Organic Halogen Compounds
Organic halogen compounds are a large class of natural and synthetic chemicals that contain one or more halogens (fluorine, chlorine, bromine, or iodine) combined with carbon and other elements. The simplest organochlorine compound is chloromethane, also called methyl chloride (CH 3 Cl). Other simple organohalogens include bromomethane (CH 3 Br), chloroform (CHCl 3 ), and carbon tetrachloride (CCl 4 ). Some examples of organohalogens are shown in Figure 1.
Organohalogens can be made in various ways. Direct halogenation of hydrocarbons with chlorine gives organochlorines; with bromine, organobromines. Alcohols can be converted into organohalogens by reaction with hydrogen halides.
Aromatic organohalogens such as chlorobenzene are synthesized by treatment of benzene with halogen and a Lewis acid catalyst such as aluminum chloride.
Organohalogen compounds are also produced by adding halogen or hydrogen halide to alkenes and alkynes.
Organoiodines and organofluorines are prepared by displacement reactions.
The reactivity of organohalogens varies enormously. The war gases phosgene (ClCOCl) and mustard (ClCH 2 CH 2 SCH 2 CH 2 Cl) are very reactive and highly toxic, whereas most other organohalogen compounds are relatively inert . Nevertheless, organohalogens undergo many chemical transformations. One common reaction is elimination, induced by the action of a strong base.
As with all chemicals, "the dose makes the poison." The chlorine-containing insecticide dichlorodiphenyltrichloroethane (DDT) is highly effective in killing disease-ridden mosquitoes, ticks, and fleas, but it is virtually nontoxic to mammals. The fluorine-containing pesticide "1080," or fluoroacetic acid (FCH 2 CO 2 H), is highly toxic and often lethal to all mammals. The industrial and combustion by-product dioxin is highly toxic to some animals but not to others; in humans, dioxin causes the skin disease chloracne.
Organohalogens are widely used in industry and society. Chloromethane is used as a solvent in rubber polymerization. Bromomethane is an important fumigant; the related halons (CBrClF 2 and CBrF 3 ) are better fire extinguishants than carbon dioxide.
Eighty-five percent of all pharmaceutical agents and vitamins involve chlorine chemistry; many drugs require chlorine, fluorine, or bromine to be effective. Ceclor and Lorabid are used to treat ear infections, Toremifene is a breast-cancer drug, and the natural antibiotic vancomycin is used to fight penicillin-resistant infections. Benzyl chloride is used to synthesize the drugs phenobarbital, benzedrine, and demerol. Inhalation anesthetics include the organofluorines desflurane, sevoflurane, and enflurane (CHClFCF 2 OCHF 2 ). Perfluorocarbons, such as perfluorotributylamine ([CF 3 CF 2 CF 2 CF 2 ] 3 N), are used as blood substitutes or blood extenders ("artificial blood") and are used for coronary angioplasty. The insecticide DDD (mitotane), related to DDT, is used to treat inoperable adrenal cancer. The chemical advantages to some of these halogenated drugs are shown in Figure 2.
Vinyl chloride (CH 2 =CHCl), a carcinogenic gas, is polymerized to polyvinyl chloride (PVC), a plastic of great versatility and safety. PVC is an invaluable component of building materials, consumer goods, medical equipment, and many other everyday products. More than 2.2 billion kilograms (5 billion pounds) of PVC are used annually for wire, cable, and other electrical applications. The chlorine in PVC makes this plastic flame retardant and ideal for construction and furnishing applications. Polytetrafluoroethylene (Teflon) is the polymer of tetrafluoroethylene (CF 2 =CF 2 ). Because of its chemical stability (very strong carbon-fluorine bonds), it has many diverse applications in our society; best known perhaps are the coatings used to make "nonstick" cookware. Trichloroethylene (CHCl=CCl 2 ) and tetrachloroethylene (CCl 2 =CCl 2 , "Perc") are widely used solvents in the dry cleaning industry.
Organohalogens are essential for crop production and protection as herbicides and insecticides. Ninety percent of grain farms utilize these chemicals in food production. The chemical structures of some of these organohalogens are shown in Figure 3.
Polychlorinated biphenyls (PCBs) were introduced in 1929 as insulators in capacitors and transformers in the electric power industry, as lubricants
and coolants in vacuum pumps, as paint additives, and in food packaging. The manufacture and use of PCBs were discontinued in 1977 because of their adverse effects on the environment. Their effect on humans is still unknown. An example of a PCB is shown in Figure 1.
Chlorofluorocarbons (CFCs or freons) are strongly implicated in causing the ozone hole, and are being phased out of use as refrigerants, dry cleaning solvents, propellants, fire extinguishants, and foam-blowing agents. These chemicals include CFC-11 (CCl 3 F), CFC-13 (CClF 3 ), and CFC-112 (CCl 2 FCCl 2 F). Replacements for CFCs are the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs), both of which have no impact on stratospheric ozone and have low global warming potential. Examples include HCFC-21 (CHCl 2 F) and HFC-152 (FCH 2 CH 2 F).
The number of known natural organohalogens has grown from thirty in 1968 to nearly 3,900 during the early 2000s. Many are the same as synthetic chemicals. They are biosynthesized by marine organisms, bacteria, fungi, plants, insects, and some mammals, including humans. Algae, wood-rotting fungi, mushrooms, several trees, phytoplankton, and even potatoes produce chloromethane. Termites are a major producer of chloroform, and several vegetables produce bromomethane. One hundred organohalogens have been found in the favorite edible seaweed of native Hawaiians.
Chloride and bromide salts are normally present in plants, wood, soil, and minerals; as a result, forest fires and volcanoes produce organohalogens. Meteorites contain organochlorines. Global emissions of chloromethane from the biomass are 5 million tons per year, whereas synthetic emissions are only 26,000 tons per year. Volcanoes also emit hydrogen chloride (3 million tons/year) and hydrogen fluoride (11 million tons/year), both of which can react with organic compounds to produce organohalogens. Chlorofluorocarbons have been detected in volcanic emissions in Guatemala and Siberia, but a study of volcanoes in Italy and Japan indicates that they may not be a major source of environmental CFCs.
Seaweeds contain hundreds of organohalogens (see Figure 4). Telfairine, like the synthetic insecticide lindane, is a powerful insecticide. These organohalogens are used by marine life in chemical defense (natural pesticides). The "smell" of the ocean is likely due to the myriad volatile organohalogens produced by seaweeds.
Organohalogens also serve as hormones. Vegetables such as lentils, beans, and peas synthesize the growth hormone 4-chloro-3-indoleacetic acid. A cockroach produces two chlorine-containing steroids as aggregation pheromones. Female ticks use 2,6-dichlorophenol as a sex attractant. Thyroid hormones (see Figure 1) contain iodine, and an organobromine is involved in the mammalian sleep phenomenon.
Just as iodine is used to treat cuts and chlorine (bleach) to disinfect bathrooms, our white blood cells generate chlorine and bromine to kill germs and fight infection. The sponge metabolite spongistatin and the blue-green
alga cryptophycin, both of which contain a chlorine atom essential for biological activity, are powerful anticancer drugs. Ambigol, found in terrestrial blue-green alga, is active against human immunodeficiency virus (HIV). An Ecuadorian frog produces an organochlorine that is 500 times more potent than morphine; a synthetic analog is under development as a new anesthetic.
Although some synthetic organohalogens are toxic contaminants that need to be removed from the environment, the vast majority of organohalogens have little or no toxicity. Organic halogen compounds continue to play an essential role in human health and well being as chemists pursue the study of these fascinating chemicals.
SEE ALSO Organic Chemistry .
Gordon W. Gribble
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