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Fluorine
Fluorine (from , meaning "to flow") is the chemical element with the symbol '''F''' and atomic number 9. Fluorine forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule. F2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the most chemically reactive and electronegativity|electronegative of all the elements. For example, it will readily 'burn' hydrocarbons at room temperature, in contrast to the combustion of hydrocarbons by oxygen, which requires an activation energy|input of energy with a spark. Therefore, molecular fluorine is highly dangerous, more so than other halogens such as the poisonous chlorine gas.
Fluorine's highest electronegativity and small atomic radius give unique properties to many of its compounds. For example, the enrichment of 235uranium|U, the principal nuclear fuel, relies on the volatility of Uranium hexafluoride|UF6. Also, the carbon–fluorine bond is considered as the strongest bond in organic chemistry. This is why teflon, a polymer made of nothing but carbon-fluorine monomers is so unreactive and so nothing sticks to it. This contributes to the stability and persistence of fluorocarbon-based organofluorine compounds, such as PTFE and PFOS. The carbon–fluorine bond also sharply increases the efficacy of many pharmaceuticals and results in the strength of many superacids.
Characteristics
F2 is a corrosive pale yellow or brown Applications
Elemental fluorine, F2, is mainly used for the production of two compounds of commercial interest, uranium hexafluoride and sulfur hexafluoride.[M. Jaccaud, R. Faron, D. Devilliers, R. Romano “Fluorine” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.]
- '''Industrial use of fluorine-containing compounds:'''
- Atomic fluorine and molecular fluorine are used for plasma etching in semiconductor manufacturing, flat panel display production and MEMS (microelectromechanical systems) fabrication.[Leonel R Arana, Nuria de Mas, Raymond Schmidt, Aleksander J Franz, Martin A Schmidt and Klavs F Jensen, , J. Micromech. Microeng. '''17''' , 2007, pp. 384-392.] Xenon difluoride is also used for this last purpose.
- Hydrofluoric acid (chemical formula hydrogen|HF) is used to etch glass in light bulbs and other products.
- Tetrafluoroethylene and perfluorooctanoic acid (PFOA) are directly used in the production of low friction plastics such as Teflon (or polytetrafluoroethylene).
- Fluorine is used indirectly the production of halons such as freon.
- Along with some of its compounds, fluorine is used in the production of pure uranium from uranium hexafluoride and in the synthesis of numerous commercial fluorochemicals, including vitally important pharmaceuticals, agrochemical compounds, lubricants, and textiles.
- Hydrochlorofluorocarbon|Fluorochlorohydrocarbons are used extensively in air conditioning and in refrigeration. Chlorofluorocarbons have been banned for these applications because they contribute to ozone destruction and the ozone hole. Interestingly, since it is chlorine and bromine radicals which harm the ozone layer, not fluorine, compounds which do not contain chlorine or bromine but contain only fluorine, carbon and hydrogen (called hydrofluorocarbons) are not on the United States Environmental Protection Agency list of ozone-depleting substances,
- Sodium hexafluoroAluminium|aluminate (cryolite), is used in the electrolysis of aluminium.
- In much higher concentrations, sodium fluoride has been used as an insecticide, especially against cockroaches.
- Fluorides have been used in the past to help molten metal flow, hence the name.
- Some researchers including US space scientists in the early 1960s have studied elemental fluorine gas as a possible rocket fuel|rocket propellant due to its exceptionally high specific impulse. The experiments failed because fluorine proved difficult to handle, and its combustion products proved extremely toxic and corrosive.
- Compounds of fluorine such as fluoropolymers, potassium fluoride and cryolite are utilized in applications such as anti-reflective coatings and dichroic mirrors on account of their unusually low refractive index.
Dental and medical uses
- Inorganic compounds of fluoride, including sodium fluoride (NaF), stannous fluoride (SnF2) and Sodium monofluorophosphate|sodium MFP, are used in toothpaste to prevent dental cavities. These or related compounds are also added to some municipal water supplies, a process called water fluoridation, although the practice is sometimes controversial, see Water fluoridation controversy|controversy.
- Many important agents for general anesthesia such as sevoflurane, desflurane, and isoflurane are hydrofluorocarbon derivatives.
- The fluorinated antiinflammatories dexamethasone and triamcinolone are among the most potent of the synthetic corticosteroids class of drugs.[eMedicine - Corticosteroid-Induced Myopathy : Article by Steve S Lim, MD]
- Fludrocortisone ("Florinef") is one of the most common mineralocorticoids, a class of drugs which mimics the actions of aldosterone.
- Fluconazole is a triazole antifungal drug used in the treatment and prevention of superficial and systemic fungal infections.
- Quinolone|Fluoroquinolones are a family of broad-spectrum antibiotics.
- SSRI antidepressants, except in a few instances, are fluorinated molecules. These include citalopram, escitalopram oxalate, fluoxetine, fluvoxamine maleate, and paroxetine. A notable exception is sertraline. Because of the difficulty of biological systems in dealing with metabolism of fluorinated molecules, fluorinated antibiotics and antidepressants are among the major fluorinated organics found in treated city sewage and wastewater.
- Compounds containing Fluorine-18|18F, a radioactive isotope that emits positrons, are often used in positron emission tomography, because its half-life of 110 minutes is long by the standards of positron-emitters. One such species is fluorodeoxyglucose.
Chemistry of fluorine
Fluorine forms a variety of very different compounds, owing to its small atomic size and covalent behavior. Elemental fluorine is a dangerously powerful oxidant, reflecting the extreme electronegativity of fluorine. Hydrofluoric acid is extremely dangerous, whereas in synthetic drugs incorporating an aromatic ring (e.g. flumazenil), fluorine is used to help prevent toxication or to delay metabolism.
The fluoride ion is basic, therefore hydrofluoric acid is a weak acid in water solution. However, water is not an inert solvent in this case: when less basic solvents such as anhydrous acetic acid are used, hydrofluoric acid is the strongest of the hydrohalogenic acids. Also, owing to the basicity of the fluoride ion, soluble fluorides give basic water solutions. The fluoride ion is a Lewis base, and has a high affinity to certain elements such as calcium and silicon. For example, deprotection of silicon protecting groups is achieved with a fluoride. The fluoride ion is poisonous.
Fluorine as a freely reacting oxidant gives the strongest oxidants known. Chlorine trifluoride, for example, can burn water and sand, both compounds of a weaker oxidant, oxygen.
Fluorine compounds involving noble gases were first synthesised by Neil Bartlett in 1962—xenon hexafluoroplatinate, XePtF6, being the first. Fluorides of krypton and radon have also been prepared. Also argon fluorohydride has been prepared, although it is only stable at cryogenic temperatures.
The carbon-fluoride bond is covalent and very stable. The use of a fluorocarbon polymer, poly(tetrafluoroethene) or Teflon, is an example: it is thermostable and waterproof enough to be used in frying pans. Organofluorines may be safely used in applications such as drugs, without the risk of release of toxic fluoride. In synthetic drugs, toxication can be prevented. For example, an aromatic ring is useful but presents a safety problem: enzymes in the body metabolize some of them into poisonous epoxides. When the ''para'' position is substituted with fluorine, the aromatic ring is protected and epoxide is no longer produced.
The substitution of hydrogen for fluorine in organic compounds offers a very large number of chemical compound|compounds. An estimated fifth of pharmaceutical compounds and 30% of agrochemical compounds contain fluorine.[Bernhard Stump, Christian Eberle, W. Bernd Schweizer, Marcel Kaiser, Reto Brun, R. Luise Krauth-Siegel, Dieter Lentz, François Diederich, , ChemBioChem '''10''' (1), 2009, pp. 79-83. PDF-Document ]
Fluorine might form a molecule with unbihexium (Ubh), the theorized island of stability. Specifically, the stable monofluoride UbhF may result from a bonding interaction between the 5g orbital on Ubh and the 2p orbital on fluorine.Production
Industrial production of fluorine entails the electrolysis of hydrogen fluoride in the presence of potassium fluoride. This method is based on the pioneering studies by Moissan in the 1880s. Fluorine gas forms at the anode, and hydrogen gas at the cathode. Under these conditions, the potassium fluoride (KF) converts to potassium bifluoride (KHF2), which is the actual electrolyte, This potassium bifluoride aids electrolysis by greatly increasing the electrical conductivity of the solution.
- HF + KF → KHF2
- 2 KHF2 → 2 KF + H2 + F2
The HF required for the electrolysis is obtained as a byproduct of the production of phosphoric acid. Phosphate-containing minerals contain significant amounts of calcium fluorides, such as fluorite. Upon treatment with sulfuric acid, these minerals release hydrogen fluoride:
- CaF2 + H2SO4 → 2 HF + CaSO4
In 1986, when preparing for a conference to celebrate the 100th anniversary of the discovery of fluorine, Karl Christe discovered a purely chemical preparation involving the reaction of solutions in anhydrous HF, K2MnF6, and antimony pentafluoride|SbF5 at 150 °C:
- F6 + 2F5 → 2F6 + F3 + ½F2
Though not a practical synthesis on the large scale, it demonstrates that electrolysis is not essential.
History
Fluorine in the form of fluorite|fluorspar (also called fluorite, calcium fluoride) was described in 1530 by Georg Agricola|Georgius Agricola for its use as a Flux (metallurgy)|flux,[Fluoride History Discovery of fluorine ] which is a substance that is used to promote the fusion of metals or minerals. In 1670 Schwanhard found that glass was etched when it was exposed to fluorspar that was treated with acid. Carl Wilhelm Scheele and many later researchers, including Humphry Davy, Caroline Menard, Joseph Louis Gay-Lussac|Gay-Lussac, Antoine Lavoisier, and Louis Jacques Thenard|Louis Thenard all would experiment with hydrofluoric acid, easily obtained by treating calcium fluoride (fluorite|fluorspar) with concentrated sulfuric acid.
It was eventually realized that hydrofluoric acid contained a previously unknown element. This element was not isolated for many years after this, due to its extreme reactivity; fluorine can only be prepared from its compounds electrolytically, and then it immediately attacks any susceptible materials in the area. Finally, in 1886, elemental fluorine was isolated by Henri Moissan after almost 74 years of continuous effort by other chemists.Biological role
While F2 is too reactive to have any natural biological role, the fluorine atom is incorporated into compounds with biological roles. However, organofluorine compounds are rare in nature. The most notable example is fluoroacetate, which functions as a plant defence against herbivores in at least 40 plants in Australia, Brazil and Africa.Precautions
Elemental fluorine
Elemental fluorine (fluorine gas) is a highly toxic, corrosive oxidant, which can cause organic material, combustibles, or other flammable materials to ignite. It must be handled with great care and any contact with skin and eyes should be strictly avoided. Fluorine gas has a characteristic pungent odor that is detectable in concentrations as low as 20 ppb. As it is so reactive, all materials of construction must be carefully selected. All metal surfaces must be passivation|passivated before exposure to fluorine.
Fluoride ion
Fluoride ions are also toxic and must also be handled with great care and any contact with skin and eyes should be strictly avoided.
Hydrogen fluoride and hydrofluoric acid
Contact of exposed skin with hydrofluoric acid solutions poses one of the most extreme and insidious industrial threats—one which is exacerbated by the fact that hydrofluoric acid damages nerves in such a way as to make such burns initially painless. The HF molecule is a weaker acid which is significantly non-dissociated in water, and the intact molecule is capable of rapidly migrating through lipid layers of cells which would ordinarily stop an ion or partly ionized acid, and the burns it produces are typically deep. HF may react with calcium, permanently damaging the bone. More seriously, HF reaction with the body's calcium inside cells can cause cardiac arrhythmias, followed by cardiac arrest brought on by sudden chemical changes within the body (hypocalcaemia). These cannot always be prevented with local or intravenous injection of calcium salts. Hydrofluoric acid spills over just 2.5% of the body's surface area (about 75 in2 or 5 dm2), despite copious immediate washing, have been fatal.[Hydrogen fluoride (PIM 268)] If the patient survives, hydrofluoric acid burns typically produce open wounds of an especially slow-healing nature.
Anhydrous hydrogen fluoride will rapidly form hydrofluoric acid on contact with moisture; its physiological effects are then the same.
Organofluorines
Organofluorines are naturally rare compounds. They can be nontoxic (perflubron and perfluorodecalin) or highly toxic (perfluoroisobutylene and fluoroacetic acid). Many pharmacuticals are organofluorines, such as the anti-cancer fluorouracil. Perfluorooctanesulfonic acid (PFOS) is a proposed persistent organic pollutant.
See also
- Fluorocarbon
- Isotopes of fluorine
- :Category:Halide minerals|Halide minerals
- Water fluoridation
Notes
References
- Los Alamos National Laboratory – Fluorine
External links
- WebElements.com – Fluorine
- It's Elemental – Fluorine
- Picture of liquid fluorine – chemie-master.de
- Chemsoc.org
Category:Chemical elements
Category:Halogens
Category:Fluorine|*
Category:Fluorinating agents
zh-min-nan:Fluorine
simple:Fluorine
Related Images- Fluorite (CaF2) crystals
- Fluorine cell room at F2 Chemicals Ltd, Preston, UK
Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL
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