Editing Halogenation (section)

=== Addition of halogens to alkenes and alkynes ===
[[File:96. Адиција на хлор на етин.ogg|thumb|right|Double-addition of [[chlorine gas]] to [[ethyne]]]]
[[Unsaturated compound]]s, especially [[alkenes]] and [[alkynes]], ''add'' halogens:
:{{chem2|R\sCH\dCH\sR' + X2 → R\sCHX\sCHX\sR'}}
In [[oxychlorination]], the combination of [[hydrogen chloride]] and [[oxygen]] serves as the equivalent of [[chlorine]], as illustrated by this route to [[1,2-dichloroethane]]:
:{{chem2|4 HCl + 2 CH2\dCH2 + O2 → 2 Cl\sCH2\sCH2\sCl + 2 H2O}}

[[File:Biadamantylidene-bromonium-ion-from-xtal-1994-2D-skeletal.png|170px|thumb|right|Structure of a [[bromonium ion]]]]
The addition of halogens to alkenes proceeds via [[Reaction intermediate|intermediate]] [[halonium ion]]s. In special cases, such intermediates have been isolated.<ref>{{cite journal | journal = [[Chem. Commun.]] | year = 1998 | pages = 927–928 | doi = 10.1039/a709063c | title = X-Ray structure of bridged 2,2′-bi(adamant-2-ylidene) chloronium cation and comparison of its reactivity with a singly bonded chloroarenium cation |author1=T. Mori |author2=R. Rathore | issue = 8 }}</ref>

Bromination is more [[chemical selectivity|selective]] than chlorination because the reaction is less [[exothermic]]. Illustrative of the bromination of an alkene is the route to the [[anesthetic]] [[halothane]] from [[trichloroethylene]]:<ref>''Synthesis of Essential Drugs'', Ruben Vardanyan, Victor Hruby; Elsevier 2005 {{ISBN|0-444-52166-6}}</ref>
:[[Image:Halothane synthesis.png|600px|Halothane synthesis]]

Iodination and bromination can be effected by the addition of [[iodine]] and [[bromine]] to alkenes. The reaction, which conveniently proceeds with the discharge of the color of {{chem2|I2 and Br2}}, is the basis of the [[analytical method]]. The [[iodine number]] and [[bromine number]] are measures of the [[degree of unsaturation]] for [[fat]]s and other organic compounds.