fluoride

[floor-ahyd, flawr-, flohr-]

fluoride, a salt of hydrofluoric acid; see hydrogen fluoride. See also fluoridation; fluorine.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

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 HF₂⁻. 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 (UF₆). 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 is 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. The volatility of fluorides of uranium and other elements may also be used for nuclear fuel reprocessing.

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 (H₂SiF₆) and its salt sodium hexafluorosilicate (Na₂SiF₆) 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 is not 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

Fluoride-containing compounds are so diverse that it is not possible to generalize on their 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. While the minimum fatal dose in humans is not known, a case of a fatal poisoning of an adult with 4 grams of NaF is documented. Sodium fluorosilicate, Na₂SiF₆ with its higher fluorine content, can cause death with as little as 0.2 g. The fatal period ranges from 5 min to 12 hours. The mechanism of toxicity involves the combination of the fluoride anion with the calcium ions in the blood to form insoluble calcium fluoride, resulting in hypocalcemia; calcium is indispensable for the function of the nervous system, and the condition can be fatal. Treatment may involve oral administration of dilute calcium hydroxide or calcium chloride to prevent further absorption, and injection of calcium gluconate to increase the calcium levels in the blood. Hydrogen fluoride is more dangerous than salts such as NaF because it is 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. In comparison, cooking fats, oils, and butter will begin to scorch and smoke at about 392 °F (200 °C), and meat is usually fried between 400–450 °F (200–230 °C), but empty cookware can exceed this temperature if left unattended on a hot burner.

A 1959 study, (conducted before the Food and Drug Administration approved the material for use in food processing equipment) showed that the toxicity of fumes given off by the coated pan on dry heating was less than that of fumes given off by ordinary cooking oils.