Halogenation

In chemistry, halogenation is a chemical reaction which introduces one or more halogens into a chemical compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, drugs. This kind of conversion is in fact so common that a comprehensive overview is challenging. This article mainly deals with halogenation using elemental halogens ( ). Halides are also commonly introduced using halide salts and hydrogen halide acids. Many specialized reagents exist for introducing halogens into diverse substrates, e.g. thionyl chloride.

In chemistry, halogenation is a chemical reaction which introduces one or more halogens into a chemical compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polymers, drugs. This kind of conversion is in fact so common that a comprehensive overview is challenging. This article mainly deals with halogenation using elemental halogens ( ). Halides are also commonly introduced using halide salts and hydrogen halide acids. Many specialized reagents exist for introducing halogens into diverse substrates, e.g. thionyl chloride. ==Organic chemistry== Several pathways exist for the halogenation of organic compounds, including free radical halogenation, ketone halogenation, electrophilic halogenation, and halogen addition reaction. The nature of the substrate determines the pathway. The facility of halogenation is influenced by the halogen. Fluorine and chlorine are more electrophilic and are more aggressive halogenating agents. Bromine is a weaker halogenating agent than both fluorine and chlorine, while iodine is the least reactive of them all. The facility of dehydrohalogenation follows the reverse trend: iodine is most easily removed from organic compounds, and organofluorine compounds are highly stable. ===Fluorination=== Because of its extreme reactivity, fluorine ( ) represents a special category with respect to halogenation. Most organic compounds, saturated or otherwise, burn upon contact with , ultimately yielding carbon tetrafluoride. By contrast, the heavier halogens are far less reactive toward saturated hydrocarbons. Highly specialised conditions and apparatus are required for fluorinations with elemental fluorine. Commonly, fluorination reagents are employed instead of . Such reagents include cobalt trifluoride, chlorine trifluoride, and iodine pentafluoride. The method electrochemical fluorination is used commercially for the production of perfluorinated compounds. It generates small amounts of elemental fluorine in situ from hydrogen fluoride. The method avoids the hazards of handling fluorine gas. Many commercially important organic compounds are fluorinated using this technology. ===Halogenation of aromatic compounds=== Aromatic compounds are subject to electrophilic halogenation: This kind of reaction typically works well for chlorine and bromine with electron-rich aromatic substrates. Often a Lewis acidic catalyst is used, such as ferric chloride. Many detailed procedures are available. Lewis bases containing N, O, or S centers can also promote the halogenation of arenes under relatively mild conditions. DMSO-catalyzed chlorination (Jiao chlorination DMSO/NCS) enables the modification of complex molecules. When the aromatic substrate contains electron-withdrawing groups, halogenation does not proceed with the halogens. Potassium bromate in the presence of acid can, however, be used to brominate otherwise recalcitrant aromatic substrates, such as nitrobenzene. The halogenation of electron-deficient arenes can be efficiently catalyzed by m-nitrobenzenesulfonic acid (m-NBSA) under relatively mild conditions. Because fluorine is so reactive, other methods, such as the Balz–Schiemann reaction, are used to prepare fluorinated aromatic compounds. ===Other halogenation methods=== In the Hunsdiecker reaction, carboxylic acids are converted to organic halide, whose carbon chain is shortened by one carbon atom with respect to the carbon chain of the particular carboxylic acid. The carboxylic acid is first converted to its silver salt, which is then oxidized with halogen: Many organometallic compounds react with halogens to give the organic halide: ==Inorganic chemistry== All elements aside from argon, neon, and helium form fluorides by direct reaction with fluorine. Chlorine is slightly more selective, but still reacts with most metals and heavier nonmetals. Following the usual trend, bromine is less reactive and iodine least of all. Of the many reactions possible, illustrative is the formation of gold(III) chloride by the chlorination of gold. The chlorination of metals is usually not very important industrially since the chlorides are more easily made from the oxides and hydrogen chloride. Where chlorination of inorganic compounds is practiced on a relatively large scale is for the production of phosphorus trichloride and disulfur dichloride. ==See also== Dehalogenation Haloalkane (Alkyl halide) Halogenoarene (Aryl halide) Free radical halogenation Haloketone Electrophilic substitution ==References==

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