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Halocarbon compounds are chemical compounds in which one or more carbon are linked by with one or more halogen (fluorine, chlorine, bromine or iodine – ) resulting in the formation of organofluorine compounds, organochlorine compounds, organobromine compounds, and organoiodine compounds. Chlorine halocarbons are the most common and are called .Yoel Sasson. "Formation of Carbon–Halogen Bonds (Cl, Br, I)" in Patai's Chemistry of Functional Groups (2009). Wiley-VCH, Weinheim.
Many synthetic organic compounds such as plastic polymers, and a few natural ones, contain halogen atoms; they are known as halogenated compounds or organohalogens. Organochlorides are the most common industrially used organohalides, although the other organohalides are used commonly in organic synthesis. Except for extremely rare cases, organohalides are not produced biologically, but many pharmaceuticals are organohalides. Notably, many pharmaceuticals such as Fluoxetine have trifluoromethyl groups.
For information on inorganic halide chemistry, see halide.
The thyroxin hormones are essential for human health, hence the usefulness of iodized salt.
Six mg of iodide a day can be used to treat patients with hyperthyroidism due to its ability to inhibit the organification process in thyroid hormone synthesis, the so-called Wolff–Chaikoff effect. Prior to 1940, iodides were the predominant antithyroid agents. In large doses, iodides inhibit proteolysis of thyroglobulin, which permits TH to be synthesized and stored in colloid, but not released into the bloodstream. This mechanism is referred to as Plummer effect.
This treatment is seldom used today as a stand-alone therapy despite the rapid improvement of patients immediately following administration. The major disadvantage of iodide treatment lies in the fact that excessive stores of TH accumulate, slowing the onset of action of thioamides (TH synthesis blockers). In addition, the functionality of iodides fades after the initial treatment period. An "escape from block" is also a concern, as extra stored TH may spike following discontinuation of treatment.
Common uses for halocarbons have been as , , , fire-resistant oils, ingredients of , and sealants, electrically insulating coatings, , and . Many halocarbons have specialized uses in industry. One halocarbon, sucralose, is a sweetener.
Before they became strictly regulated, the general public often encountered as paint and cleaning solvents such as trichloroethane (1,1,1-trichloroethane) and carbon tetrachloride (tetrachloromethane), pesticides like 1,2-dibromoethane (EDB, ethylene dibromide), and refrigerants like Freon-22 (duPont trademark for chlorodifluoromethane). Some haloalkanes are still widely used for industrial cleaning, such as methylene chloride (dichloromethane), and as refrigerants, such as R-134a (1,1,1,2-tetrafluoroethane).
Haloalkenes have also been used as , including perchloroethylene (Perc, tetrachloroethene), widespread in dry cleaning, and trichloroethylene (TCE, 1,1,2-trichloroethene). Other haloalkenes have been chemical building blocks of plastics such as polyvinyl chloride ("vinyl" or PVC, polymerized chloroethene) and Teflon (duPont trademark for polymerized tetrafluoroethene, PTFE).
Haloaromatics include the former (Monsanto Company trademark for polychlorinated biphenyls, PCBs), once widely used in power transformers and capacitors and in building caulk, the former (Union Carbide trademark for polychlorinated naphthalenes, PCNs), once used for electrical insulation, and the and their derivatives, used for , such as dichloro-diphenyl-trichloroethane (DDT, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane), herbicides such as 2,4-D (2,4-dichlorophenoxyacetic acid), askarel (mixed with PCBs, no longer used in most countries), and chemical feedstocks.
A few halocarbons, including acid halides like acetyl chloride, are highly reactive; these are rarely found outside chemical processing. The widespread uses of halocarbons were often driven by observations that most of them were more stable than other substances. They may be less affected by acids or alkalis; they may not burn as readily; they may not be attacked by bacteria or molds; or they may not be affected as much by sun exposure.
In 1962 a book by U.S. biologist Rachel Carson started a storm of concerns about environmental pollution, first focused on DDT and other , some of them also halocarbons. These concerns were amplified when in 1966 Danish chemist Soren Jensen reported widespread residues of PCBs among Arctic and sub-Arctic fish and birds . In 1974, Mexican chemist Mario Molina and U.S. chemist Sherwood Rowland predicted that common halocarbon , the chlorofluorocarbons (CFCs), would accumulate in the upper atmosphere and destroy protective ozone . Within a few years, ozone depletion was being observed above Antarctica, leading to bans on production and use of chlorofluorocarbons in many countries. In 2007, the Intergovernmental Panel on Climate Change (IPCC) said halocarbons were a direct cause of global warming. Climate Change 2007: The Physical Science Basis. Summary for Policymakers , page 3
Since the 1970s there have been longstanding, unresolved controversies over potential health hazards of trichloroethylene (TCE) and other halocarbon that had been widely used for industrial cleaning . More recently perfluorooctanoic acid (PFOA), a precursor in the most common manufacturing process for Teflon and also used to make coatings for fabrics and food packaging, became a health and environmental concern starting in 2006 , suggesting that halocarbons, though thought to be among the most inert, may also present hazards.
Halocarbons, including those that might not be hazards in themselves, can present waste disposal issues. Because they do not readily degrade in natural environments, halocarbons tend to accumulate. Incineration and accidental fires can create corrosive byproducts such as hydrochloric acid and hydrofluoric acid, and like halogenated dioxins and . Species of Desulfitobacterium are being investigated for their potential in the bioremediation of halogenic organic compounds.
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