Organohalogens
Organohalogens are organic compounds that contain one or more halogen atoms (fluorine, chlorine, bromine or iodine). The halogen atom(s) is covalently bonded to a carbon atom. Relatively few organohalogen compounds are found in the terrestrial organisms of nature, but organohalogen compounds are much more common within marine organisms. An organohalogen with an essential role in human biochemistry is thyroxin, the iodine-containing hormone that helps to regulate human metabolism. Simple organohalogens may be produced synthetically from the radical chlorination/bromination of saturated alkanes:
CH4 + Cl2 CH3Cl + HCl
CH4 + Br2 CH3Br + HBr
They may also be produced from the chlorination of alkylbenzenes: C6H5CH3 + Cl2 C6H5CH2Cl + HCl
The electrophilic substitution of aromatic hydrocarbons using ferric chloride catalyst also produces organohalogens: C6H6 + Cl2 C6H5Cl + HCl
And orgaohalogens can be produced from addition reactions involving chlorine/bromine gas (and/or hydrogen chloride/hydrogen bromide) and unsaturated alkene and alkyne hydrocarbons: CH2=CH2 + Cl2 ClCH2CH2Cl
CH2=CH2 + Br2 BrCH2CH2Br
CH2=CH2 + HCl CH3CH2Cl
The corresponding fluoro- and iodo-derivatives are prepared by displacement reactions: CH3CH2CH2Br + KF CH3CH2CH2F + KBr, where the bromine (or chlorine) atom is replaced with a fluorine (or iodine) atom.
Saturated methanes with the general molecular formula of CFxCl4-x containing both fluorine and chlorine atoms are formally referred to as chlorofluorocarbons. These substances, also known as freons, were once used extensively as coolant fluids in refrigerators and air conditioners, and as propellants in aerosol cans. Freons are unreactive, and they remain in the atmosphere for long periods of time. Eventually freons reach altitudes where they are decomposed into chlorine atoms by strong ultraviolet radiation. The resulting chlorine atoms that are generated catalyze the depletion of the ozone layer in the stratosphere. Government regulations have required industries to replace freons with chemicals that do less harm to the ozone layer. The appliance, automotive and building industries have switched from freons to hydrofluorocarbons (i.e., CH2FCH3 and CHCl2CF3) for home refrigerators, car air conditioners and cooling systems in commercial buildings.
Organohalogens are synthesized by the chemical industry to be utilized as chemical intermediates, solvents, and specialty chemicals such as refrigerants, pesticides for crop protection, and herbicides for weed control. Several organochlorine and organobromine compounds are used as medicines in treating diseases and infections. Halothane, CHClBrCF3, is a nonirritating and nonflammable anesthetic used in very low concentrations with oxygen gas or with a nitrous oxide-oxygen gas mixture. Highly substituted alkyl fluorides, perfluoroalkanes, are now used as artificial blood substitutes because of their oxygen transporting properties. The organochlorine polymer polyvinyl chloride (with a -(CH2CClH)n- backbone) is used in the manufacture of records, packing materials, floor tiles, plumbing materials, and raincoats.
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