halogenation

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halogenation

[‚hal·ə·jə′nā·shən]
(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.

Halogenation

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.

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Halogenated butyl rubber is a key ingredient for manufacturing inner liner of tubeless tyre.
To examine DNA damage induced by BT, we used an immuno-cytometric method to evaluate halogenated DNA, and we determined 8-oxo-deoxy-guanosine (8-oxo-dG) levels using high-performance liquid chromatography (HPLC) coupled with electrochemical detection (ECD).
Based on comparable calculations from halogenated gas emissions in the atmosphere from present-day salt seas in the south of Russia, the scientists calculated that from the Zechstein Sea alone an annual VHC emissions rate of at least 1.
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.
For the most part, halogenated compounds are found in PCBs, solder masks, mold compounds, connectors, cable insulation and wiring conduit.
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.
That raised alarms because the compounds, as complex halogenated chemicals, structurally resemble the pesticide DDT and polychlorinated biphenyls (PCBs), which were once used in flame retardants.
The flame retardants are based in inorganic materials, which avoid the environmental impact and toxicity concerns associated with halogenated flame retardants.
25th International Symposium on Halogenated Environmental Organic Pollutants and POPs (Dioxin 2005), National Water Research Institute, Toronto, ON.
NIST scientists, in collaboration with researchers from the Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, have developed a simplified method of calculating global warming potentials (GWPs) of halogenated compounds.