Halocarbon compounds arechemical compounds in which one or morecarbonatoms are linked bycovalent bonds with one or morehalogenatoms (fluorine,chlorine,bromine,iodine, orastatine –group 17) resulting in the formation oforganofluorine compounds,organochlorine compounds,organobromine compounds,organoiodine compounds, andorganoastatine compounds. Chlorine halocarbons are the most common and are calledorganochlorides.^[1]

Many synthetic organic compounds such asplasticpolymers, and a few natural ones, contain halogen atoms; they are known ashalogenated compounds ororganohalogens. 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 asProzac havetrifluoromethyl groups.

For information on inorganic halide chemistry, seehalide.

Halocarbons are typically classified in the same ways as the similarlystructuredorganic compounds that havehydrogenatoms occupying themolecular sites of thehalogenatoms in halocarbons. Among the chemical families are:^[2]

• haloalkanes—compounds withcarbonatoms linked bysingle bonds
• haloalkenes—compounds with one or moredouble bonds betweencarbonatoms
• haloaromatics—compounds withcarbons linked in one or morearomatic rings with a delocalised donut shaped pi cloud.

Thehalogenatoms in halocarbonmolecules are often called "substituents," as though those atoms had been substituted forhydrogen atoms. However halocarbons are prepared in many ways that do not involve direct substitution ofhalogens forhydrogens.

A few halocarbons are produced in massive amounts by microorganisms. For example, several million tons ofmethyl bromide are estimated to be produced by marine organisms annually. Most of the halocarbons encountered in everyday life – solvents, medicines, plastics – are man-made. The first synthesis of halocarbons was achieved in the early 1800s. Production began accelerating when their useful properties as solvents and anesthetics were discovered. Development of plastics and synthetic elastomers has led to greatly expanded scale of production. A substantial percentage of drugs are halocarbons.

A large amount of the naturally occurring halocarbons, such asdioxins, are created by wood fire andvolcanic activity. A third major source is marine algae, which produce several chlorinatedmethane andethane containing compounds. Several thousand complex halocarbons are known to be produced mainly by marine species. Although chlorine compounds are the majority of the discovered compounds, bromides, iodides and fluorides have also been found in nature.Tyrian purple is a bromide and is produced by certain sea snails.Thyroxine is secreted by thethyroid gland and is an iodide. The highly toxicfluoroacetate is one of the rare natural organofluorides and is produced by certain plants.^[3][4][5]

Organoiodine compounds, calledorganic iodides, are similar in structure to organochlorine and organobromine compounds, but the C-I bond is weaker. Many organic iodides are known, but few are of major industrial importance. Iodide compounds are mainly produced as nutritional supplements.^[6]

Thethyroxin hormones are essential for human health, hence the usefulness ofiodized salt.

Six mg of iodide a day can be used to treat patients withhyperthyroidism due to its ability to inhibit the organification process in thyroid hormone synthesis, the so-calledWolff–Chaikoff effect. Prior to 1940, iodides were the predominant antithyroid agents. In large doses, iodides inhibitproteolysis ofthyroglobulin, which permits TH to be synthesized and stored incolloid, but not released into the bloodstream. This mechanism is referred to asPlummer 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 ofthioamides (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.

The first halocarbon commercially used wasTyrian purple, a natural organobromide of theMurex brandaris marine snail.

Common uses for halocarbons have been assolvents,pesticides,refrigerants, fire-resistant oils, ingredients ofelastomers,adhesives and sealants, electrically insulating coatings,plasticizers, andplastics. Many halocarbons have specialized uses in industry. One halocarbon,sucralose, is a sweetener.

Before they became strictly regulated, the general public often encounteredhaloalkanes as paint and cleaning solvents such astrichloroethane (1,1,1-trichloroethane) andcarbon tetrachloride (tetrachloromethane), pesticides like1,2-dibromoethane (EDB, ethylene dibromide), andrefrigerants likeFreon-22 (duPont trademark forchlorodifluoromethane). Some haloalkanes are still widely used for industrial cleaning, such asmethylene chloride (dichloromethane), and as refrigerants, such as R-134a (1,1,1,2-tetrafluoroethane).

Haloalkenes have also been used assolvents, includingperchloroethylene (Perc, tetrachloroethene), widespread in dry cleaning, andtrichloroethylene (TCE, 1,1,2-trichloroethene). Other haloalkenes have been chemical building blocks of plastics such aspolyvinyl chloride ("vinyl" or PVC, polymerized chloroethene) and Teflon (duPont trademark for polymerized tetrafluoroethene,PTFE).

Haloaromatics include the formerAroclors (Monsanto Company trademark forpolychlorinated biphenyls, PCBs), once widely used in power transformers and capacitors and in building caulk, the formerHalowaxes (Union Carbide trademark forpolychlorinated naphthalenes, PCNs), once used for electrical insulation, and thechlorobenzenes and their derivatives, used fordisinfectants,pesticides such as dichloro-diphenyl-trichloroethane (DDT, 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane),herbicides such as2,4-D (2,4-dichlorophenoxyacetic acid),askareldielectrics (mixed with PCBs, no longer used in most countries), and chemical feedstocks.

A few halocarbons, including acid halides likeacetyl chloride, are highlyreactive; 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 bybacteria ormolds; or they may not be affected as much by sun exposure.

The stability of halocarbons tended to encourage beliefs that they were mostly harmless, although in the mid-1920s physicians reported workers inpolychlorinated naphthalene (PCN) manufacturing suffering fromchloracne (Teleky 1927), and by the late 1930s it was known that workers exposed to PCNs could die fromliver disease (Flinn & Jarvik 1936) and thatDDT would killmosquitos and otherinsects (Müller 1948). By the 1950s, there had been several reports and investigations of workplace hazards. In 1956, for example, after testinghydraulic oils containingpolychlorinated biphenyl (PCB)s, the U.S. Navy found that skin contact caused fatalliver disease in animals and rejected them as "too toxic for use in asubmarine" (Owens v. Monsanto 2001).

In 1962 a book by U.S. biologistRachel Carson (Carson 1962) started a storm of concerns about environmentalpollution, first focused onDDT and otherpesticides, 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 (Jensen 1966). In 1974, Mexican chemistMario Molina and U.S. chemistSherwood Rowland predicted that common halocarbonrefrigerants, thechlorofluorocarbons (CFCs), would accumulate in the upperatmosphere and destroy protectiveozone (Molina & Rowland 1974). Within a few years,ozone depletion was being observed aboveAntarctica, leading to bans on production and use ofchlorofluorocarbons in many countries. In 2007, theIntergovernmental Panel on Climate Change (IPCC) said halocarbons were a direct cause ofglobal warming.^[7]

Since the 1970s there have been longstanding, unresolved controversies over potential health hazards oftrichloroethylene (TCE) and other halocarbonsolvents that had been widely used for industrial cleaning (Anderson v. Grace 1986) (Scott & Cogliano 2000) (U.S. National Academies of Science 2004) (United States 2004). More recentlyperfluorooctanoic acid (PFOA), a precursor in the most common manufacturing process for Teflon and also used to make coatings for fabrics andfood packaging, became a health and environmental concern starting in 2006 (United States 2010), 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 presentwaste disposal issues. Because they do not readily degrade in natural environments, halocarbons tend to accumulate.Incineration and accidental fires can createcorrosive byproducts such ashydrochloric acid andhydrofluoric acid, andpoisons like halogenateddioxins andfurans. Species of Desulfitobacterium are being investigated for their potential in thebioremediation of halogenic organic compounds.^[8]

• Halogenation
• Carbon–fluorine bond
• Fluorinated gases
• List of refrigerants

• Anderson v. Grace (1986),628 F. Supp. 1219, Massachusetts, USA{{citation}}: CS1 maint: location missing publisher (link), settled between the parties, reviewed inHarr, J., Ed.; Asher, M., Ed. (1996),A Civil Action, Minneapolis, MN, USA: Sagebrush Education Resources{{citation}}: CS1 maint: multiple names: authors list (link)
• Carson, R. (1962),Silent Spring, Boston, MA, USA: Houghton Mifflin
• Flinn, F.B.; Jarvik, N.E. (1936), "Action of certain chlorinated naphthalenes on the liver",Proceedings of the Society for Experimental Biology and Medicine,35:118–120,doi:10.3181/00379727-35-8879p,S2CID 87157158
• Jensen, S. (1966), "Report of a new chemical hazard",New Scientist,32: 612
• Molina, M.J.; Rowland, F.S. (1974), "Stratospheric sink for chlorofluoromethanes: chlorine atom-catalysed destruction of ozone",Nature,249 (5460):810–812,Bibcode:1974Natur.249..810M,doi:10.1038/249810a0,S2CID 32914300
• Müller, P.H. (1948),"Dichloro-diphenyl-trichloroethane and newer insecticides"(PDF),Nobel Lecture
• Owens v. Monsanto (2001),96-CV-440, Exhibit 3A03F(PDF), Alabama, USA, archived fromthe original(PDF) on 2006-09-28, retrieved2006-01-26{{citation}}: CS1 maint: location missing publisher (link), cited inChemical Industry Archives, Anniston CaseArchived 2005-07-18 at theWayback Machine, by Environmental Working Group, Washington, DC, 2002
• Scott, C.S., Ed.; Cogliano, V.J., Ed. (2000),"Trichloroethylene Health Risks--State of the Science",Environmental Health Perspectives,108 (S2):159–60,doi:10.1289/ehp.00108s2159,PMC 1637768,PMID 10928830, archived fromthe original on 2006-02-19{{citation}}: CS1 maint: multiple names: authors list (link)
• Teleky, L. (1927), "Die pernakrankheit",Klinische Wochenschrift, Jahrgänge 6, Berlin: Springer: 845,doi:10.1007/BF01728520,S2CID 30035538
• U.S. National Academies of Science, Current Projects System (2004),Assessing the Human Health Risks of Trichloroethylene
• United States, Environmental Protection Agency (2004),Integrated Risk Information System, Trichloroethylene (CASRN 79-01-6), archived fromthe original on October 5, 1999
• United States, Environmental Protection Agency (2010),PFOA Stewardship Program (begun in 2006), archived fromthe original on May 9, 2009

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