Many synthetic organic compounds such as plasticpolymers, 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 Prozac have trifluoromethyl groups.
For information on inorganic halide chemistry, see halide.
The halogenatoms 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.
History and context
A few halocarbons are produced in massive amounts by microorganisms. For example, several million tons of methyl 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.
Natural halocarbons
A large amount of the naturally occurring halocarbons, such as dioxins, are created by wood fire and volcanic activity. A third major source is marine algae, which produce several chlorinated methane and ethane 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 the thyroid gland and is an iodide. The highly toxic fluoroacetate is one of the rare natural organofluorides and is produced by certain plants.[3][4][5]
Organoiodine compounds, including biological derivatives
Organoiodine compounds, called organic 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]
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.
Uses
The first halocarbon commercially used was Tyrian purple, a natural organobromide of the Murex brandaris marine snail.
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).
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 polychlorinated naphthalene (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 polychlorinated biphenyls (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).
Atmospheric concentration of several halocarbons, years 1978–2015
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 poisons like halogenated dioxins and furans. Species of Desulfitobacterium are being investigated for their potential in the bioremediation of halogenic organic compounds.[8]
↑Yoel Sasson. "Formation of Carbon–Halogen Bonds (Cl, Br, I)" in Patai's Chemistry of Functional Groups (2009). Wiley-VCH, Weinheim. doi:10.1002/9780470682531.pat0011
↑M. Rossberg et al. “Chlorinated Hydrocarbons” in Ullmann’s Encyclopedia of Industrial Chemistry 2006, Wiley-VCH, Weinheim. doi:10.1002/14356007.a06_233.pub2
↑Gordon W. Gribble (2002), Neilson, A. H. (ed.), "Naturally Occurring Organofluorines", Organofluorines, The Handbook of Environmental Chemistry, 3n: 121–136, doi:10.1007/10721878, ISBN3-540-42064-9{{citation}}: CS1 maint: work parameter with ISBN (link).
↑Phyllis A. Lyday "Iodine and Iodine Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.doi:10.1002/14356007.a14_381
Anderson v. Grace (1986), 628 F. Supp. 1219, Massachusetts, USA{{citation}}: CS1 maint: location missing publisher (link), settled between the parties, reviewed in Harr, 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, S2CID87157158
Jensen, S. (1966), "Report of a new chemical hazard", New Scientist, 32: 612
