Halocarbon

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.

Many synthetic organic compounds such as plastic polymers, and a few natural ones, contain halogen atoms; they are known as halogenated compounds. Chlorine is by far the most abundant of the halogens, and the only one needed in relatively large amounts (as chloride ions) by humans. For example, chloride ions play a key role in brain function by mediating the action of the inhibitory transmitter GABA and are also used by the body to produce stomach acid. Iodine is needed in trace amounts for the production of thyroid hormones such as thyroxine. On the other hand, neither fluorine nor bromine are believed to be really essential for humans, although small amounts of fluoride does make teeth enamel somewhat more resistant to attack.

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.
 * haloalkanes -- compounds with carbon atoms linked by single bonds
 * haloalkenes -- compounds with one or more double bonds between carbon atoms
 * haloaromatics -- compounds with carbons linked in one or more aromatic rings with alternating single and double bonds

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.

Natural halocarbons
A large abount of the naturally occurring halocarbons are created by wood fire, dioxine for example, or vulcanic activities. A second large source are marine algae which produce several chlorinated methane and ethane derivates. There are several thousand more complex halocarbons known, produced mainly by marine species. Although clorine compounds are the majority of the discovered compounds, bromides iodides and fluorides have also been found. The tyrian purple, which is a dibromoindigo, is 	representative of the bromides, while the thyroxine secreted from the thyroid gland, is an iodide, and the highly toxic fluoroacetate is one of the rare organofluorides. These three representatives, thyroxine from humans, tyrian purple from snails and fluoroacetate from plants, also show that unrelated species use halocarbons for several purposes.

Uses
The first halocarbon comercially used was Tyrian purple an natural organobromide of the Murex brandaris marine snail.

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 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 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.

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, and by the late 1930s it was known that workers exposed to PCNs could die from liver disease and that DDT would kill mosquitos and other insects. 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".

In 1962 a book by U.S. biologist Rachel Carson 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. 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. 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.

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. 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, 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.