organohalogen compounds

Organohalogen compounds are organic compounds that contain at least one halogen (fluorine [F], chlorine [Cl], bromine [Br], or iodine [I]) bonded to carbon. They are subdivided into alkyl, vinylic, aryl, and acyl halides. In alkyl halides, all four bonds to the carbon that bears the halogen are single bonds. In vinylic halides, the carbon that bears the halogen is doubly bonded to another carbon; in aryl halides the halogen-bearing carbon is part of an aromatic ring. In acyl halides (also called acid halides), the halogen-bearing carbon is doubly bonded to oxygen. Acyl halides are treated elsewhere in the menu section Carboxylic Acids And Their Derivatives. The type of carbon to which the halogen is bonded is primarily responsible for the characteristic properties of the members of each class. The great variety in the chemical reactivity of organohalogen compounds depends on the halogen, the class to which they belong, and they may even differ within a class. Many organohalogen compounds, especially organochlorine compounds, are important industrial chemicals; they are used as solvents and pesticides and as intermediates in the preparation of dyes, drugs, and synthetic polymers. More than 1,500 organohalogen compounds have been identified as naturally occurring materials and are produced by various plants, fungi, bacteria, and marine organisms. Alkyl halides (RX, where R is an alkyl group and X = F, Cl, Br, or I) are classified as primary, secondary, or tertiary according to the degree of substitution at the carbon to which the halogen is attached. In a primary alkyl halide, the carbon that bears the halogen is directly bonded to one other carbon, in a secondary alkyl halide to two, and in a tertiary alkyl halide to three. Several alkyl halides are high-volume industrial chemicals, and include the chlorinated derivatives of methane: chloromethane (methyl chloride, CH3Cl), dichloromethane (methylene chloride, CH2Cl2), trichloromethane (chloroform, CHCl3), and tetrachloromethane (carbon tetrachloride, CCl4). A primary use of chloromethane is its reaction with silicon (Si) in the presence of a copper (Cu) catalyst to provide dichlorodimethylsilane, (CH3)2SiCl2, for the preparation of silicone polymers. Dichloromethane is used principally as a solvent, paint remover, aerosol propellant, and foaming agent. Trichloromethane, which is now employed as an industrial solvent, was once used as an inhalation anesthetic but has since been replaced by much safer substances such as halothane (2-bromo-2-chloro-1,1,1-trifluoroethane, BrCHClCF3). The extent to which dichloromethane and trichloromethane are employed is now carefully monitored because of continuing concerns about their toxicity and carcinogenicity. A small proportion of tetrachloromethane is made by chlorination of methane, but most comes from the treatment of carbon disulfide with chlorine. Most tetrachloromethane is as a starting material for the preparation of chlorofluorocarbons, or CFCs. In a reaction using antimony pentafluoride as a catalyst, the chlorine atoms of tetrachloromethane may be sequentially replaced by fluorine. Alkyl halides may easily be converted to other classes of compounds. The three most important reactions of alkyl halides are nucleophilic substitution, elimination, and conversion to organomagnesium compounds. Various families of organic compounds that can be prepared by the appropriate choice of nucleophile include ethers, esters, nitriles, and sulfides. Elimination, which always accompanies nucleophilic substitution, is the primary limitation on efficient synthetic applications of nucleophilic substitution. The use of a sufficiently strong base usually makes it possible to cause elimination to predominate over substitution. Dehydrohalogenation of alkyl halides by the E2 mechanism is one of the main methods by which alkenes are prepared. Many metals, especially those of groups 1 and 2, reduce alkyl halides, converting the carbon-halogen bond to a carbon-metal bond (these are organometalllic compounds, q. v.). The most generally useful organometallic compounds are those of magnesium (Mg), formulated as alkylmagnesium halides, RMgX. Called Grignard reagents, organomagnesium compounds are versatile in synthetic organic chemistry. The most useful reaction of Grignard reagents is their reaction with aldehydes and ketones to form alcohols. Vinylic chlorides and bromides constitute a diverse class of marine natural products. The vinylic halide prepared in greatest amount as an industrial chemical is vinyl chloride (CH2 =CHCl), which is prepared from 1,2-dichloroethane (ClCH2CH2Cl). Chloroprene is the monomer used in the formation of the elastomer neoprene. Tetrafluoroethylene (CF2 =CF2) is the monomer from which the polymer poly(tetrafluoroethylene), PTFE, familiarly known by its trade name Teflon, is prepared. Polymerization of certain vinylic halides yields economically valuable materials. Among synthetic polymers, the annual production of poly(vinyl chloride), or PVC, used as siding for houses, shingles, gutters and downspouts, floor tiles, and pipe fittings, is second only to that of polyethylene. In many organohalogen compounds, the halogen is directly attached to a benzenoid ring occur naturally: these compounds are known as aryl halides. Unlike alkyl and vinylic halides, for which marine origins are the most common, aryl halides are found in a variety of sources. Thyroxine, an iodine-containing amino acid secreted by the thyroid gland, acts as a regulator of metabolism. Several halogen-containing aromatic compounds, although not usually considered natural products, have become widely dispersed in the environment. The most familiar example is 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), an insecticide that persists in the environment. A second chlorine-containing aromatic compound also widespread in the environment is 2,3,7,8-tetrachlorodibenzo-p-dioxin (known simply as dioxin). A group of aryl halides called polychlorinated biphenyls (PCBs), formerly prepared on a large scale for use as heat-transfer mediums and insulating materials in transformers and other electrical equipment, are now recognized to have negative environmental impact similar to that associated with DDDT and dioxin. Aryl halides (ArX), especially bromides and iodides, are converted to Grignard reagents (ArMgX) by reaction with magnesium. Called arylmagnesium halides, these are similar in reactivity and in their applications to alkylmagnesium halides.