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(organic chemistry)
A chemical process or reaction in which a halogen element is introduced into a substance, generally by the use of the element itself.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


A chemical reaction or process which results in the formation of a chemical bond between a halogen atom and another atom. Reactions resulting in the formation of halogen-carbon bonds are especially important. The halogenated compounds produced are employed in many ways, for example, as solvents, intermediates for numerous chemicals, plastic and polymer intermediates, insecticides, fumigants, sterilants, refrigerants, additives for gasoline, and materials used in fire extinguishers.

Halogenation reactions can be subdivided in several ways, for example, according to the type of halogen (fluorine, chlorine, bromine, or iodine), type of material to be halogenated (paraffin, olefin, aromatic, hydrogen, and so on), and operating conditions and methods of catalyzing or initiating the reaction.

Halogenation reactions with elemental chlorine, bromine, and iodine are of considerable importance. Because of high exothermocities, fluorinations with elemental fluorine tend to have high levels of side reactions. Consequently, elemental fluorine is generally not suitable for direct fluorination. Two types of reactions are possible with these halogen elements, substitution and addition.

Substitution halogenation is characterized by the substitution of a halogen atom for another atom (often a hydrogen atom) or group of atoms (or functional group) on paraffinic, olefinic, aromatic, and other hydrocarbons. A chlorination reaction of importance that involves substitution is that between methane and chlorine.

Addition halogenation involves a halogen reacting with an unsaturated hydrocarbon. Chlorine, bromine, and iodine react readily with most olefins; the reaction between ethylene and chlorine to form 1,2-dichloroethane is of considerable commercial importance, since it is used in the manufacture of vinyl chloride.

Addition reactions with bromine or iodine are frequently used to measure quantitatively the number of —CH═CH— (or ethylenic-type) bonds in organic compounds. Bromine numbers or iodine values are measures of the degree of unsaturation of the hydrocarbons.

Substitution halogenation on the aromatic ring can be made to occur via ionic reactions. The chlorination reactions with elemental chlorine are similar to those used for addition chlorination of olefins.

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Okosieme, "Impact of iodination on thyroid pathology in Africa," Journal of the Royal Society of Medicine, vol.
Dur e Shahwar talking to reporter Thursday said people can easily protect their children against low IQ levels and avoid trauma of rearing low learners or those with other physical and mental deficiencies through mandatory iodination of salt.
The results from the phenol-sulfuric acid test, a-naphthol reaction, iodination reaction, Fehling's test, carbazole reaction, Fe[Cl.sub.3] reaction, and Coomassie brilliant blue test were presented.
Klebanoff, "Iodination of bacteria: a bactericidal mechanism.," Journal of Experimental Medicine, vol.
It has long been known that tumor-derived Tg lacks iodination, is poorly glycosylated and shows reduced sulfation of chondrotin sulfate (97-102).
Oxidation of iodide results in the formation of hypoioclite, and the subsequent iodination of bacteria by hypoiodite promotes killing and degradation of the ingested bacteria.
They have scientific expertise in peptide conjugations and in immunoassay, immunohistochemistry and immunocytochemistry kit development and formulation, as well as iodination of peptides.
Sub-clinical hypothyroidism and hyperprolactinemia in infertile women: Bangladesh perspective after universal salt iodination. Internet J Endocrinol 2009; 5(1)
A sudden shift from very low to high iodine intake may induce damage to the thyroid tissue by free radicals and the enhancement of the autoimmunogenic properties of thyroglobulin by increased iodination. (1) Excessive iodine intake reduces organic binding of iodine, resulting in hypothyroidism and goiter, thyroiditis, and autonomous thyroid nodules.
Perchlorate competitively blocks iodide from entering the thyroid by an effect on the Na+/I symporter, but it has no effect on the iodination process itself.