fluorocarbon

Fluorocarbons, sometimes referred to as perfluorocarbons, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. Fluoroalkanes, that contain only single bonds, are more chemically and thermally stable than alkanes. However, fluorocarbons with double bonds (fluoroalkenes) and especially triple bonds (fluoroalkynes) are more reactive than their corresponding hydrocarbons. Fluoroalkanes can serve as oil-repellant/water-repellant fluoropolymers, solvents, liquid breathing research agents, and powerful greenhouse gases. Unsaturated fluorocarbons tend to be used as reactants.

Many chemical compounds are labeled as fluorocarbons, perfluorinated, or with the prefix perfluoro- despite containing atoms other than carbon or fluorine, such as chlorofluorocarbons and perfluorinated compounds; however, these molecules are fluorocarbon derivatives, and not true fluorocarbons. Fluorocarbon derivatives share many of the properties of fluorocarbons, while also possessing new properties due to the inclusion of new atoms. For example, fluorocarbon derivatives can function as fluoropolymers, refrigerants, solvents, anesthetics, fluorosurfactants, and ozone depletors.

Contents 1 Usage of term 2 Properties 2.1 Physical properties 2.1.1 Van der Waals force reduction 2.2 Fluoroalkane stability 2.3 Fluoroalkene and fluoroalkyne reactivity 3 Manufacture 3.1 The Fowler Process 3.2 Electrochemical Fluorination 4 Derivatives 4.1 Fluorosurfactants 4.2 Anesthetics 4.3 Halogenated derivatives 4.4 Hydrofluorocarbons 5 Environmental and Health Concerns 6 Chemical properties 6.1 Pentakis(trifluoromethyl)cyclopentadiene 6.2 Hexafluoroacetone and its imine 6.3 Aliphatic vs. Aromatic Organofluorines 7 Methods for preparation of organofluorines 8 See also 9 External links 10 References

Usage of term

The formal IUPAC definition of a fluorocarbon is a molecule consisting wholly of fluorine and carbon.^[1] However, other fluorocarbon based molecules that are not technically fluorocarbons are commonly referred to as fluorocarbons,^[2] because of similar structures and identical properties. Compounds with atoms other than carbon and fluorine are not true fluorocarbons and they are considered as fluorocarbon derivatives in a separate section below.

Properties

Physical properties

Fluorocarbon liquids are colorless. They have high density, up to over twice that of water, due to their high molecular weight. Low intermolecular forces give the liquids low viscosities when compared to liquids of similar boiling points. Also, low surface tension, heats of vaporization, and refractive indices are notable. They are not miscible with most organic solvents (eg, ethanol, acetone, ethyl acetate and chloroform), but are miscible with some hydrocarbons (eg, 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). The number of carbon atoms in a fluorocarbon molecule largely determines most physical properties. The greater the number of carbon atoms, the higher the boiling point, density, viscosity, surface tension, critical properties, vapor pressure and refractive index. Gas solubility decreases as carbon atoms increase, while melting point is determined by other factors as well, so is not readily predicted.

Van der Waals force reduction

As the high electronegativity of fluorine reduces the polarizability of the atom,^[2] fluorocarbons are only weakly susceptible to the fleeting dipoles that form the basis of the Van der Waals/London dispersion 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 London dispersion force makes the solid polytetrafluoroethylene (PTFE) slippery as it has a very low coefficient of friction. Also, the low attractive forces in fluorocarbon liquids make them compressible and gas soluble while smaller fluorocarbons are extremely volatile.^[2] There are five fluoroalkane gases; tetrafluoromethane (bp −128 °C), hexafluoroethane (bp −78.2 °C), octafluoropropane (bp −36.5 °C), perfluoro-n-butane (bp −2.2 °C) and perfluoro-iso-butane (bp −1 °C). Nearly all other fluoroalkanes are liquids with the exception of perfluorocyclohexane, which sublimes at 51 °C.^[3] As a result of the high gas solubility of fluorocarbon liquids, they have been the subject of medical research as blood carriers because of their oxygen solubility.^[4] Fluorocarbons also have low surface energies and high dielectric strengths.^[2]

Fluoroalkane stability

Fluorocarbons with only single bonds are very stable because of the strength and nature of the carbon–fluorine bond. It is called the strongest bond in organic chemistry.^[5] Its strength is a result of the electronegativity of fluorine imparting partial ionic character through partial charges on the carbon and fluorine atoms.^[5] The partial charges shorten and strengthen the bond through favorable coulombic 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.^[2] Furthermore, multiple carbon–fluorine bonds also strengthen the "skeletal" carbon–carbon bonds from the inductive effect.^[2] Therefore, saturated fluorocarbons are more chemically and thermally stable than their corresponding hydrocarbon counterparts. However, fluoroalkanes are not inert. They are suceptible to reduction through the Birch reduction.

Fluoroalkene and fluoroalkyne reactivity

When fluorocarbons are unsaturated, they are less stable and more reactive than fluoroalkanes, or comparable hydrocarbons, due to the electronegativity of fluorine. The reactivity of the simplest fluoroalkyne, difluoroacetylene, is an example of this instability; difluoroacetylene easily polymerizes.^[2] Another example is fluorofullerene, which has weaker and longer carbon–fluorine bonds than saturated fluorocarbons.^[6] It is reactive towards nucleophiles and hydrolyzes in solution.^[6] Additionally, the polymerization of the fluoroalkene tetrafluoroethylene (which results in PTFE) is more energetically favorable than that of ethylene.^[2] 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.^[2]

Manufacture

Prior to World War II, the only known route to fluorocarbons was by direct reaction of fluorine with the hydrocarbon. This highly exothermic process was capable only of synthesising tetrafluoromethane, hexafluoroethane and octafluoropropane; larger hydrocarbons decomposed in the extreme conditions. The Manhattan project saw the need for some very robust chemicals, including a wider range of fluorcarbons, requiring new manufacturing methods. The so-called "catalytic" method involved reacting fluorine and hydrocarbon on a bed of gold-plated copper turnings, the metal removing the heat of the reaction (so not really acting as a catalyst at all), allowing larger hydrocarbons to survive the process. However, it was the Fowler process that allowed the large scale manufacture of fluorcarbons required for the Manhattan project.

The Fowler Process

The Fowler process uses cobalt fluoride to moderate the reaction. In the laboratory, this is typically done in two stages, the first stage being fluorination of cobalt difluoride to cobalt trifluoride.

2 CoF₂ + F₂ → 2 CoF₃

During the second stage, in this instance to make perfluorohexane, the hydrocarbon feed is introduced and is fluorinated by the cobalt trifluoride, which is converted back to cobalt difluoride. Both stages are performed at high temperature.

C₆H₁₄ + 28 CoF₃ → C₆F₁₄ + 14 HF + 28 CoF₂

Industrially, both steps are combined, for example in the manufacture of the Flutec range of fluorocarbons, using a vertical stirred bed reactor, with hydrocarbon introduced at the bottom, and fluorine introduced half way up the reactor. The fluorocarbon vapor is recovered from the top.

Electrochemical Fluorination

An alternative technique, 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, this is a rather more direct route to fluorocarbons. The process is run 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:

2 N(C₆H₁₃)₃ + 90 HF → 6 C₆F₁₄ + 2 NF₃ + 45 H₂

The perfluorinated amine will also be produced:

N(C₆H₁₃)₃ + 42 HF → 2 N(C₆F₁₃)₃ + 21H₂

Both of these products, and others, are manufactured by 3M as part of the Fluorinert range.

Derivatives

Fluorocarbon derivatives are highly fluorinated molecules that are commonly referred to as fluorocarbons. They are economically useful because they share part or nearly all of the properties of fluorocarbons. Some fluorocarbon derivatives have markedly different properties than fluorocarbons. For example, fluorosurfactants powerfully reduce surface tension by concentrating at the liquid-air interface due to the lipophobicity of fluorocarbons,^[7] due to the polar functional group added to the fluorocarbon chain. Other groups or atoms for fluorocarbon based compounds the oxygen atom incorporated into an ether group for anesthetics, and the chlorine atom for chlorofluorocarbons (CFCs). In a sharp contrast to true fluorocarbons, the chlorine atom produces a chlorine radical which degrades ozone.

Fluorosurfactants

• Perfluorooctanesulfonic acid (PFOS)
• Perfluorooctanoic acid (PFOA)
• Perfluorononanoic acid

Anesthetics

• Methoxyflurane (contains chlorine)
• Enflurane (contains chlorine)
• Isoflurane (contains chlorine)
• Sevoflurane
• Desflurane

Halogenated derivatives

• Polychlorotrifluoroethylene ([CFClCF₂]_n)
• Perfluorooctyl bromide (Perflubron)
• Dichlorodifluoromethane
• Chlorodifluoromethane

Hydrofluorocarbons

• Polyvinylidene fluoride ([CH₂CF₂]_n)
• Tetrafluoroethane

Environmental and Health Concerns

Despite the presence of some natural fluorocarbons and fluorocarbon-derivatives, such as tetrafluoromethane and CFCs, which have been reported in igneous and metamorphic rock,^[8] man-made fluorocarbon based compounds are implicated in a variety of environmental and health related issues. For example, CFCs deplete the ozone layer while fluoroalkanes, commonly referred to as perfluorocarbons, are potent greenhouse gases. Also, the fluorosurfactants PFOS and PFOA, and other related chemicals, are persistent global contaminants. PFOS is a proposed persistent organic pollutant and may be currently harming the health of wildlife.

Chemical properties

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.

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.^[9] 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 London dispersion forces between molecules. Further demonstrating the remarkable effects of fluorination, (CF₃)₂CO forms a stable, distillable hydrate,^[10] (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.^[11] Compounds of the type R₂C=NH are otherwise quite rare.

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 over fluorocarbon-based molecular surfaces.^[2] This behavior is evidenced by the following crystal structures.^[12][13]

Methods for preparation of organofluorines

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

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

R₃CH + F₂ → R₃CF + HF

Such reactions are important in the preparation 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 ^[14].

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.^[15]
• Decomposition of aryldiazonium tetrafluoroborates in the Sandmeyer reaction^[16] 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:^[17]

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

See also

• Trifluoromethyl

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

References

1. ^ International Union of Pure and Applied Chemistry. "fluorocarbons". Compendium of Chemical Terminology Internet edition.
2. ^ ^{a b c d e f g h i j} Lemal DM (January 2004). "Perspective on fluorocarbon chemistry". J. Org. Chem. 69 (1): 1–11. doi:10.1021/jo0302556. PMID 14703372. 
3. ^ http://www.ornl.gov/~webworks/cpr/v823/rpt/108771.pdf
4. ^ Lewandowski G, Meissner E, Milchert E (August 2006). "Special applications of fluorinated organic compounds". J. Hazard. Mater. 136 (3): 385–91. doi:10.1016/j.jhazmat.2006.04.017. PMID 16759798. 
5. ^ ^{a b} 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. 
6. ^ ^{a b} Kiplinger JL, Richmond TG, Osterberg CE (1994). "Activation of Carbon-Fluorine Bonds by Metal Complexes". Chem. Rev. 94 (2): 373–431. doi:10.1021/cr00026a005. http://pubs.acs.org/doi/abs/10.1021/cr00026a005. 
7. ^ Mason Chemical Company: "Fluorosurfactant - Structure / Function" Accessed November 1, 2008.
8. ^ 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.
9. ^ 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).
10. ^ Van Der Puy, M. ; Anello, L. G.. "Hexafluoroacetone". Org. Synth.; Coll. Vol. 7: 251. 
11. ^ Middleton, W. J.; Carlson, H. D.. "Hexafluoroacetoneimine". Org. Synth.; Coll. Vol. 6: 664. 
12. ^ J. Lapasset, J. Moret, M. Melas, A. Collet, M. Viguier, H. Blancou, Z. Kristallogr. 1996, 211, 945. CSD entry TULQOG.
13. ^ 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.
14. ^ See: Gryszkiewicz-Trochimowski and McCombie method
15. ^ 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. 
16. ^ Flood, D. T.. "Fluorobenzene". Org. Synth.; Coll. Vol. 2: 295. 
17. ^ 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