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Halogenated Hydrocarbons

Halogenated Hydrocarbons

Halogenated hydrocarbons are derivatives of hydrocarbons (that is, organic compounds that only contain carbon and hydrogenatoms) that include some halogen atoms within their chemical structure. The most commonly encountered halogens in halogenated hydrocarbons are fluorine and chlorine, but sometimes bromine or iodine occur, or combinations of any of these.

Some halogenated hydrocarbons are naturally synthesized by halogenation reactions during combustion of biomass containing the constituent atoms (that is, carbon, hydrogen, and halogens). For example, these syntheses occur commonly (but at low rates) during forest fires. However, most species of halogenated hydrocarbons are synthetic and are manufactured by humans as industrially useful materials, or are incidentally created either as a byproduct during industrial chemical reactions or when municipal waste is incinerated.

Chlorinated hydrocarbons are a well known group, with a wide variety of uses. A number of these chemicals have been used as insecticides, including DDT, DDD, lindane, chlordane, aldrin, and dieldrin. Others have been used as herbicides, especially 2,4-D and 2,4,5-T.

Polychlorinated biphenyls (PCBs) have been widely used as dielectric fluids in electrical transformers and for other purposes. Dioxins, including the deadly TCDD, are trace contaminants synthesized during the manufacture of other chlorinated hydrocarbons and in spontaneous chlorination reactions in incinerators and pulp mills. Chlorinated hydrocarbons are associated with some well known environmental problems because most of these chemicals are persistent in the environment, and they accumulate in organisms, sometimes causing toxicity.

Chlorofluorocarbons are another group of halogenated hydrocarbons that have been used extensively in refrigeration, air-conditioning, and for dry cleaning. After their use these chemicals are often discharged to the atmosphere, where they are very persistent, and react with sunlight to destroy ozone in the stratosphere. This is an important environmental problem, because ozone helps screen harmful ultraviolet radiation, which can cause skin cancers, cataracts, and other problems. In recognition of the environmental problems associated with these chemicals, the manufacture and use of chlorofluorocarbons has been eliminated through international agreements such as the Montreal Protocol.

See also Bioaccumulation; Dioxin; Hydrocarbon; Hydrochlorofluorocarbons; Ozone layer depletion.

Bill Freedman

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trichloroethylene

trichloroethylene (chloroform, CHCl3) Colourless, volatile, sweet-smelling liquid prepared by the chlorination of methane. Formerly a major anaesthetic, trichloroethylene is used in the manufacture of fluorocarbons, in cough medicines, for insect bites and as a solvent. Properties: r.d. 1.48; m.p. −63.5 °C (−82.3°F); b.p. 61.2°C (142.2°F).

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