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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 ?
Halogenated butyl rubber is a key ingredient for manufacturing inner liner of tubeless tyre.
The systemic use of halogenated derivatives of Quinolin-8-ol gained great worldwide acceptance, especially in those countries where amoebiasis was common.
Because myeloid cells copiously express MPO and because halogenated DNA may induce both genetic and epigenetic changes that contribute to carcinogenesis, halogenative stress may account for benzene-induced bone marrow disorders and myeloid leukemia.
Second Nature Medium Temperature systems--designed to keep refrigerated foods at optimum temperatures--replace R404A, a halogenated fluorocarbon (HFC) and contributor to greenhouse gases, with a 35% aqueous propylene glycol fluid and R407C, a non-halogenated refrigerant with less than half of the global warming potential of traditional HFC refrigerants.
The move followed pressure from the European Environmental Bureau (EEB) which highlighted the exposure of consumers through textile products to deca-BDE noting "general concerns about [the] harmful properties for human health and the environment" of halogenated and brominated flame retardants.
Berlin, March 31 (ANI): An international team of scientists has suggested that the largest known mass extinction in the history of the earth could have been triggered off by giant salt lakes, whose emissions of halogenated gases changed the atmospheric composition so dramatically that vegetation was irretrievably damaged.
The latest concern is the inclusion of halogenated materials in flux products.
* Availability in specialty grade polyurethanes, including halogenated and non-halogenated fire retardant, static dissipative and versions certified to many NSF, FDA and European standards.
In the framework of Regulation 2037/2000 EC on substances that deplete the ozone layer, and in compliance with commitments made under the Montreal Protocol on these same products and the protection of the ozone layer, the EU has scheduled the gradual abandonment of the production and use of chlorofluorocarbons, other fully halogenated chlorofluorocarbons, halons, carbon tetrachloride, 1,1,1-trichloroethane, hydrobromofluorocarbons and bromochloromethane.
Published earlier this year by the environmental group, "Killer Couches: Protecting Infants & Children from Toxic Exposure" reports that a high percentage of household furniture in California contain halogenated fire retardants which are toxic to humans and animals.
The research demonstrates that so-called halogenated organic compounds are also produced naturally and "were bioaccumulating in marine mammals--just as PCBs do now--before Monsanto, Dupont, and 3M were making halogenated organic compounds for industrial use," said Emma Teuten and Chris Reddy.