Everything about Fluoride totally explained
» This article is about the chemical ion F −. For the addition of fluoride ions to water supplies, see Water fluoridation.
Fluoride is the reduced form of
fluorine. Both
organic and
inorganic compounds containing the
element fluorine are considered fluorides. As a
halogen, fluorine forms a monovalent ion (−1 charge). The range of fluorides is considerable as fluorine forms compounds with all elements except He and Ne. Fluorides range from severe toxins such as
sarin to life-saving pharmaceuticals such as
efavirenz and from refractory materials such as
calcium fluoride to highly reactive
sulfur tetrafluoride.
Occurrence
Solutions of inorganic fluorides in water contain F
− and
bifluoride HF
2−. Few inorganic fluorides are soluble in water without undergoing significant hydrolysis. Examples of inorganic fluorides include
hydrofluoric acid (HF),
sodium fluoride (NaF), and
uranium hexafluoride (UF
6). In terms of its reactivity, fluoride differs significantly from
chloride and other halides, and is more strongly solvated due to its smaller radius/charge ratio. Its closest chemical relative is
hydroxide. The Si-F linkage is one of the strongest single bonds. In contrast, other silyl halides are easily hydrolyzed.
Many fluoride minerals are known, but paramount in commercial importance are
fluorite and
fluoroapatite. Fluoride is found naturally in low concentration in drinking water and foods. Water with underground sources is more likely to have higher levels of fluoride, whereas the concentration in seawater averages 1.3
parts per million (ppm). Fresh water supplies generally contain between 0.01-0.3 ppm, while the ocean contains between 1.2 and 1.5 ppm.
Applications
Fluorides are pervasive in modern technology.
Hydrofluoric acid is the most important fluoride synthesized. It is principally used in the production of fluorocarbons and aluminium fluorides. Hydrofluoric acid has a variety of specialized applications, including its ability to dissolve glass.
Organic synthesis
Fluoride reagents are significant in
synthetic organic chemistry. Due to the affinity of
silicon for fluoride, and the ability of silicon to expand its coordination number,
silyl ether protecting groups can be easily removed by the fluoride sources such as
sodium fluoride and
tetra-n-butylammonium fluoride (TBAF).
Enzyme inhibitors
In
biochemistry, fluoride salts are commonly used to
inhibit the activity of
phosphatases, such as
serine/
threonine phosphatases. It may do this by replacing the
nucleophilic hydroxyl ion in these enzymes' active sites.
Beryllium fluoride and
aluminium fluoride are also used as phosphatase inhibitors, since these compounds are structural mimics of the
phosphate group and can act as analogues of the
transition state of the reaction.
Inorganic materials
Sulfur hexafluoride is an inert, nontoxic insulator that's used in electrical transformers.
Uranium hexafluoride is used in the separation of isotopes of uranium between the
fissile isotope
U-235 and the non-fissile isotope
U-238 in preparation of
nuclear reactor fuel and
atomic bombs.
Fluoropolymers
Fluoropolymers such as polytetrafluoroethylene,
Teflon, are used as chemically inert and
biocompatible materials for a variety of applications, including as
surgical implants such as
coronary bypass grafts, and a replacement for
soft tissue in
cosmetic and
reconstructive surgery. These compounds are also commonly used as
non-stick surfaces in
cookware and bakeware, and the fluoropolymer fabric
Gore-Tex used in breathable garments for outdoor use.
Cavity-prevention
Fluoride-containing compounds are used in topical and systemic
fluoride therapy for preventing
tooth decay. They are used for
water fluoridation and in many products associated with
oral hygiene. Originally,
sodium fluoride was used to fluoridate water, however,
hexafluorosilicic acid (H
2SiF
6) and its salt sodium hexafluorosilicate (Na
2SiF
6) are more commonly used additives, especially in the United States. The fluoridation of water prevents tooth decay and is considered by the U.S.
Centers for Disease Control and Prevention as "one of 10 great public health achievements of the 20th century". In some countries where large, centralized water systems are uncommon, fluoride is delivered to the populace by fluoridating table salt. Fluoridation of water isn't without critics, however (see
water fluoridation opposition).
Biomedical applications
Positron emission tomography is commonly carried out using fluoride-containing pharmaceuticals such as
fluorodeoxyglucose, which is labelled with the
radioactive isotope fluorine-18 that emits
positrons when it decays into
18O.
Fluorine is also part of a large variety of drugs including:
antipsychotics such as
fluphenazine,
HIV protease inhibitors such as
tipranavir,
antibiotics such as
ofloxacin and
trovafloxacin, and
anesthetics such as
halothane. These atoms are incorporated in the drug structures to reduce
drug metabolism, as the strong C-F bond resists deactivation in the liver by
cytochrome P450 oxidases.
Toxicology
toxicity, which depends on their reactivity and structure, and in the case of salts, their solubility and ability to release fluoride ions.
Soluble fluoride salts, of which
NaF is the most common, are mildly toxic but have resulted in both accidental and suicidal deaths from
acute poisoning.
Hydrogen fluoride is more dangerous than salts such as NaF because it's corrosive and volatile, and can result in fatal exposure through inhalation or contact with the skin; calcium gluconate gel is the usual antidote.
A few organofluorine compounds are extremely toxic, such as
organophosphates like
sarin and
diisopropylfluorophosphate that react with the
cholinesterase enzyme at
neuromuscular junctions and thus block the transmission of nerve impulses to the muscles. Here, a reactive fluorine-phosphorous bond in the inhibitor is the site of nucleophilic attack by a
serine residue in the enzyme's active site, causing the loss of a F
- ion and
alkylation and inactivation of the enzyme.
While PTFE itself is chemically inert and non-toxic, it begins to deteriorate after the temperature of cookware reaches about 500 °F (260 °C), and decompose above 660 °F (350 °C). These degradation products can be lethal to
birds, and can cause flu-like symptoms in humans.
Further Information
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