Halocarbon

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Halocarbon compounds are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms: fluorine, chlorine, bromine or iodine. There are also compounds such as methylammonium chloride that include carbon atoms and noncovalent halogen atoms, also called inorganic halogens. Unlike halocarbon halogens, noncovalent halogen atoms will usually dissociate and ionize in water. Halocarbons are a class of organic compounds containing covalently bonded fluorine, chlorine, bromine, or iodine.

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[edit] Chemical families

Halocarbons are typically classified in the same ways as the similarly structured organic compounds that have hydrogen atoms occupying the molecular sites of the halogen atoms in halocarbons. Among the chemical families are:

The halogen atoms in halocarbon molecules are often called "substituents," as though those atoms had been substituted for hydrogen atoms. However halocarbons are prepared in many ways that do not involve direct substitution of halogens for hydrogens.

[edit] Origins

A few halocarbons, including methyl chloride, are produced in large amounts by natural interactions between halogen salts and debris from plants and animals, but most are created in anything more than minuscule traces only through human efforts. English and French chemists, among others, began to synthesize halocarbons in the 1820s and 1830s and soon discovered halocarbon polymers as well, molecules with long chains of halocarbon groups linked by covalent bonds.

[edit] Uses

Common uses for halocarbons have been as solvents, pesticides, refrigerants, fire-resistant oils, ingredients of elastomers, adhesives and sealants, electrically insulating coatings, plasticizers, and plastics. Many halocarbons have specialized uses in industry.

Before they became strictly regulated, the general public often encountered haloalkanes as paint and cleaning solvents such as trichloroethane (1,1,1-trichloroethane) and carbon tetrachloride (tetrachloromethane), pesticides like ethylene dibromide (EDB, 1,2-dibromoethane), 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-143a (1,1,1,2-tetrafluoroethane).

Haloalkenes have also been used as solvents, 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 Aroclors (Monsanto trademark for polychlorinated biphenyls, PCBs), once widely used in power transformers and capacitors and in building caulk, the former Halowaxes (Union Carbide trademark for polychlorinated naphthalenes, PCNs), once used for electrical insulation, and the chlorobenzenes and their derivatives, used for disinfectants, pesticides 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 dielectrics (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.

[edit] Hazards

The stability of halocarbons tended to encourage beliefs that they were mostly harmless, although in the mid-1920s physicians reported workers in PCN manufacturing suffering from chloracne (Teleky 1927), and by the late 1930s it was known that workers exposed to PCNs could die from liver disease (Flinn & Jarvik 1936) and that DDT would kill mosquitos and other insects (Müller 1948). By the 1950s, there had been several reports and investigations of workplace hazards. In 1956, for example, after testing hydraulic oils containing PCBs, the U.S. Navy found that skin contact caused fatal liver disease in animals and rejected them as "too toxic for use in a submarine" (Owens v. Monsanto 2001).

In 1962 a book by U.S. biologist Rachel Carson (Carson 1962) started a storm of concerns about environmental pollution, first focused on DDT and other pesticides, some of them also halocarbons. These concerns were amplified when in 1966 Swedish chemist Soren Jensen reported widespread residues of PCBs among Arctic and sub-Arctic fish and birds (Jensen 1966). In 1974, U.S. chemists Mario Molina and Sherwood Rowland predicted that common halocarbon refrigerants, the chlorofluorocarbons (CFCs), would accumulate in the upper atmosphere and destroy protective ozone (Molina & Rowland 1974). Within a few years, ozone depletion was being observed above Antarctica, leading to bans on production and use of chlorofluorocarbons in many countries.

Since the 1970s there have been longstanding, unresolved controversies over potential health hazards of trichloroethylene (TCE) and other halocarbon solvents 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 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, has become a health and environmental concern (United States 2006), suggesting that halocarbons 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 like hydrochloric acid and hydrofluoric acid and poisons like halogenated dioxins and furans.

[edit] References

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