.; Coll. Vol. 6: 664. 
10. ^ J. Lapasset, J. Moret, M. Melas, A. Collet, M. Viguier, H. Blancou, Z. Kristallogr. 1996, 211, 945. CSD entry TULQOG.
11. ^ 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.
12. ^ See: Gryszkiewicz-Trochimowski and McCombie method
13. ^ 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. 
14. ^ Flood, D. T.. "Fluorobenzene". Org. Synth.; Coll. Vol. 2: 295. 
15. ^ 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