Editing Halogenation (section)

==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 (chemistry)|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.

===Free radical halogenation ===
{{Main|Free-radical halogenation}}
Halogenation of [[saturated hydrocarbon]]s is a [[substitution reaction]]. The reaction typically involves [[free radical]] pathways. The [[regiochemistry]] of the halogenation of [[alkanes]] is largely determined by the relative weakness of the [[Carbon–hydrogen bond|C–H bonds]]. This trend is reflected by the faster reaction at [[Tertiary (chemistry)|tertiary]] and [[Secondary (chemistry)|secondary]] positions. 

Free radical chlorination is used for the industrial production of some [[solvents]]:<ref name=Ullmann>{{Ullmann|doi=10.1002/14356007.a06_233.pub2}}</ref>
:{{chem2|CH4 + Cl2 → CH3Cl + HCl}}

Naturally-occurring [[organobromine compound]]s are usually produced by free radical pathway [[catalyzed]] by the [[enzyme]] [[bromoperoxidase]]. The reaction requires [[bromide]] in combination with [[oxygen]] as an [[oxidant]]. The [[oceans]] are estimated to release 1–2 [[million]] tons of [[bromoform]] and 56,000 tons{{which|reason=Short ton, long ton or metric ton?|date=July 2023}} of [[bromomethane]] annually.<ref>{{cite journal|doi=10.1039/a900201d|title=The diversity of naturally occurring organobromine compounds|year=1999|last1=Gribble|first1=Gordon W.|journal=Chemical Society Reviews|volume=28|issue=5|pages=335–346}}</ref>

The [[iodoform reaction]], which involves degradation of [[methyl ketone]]s, proceeds by the free radical iodination.

===Fluorination===
Because of its extreme reactivity, fluorine ({{chem2|F2}}) represents a special category with respect to halogenation.  Most organic compounds, saturated or otherwise, burn upon contact with {{chem2|F2}}, 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 {{chem2|F2}}. Such reagents include [[cobalt trifluoride]], [[chlorine trifluoride]], and [[iodine pentafluoride]].<ref>{{cite book |doi=10.1002/14356007.a11_307 |chapter=Fluorine Compounds, Inorganic |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2000 |last1=Aigueperse |first1=Jean |last2=Mollard |first2=Paul |last3=Devilliers |first3=Didier |last4=Chemla |first4=Marius |last5=Faron |first5=Robert |last6=Romano |first6=René |last7=Cuer |first7=Jean Pierre |isbn=3-527-30673-0 }}</ref>

The method [[electrochemical fluorination]] is used commercially for the production of [[perfluorinated compound]]s.  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.

=== 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.

===Halogenation of aromatic compounds===
{{main|Aryl halide}}
[[Aromatic compound]]s are subject to [[electrophilic halogenation]]:
:{{chem2|R\sC6H5 + X2 → HX + R\sC6H4\sX}}
This kind of reaction typically works well for [[chlorine]] and [[bromine]]. Often a [[Lewis acid]]ic [[catalyst]] is used, such as [[ferric chloride]].<ref name=PhCl>{{cite book |doi=10.1002/14356007.o06_o03 |chapter=Chlorinated Benzenes and Other Nucleus-Chlorinated Aromatic Hydrocarbons |title=Ullmann's Encyclopedia of Industrial Chemistry |year=2011 |last1=Beck |first1=Uwe |last2=Löser |first2=Eckhard |isbn=978-3527306732 }}</ref> Many detailed procedures are available.<ref>Organic chemistry by Jonathan Clayden, Nick Grieves, Stuart Warren, Oxford University Press</ref><ref>{{OrgSynth|author=Edward R. Atkinson, Donald M. Murphy, and James E. Lufkin |year=1951|title=''dl''-4,4′,6,6′-Tetrachlorodiphenic Acid|volume=31| page=96|doi=10.15227/orgsyn.031.0096}}</ref>
Because [[fluorine]] is so [[Reactivity (chemistry)|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]]:
:{{chem2|R\sCOO−Ag+ + [[Bromine|Br2]] → R\sBr + [[carbon dioxide|CO2]] + [[Silver bromide|Ag+Br−]]}}
:{{chem2|[[Silver acetate|CH3\sCOO−Ag+]] + Br2 → [[Bromomethane|CH3\sBr]] + CO2 + Ag+Br−}}

Many [[organometallic compound]]s react with halogens to give the organic halide:
:{{chem2|RM + X2 → RX + MX}}
:{{chem2|[[n-Butyllithium|CH3CH2CH2CH2Li]] + [[Chlorine|Cl2]] → [[1-chlorobutane|CH3CH2CH2CH2Cl]] + [[Lithium chloride|LiCl]]}}