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Fluorocarbon
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Fluorocarbons, sometimes referred to as perfluorocarbons, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. The electron withdrawing nature, or electronegativity, of fluorine results in many of the unique characteristics of fluorocarbons. For example, the electronegativity of fluorine makes single bonds to carbon remarkably strong. Resultingly, fluoroalkanes are more chemically and thermally stable than alkanes.
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Fluorocarbons, sometimes referred to as perfluorocarbons, are organofluorine compounds that contain only carbon and fluorine bonded together in strong carbon–fluorine bonds. The electron withdrawing nature, or electronegativity, of fluorine results in many of the unique characteristics of fluorocarbons. For example, the electronegativity of fluorine makes single bonds to carbon remarkably strong. Resultingly, fluoroalkanes are more chemically and thermally stable than alkanes. However, fluorocarbons with double bonds (fluoroalkenes) and especially triple bonds (fluoroalkynes) are more reactive than corresponding hydrocarbons. Also, the electronegativity of fluorine also reduces the cohesive intermolecular forces of fluorocarbons by mitigating the effect of the London dispersion force. 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, as fluorocarbons with double and triple bonds are not as stable as fluorocarbons with single bonds.
Many chemical compounds are labeled as fluorocarbons, perfluorinated, or with the prefix perfluoro- despite containing atoms other than carbon or fluorine, such as chlorofluorocarbons or perfluorooctanesulfonic acid (PFOS). These highly-fluorinated compounds are fluorocarbon derivatives, and not true fluorocarbons according to the IUPAC definition. 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.
Usage of term
The formal IUPAC definition of a fluorocarbon is a molecule consisting wholly of fluorine and carbon. However, other fluorocarbon based molecules that are not technically fluorocarbons are commonly referred to as fluorocarbons, 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.
General properties of fluorocarbons
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. Its strength is a result of the electronegativity of fluorine imparting partial ionic character through partial charges on the carbon and fluorine atoms. 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. Furthermore, multiple carbon–fluorine bonds also strengthen the "skeletal" carbon–carbon bonds from the inductive effect. Therefore, saturated fluorocarbons are much more chemically and thermally stable than their corresponding hydrocarbon counterparts. However, fluoroalkanes are not inert. They are particularly 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. Another example is fluorofullerene, which has weaker and longer carbon–fluorine bonds than saturated fluorocarbons. It is reactive towards nucleophiles and hydrolyzes in solution. Additionally, the polymerization of the fluoroalkene tetrafluoroethylene (which results in PTFE) is more energetically favorable than that of ethylene. 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.
Van der Waals force reduction
The high electronegativity of fluorine reduces the polarizability of the atom. Therefore, fluorocarbons are only weakly susceptible to the fleeting dipoles that form the basis of the 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) have 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. 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. As a result of the high gas solubility of fluorocarbon liquids, they have medical applications in liquid breathing. Fluorocarbons also have low surface energies and high dielectric strengths.
Examples of fluorocarbons
Fluoroalkanes
Fluoroalkenes
Fluoroalkynes
Properties and examples of fluorocarbon 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, 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
Anesthetics
Halogenated derivatives
Hydrofluorocarbons
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, 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 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.
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, (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 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. This behavior is evidenced by the following crystal structures.
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
- R3CCl + MF ? R3CF + MCl (M = Na, K, Cs)
- 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:
- Electrophilic fluorination reagents also exist, for example F-TEDA-BF4.
See also
External links
- Freeview video provided by the Vega Science Trust.
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