# Dehalogenation

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Dehalogenation
> Markdown URL: https://mediated.wiki/source/Dehalogenation.md
> Source: https://en.wikipedia.org/wiki/Dehalogenation
> Source revision: 1239094623
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

{{Short description|Chemical reaction in which a carbon-halogen bond is cleaved}}
[[File:Scheme for dehalogenation reaction (R = alkyl or aryl group, X = I, Cl, Br, F).png|thumb|Scheme for dehalogenation reaction (R = [alkyl](/source/alkyl) or [aryl group](/source/aryl_group), X = I, Cl, Br, F)]]

In [organic chemistry](/source/organic_chemistry), '''dehalogenation''' is a set of [chemical reaction](/source/chemical_reaction)s that involve the [cleavage](/source/Bond_cleavage) of [carbon](/source/carbon)-[halogen](/source/halogen) bonds; as such, it is the inverse reaction of [halogenation](/source/halogenation). Dehalogenations come in many varieties, including defluorination (removal of [fluorine](/source/fluorine)), dechlorination (removal of [chlorine](/source/chlorine)), debromination (removal of [bromine](/source/bromine)), and deiodination (removal of [iodine](/source/iodine)). Incentives to investigate dehalogenations include both constructive and destructive goals. Complicated organic compounds such as pharmaceutical drugs are occasionally generated by dehalogenation.  Many [organohalide](/source/organohalide)s are hazardous, so their dehalogenation is one route for their detoxification.<ref>{{cite journal |doi=10.1146/annurev.micro.58.030603.123600 |title=Anaerobic Microbial Dehalogenation |year=2004 |last1=Smidt |first1=Hauke |last2=De Vos |first2=Willem M. |journal=Annual Review of Microbiology |volume=58 |pages=43–73 |pmid=15487929 }}</ref>

==Mechanistic and thermodynamic concepts==
Removal of a halogen atom from an organohalide generates a radical. Such reactions are difficult to achieve and, when they can be achieved, these processes often lead to complicated mixtures.  When a pair of halides are mutually adjacent ([vicinal](/source/Vicinal_(chemistry))), their removal is favored.  Such reactions give alkenes in the case of vicinal alkyl dihalides:<ref>{{cite journal |doi=10.15227/orgsyn.036.0019 |title=1,1-Dichloro-2,2-Difluoroethylene |journal=Organic Syntheses |year=1956 |volume=36 |page=19|author=J. C. Sauer }}</ref>
:{{chem2|1=R2C(X)C(X)R2 +  M -> R2C=CR2 +  MX2}}

Most desirable from the perspective of remediation are dehalogenations by [hydrogenolysis](/source/hydrogenolysis), i.e. the replacement of a {{chem2|C\sX}} bond by a {{chem2|C\sH}} bond. Such reactions are amenable to catalysis:
:{{chem2|R\sX  +  H2 -> R\sH  + HX}}

The rate of dehalogenation depends on the strength of the bond between the carbon and halogen atom. The [bond dissociation energies](/source/Bond-dissociation_energy) of carbon-halogen bonds are described as: {{chem2|H3C\sI}} (234&nbsp;kJ/mol), {{chem2|H3C\sBr}} (293&nbsp;kJ/mol), {{chem2|H3C\sCl}} (351&nbsp;kJ/mol), and {{chem2|H3C\sF}} (452&nbsp;kJ/mol). Thus, for the same structures the [bond dissociation rate](/source/Dissociation_constant) for dehalogenation will be: {{nowrap|F &lt; Cl &lt; Br &lt; I}}.<ref name = trost>{{cite book|last1 =Trost|first1= Barry M.|last2= Fleming|first2 = Ian| title= Comprehensive Organic Synthesis – Selectivity, Strategy and Efficiency in Modern Organic Chemistry|volume= 1-9|publisher= Elsevier |date= 1991|pages =793–809|isbn = 0080359299}}</ref> Additionally, the rate of dehalogenation for alkyl halide also varies with [steric](/source/steric) environment and follows this trend:  {{nowrap|primary > secondary > tertiary}} halides.<ref name="trost" />

==Applications==
{{main|Reductive dechlorination}}
Since organochlorine compounds are the most abundant organohalides, most dehalogenations entail manipulation of C-Cl bonds.
<!--==History==

[Organic halides](/source/Halocarbon) belong to a class of organic compounds that contain carbon-halogen bond. In 1832, scientist named [Justus von Liebig](/source/Justus_von_Liebig) synthesized the first organic halide (charcoal) via chlorination of ethanol. Since then, organohalides have gained a lot of attention.<ref>Klein, U. Experiments, models, paper tools: Cultures of organic chemistry in the nineteenth century, Stanford university press: California, 2003, 191-193</ref> Organohalides are commonly used as pesticides, biodegradables, soil fumigants, refrigerants, chemical reagents – solvents, and polymers.<ref name =moon2009>{{cite journal|last1=Moon|first1= J.|last2= Lee|first2= S.|title = Palladium catalyzed-dehalogenation of aryl chlorides and bromides using phosphite ligands|journal = J. Organomet. Chem.|date = 2009|volume= 694 |issue = 3|pages = 473-477|doi = 10.1016/j.jorganchem.2008.10.052}}</ref><ref>Ware, G.; Gunther, F. Reviews of environmental contamination and toxicology, Springer-Verlag: New York, 1998, 155, 1-67.</ref><ref name = trost>{{cite book|last1 =Trost|first1= Barry M.|last2= Fleming|first2 = Ian| title= Comprehensive organic synthesis – selectivity, strategy and efficiency in modern organic chemistry|volume= 1-9|publisher= Elsevier |date= 1991|pages =793-809|isbn = 0080359299}}</ref> It has been classified as pollutant despite of their wide use in various applications. Due to which, dehalogenation is a key reaction to convert toxic organohalides to less hazardous products.
-->

===Organic synthesis===
Of some interest in [organic synthesis](/source/organic_synthesis), electropositive metals react with many organic halides in a [metal-halogen exchange](/source/metal-halogen_exchange):
:{{chem2|RX  +  2 M  ->  RM  +  MX}}
The resulting organometallic compound is susceptible to hydrolysis:
:{{chem2|RM  +  H2O  ->  RH  +  MOH}}
Heavily studied examples are found in [organolithium chemistry](/source/organolithium_chemistry) and [organomagnesium chemistry](/source/organomagnesium_chemistry).  Some illustrative cases follow.

[Lithium-halogen exchange](/source/metal-halogen_exchange) is essentially irrelevant to remediation, but the method is useful for fine chemical synthesis.<ref>Ramón, D.; Yus, M. Masked lithium bishomoenolates: Useful intermediates in organic synthesis, J. Org. Chem. 1991, 56, 3825-3831.</ref><ref>Guijarro, A.; Ramón, D.; Yus, M. Naphthalene-catalysed lithiation of functionalized chloroarenes: regioselective preparation and reactivity of functionalized lithioarenes, Tetrahedron, 1993, 49, 469-482.</ref><ref>Yus, M.; Ramón, D. Arene-catalysed lithiation reactions with lithium at low temperature, Chem. Comm. 1991, 398-400.</ref> Sodium metal has been used for dehalogenation process.<ref>Hawari, J. Regioselectivity of dechlorination: reductive dechlorination of polychlorobiphenyls by polymethylhydrosiloxane-alkali metal. J. Organomet. Chem. 1992, 437, 91-98.</ref><ref>Mackenzie, K.; Kopinke, F.-D. Debromination of duroplastic flame-retarded polymers. Chemosphere, 1996, 33, 2423-2428.</ref>
Removal of halogen atom from arene-halides in the presence of Grignard agent and water for the formation of new compound is known as Grignard degradation. Dehalogenation using Grignard reagents is a two steps hydrodehalogenation process. The reaction begins with the formation of alkyl/arene-magnesium-halogen compound, followed by addition of proton source to form dehalogenated product. Egorov and his co-workers have reported dehalogenation of benzyl halides using atomic magnesium in 3P state at 600&nbsp;°C. Toluene and bi-benzyls were produced as the product of the reaction.<ref>Tarakanova, A.; Anisimov, A.; Egorov, A. Low-temperature dehalogenation of benzyl halides with atomic magnesium in the 3P state. Russian Chemical Bulletin, 1999, 48, 147-151.</ref> Morrison and his co-workers also reported dehalogenation of organic halides by flash vacuum pyrolysis using magnesium.<ref>Aitken, R.; Hodgson, P; Oyewale, A.’ Morrison, J. Dehalogenation of organic halides by flash vacuum pyrolysis over magnesium: a versatile synthetic method. Chem. Commun. 1997, 1163-1164.</ref>

===With transition metal complexes===
Many low-valent and electron-rich transition metals effect stoichiometric dehalogenation.<ref>Grushin, V.; Alper, H. Activation of otherwise unreactive C-Cl bonds. Top. Organomet. Chem. 1999, 3, 193-226.</ref>  The reaction achieves practical interest in the context of organic synthesis, e.g. Cu-promoted [Ullmann coupling](/source/Ullmann_coupling).

The reaction is mainly conducted as stoichiometrically.  Some metalloenzymes [Vitamin B12](/source/Vitamin_B12) and [coenzyme F430](/source/coenzyme_F430) are capable of dehalogenations catalytically.<ref>{{cite journal |doi=10.1039/C5CS00165J |title=Vitamin B12catalysed reactions |year=2015 |last1=Giedyk |first1=Maciej |last2=Goliszewska |first2=Katarzyna |last3=Gryko |first3=Dorota |journal=Chemical Society Reviews |volume=44 |issue=11 |pages=3391–3404 |pmid=25945462 }}</ref>  Of great interest are hydrodehalogenations, especially for chlorinated precursors:<ref>{{cite journal |doi=10.1021/cr0102967 |title=Metal-Mediated Reductive Hydrodehalogenation of Organic Halides |year=2002 |last1=Alonso |first1=Francisco |last2=Beletskaya |first2=Irina P. |last3=Yus |first3=Miguel |journal=Chemical Reviews |volume=102 |issue=11 |pages=4009–4092 |pmid=12428984 }}</ref>
:{{chem2|R\sCl  +  H2  ->  R\sH  +  HCl  }} 
thumb|Dehalogenation using lithium chromium(I) dihydride

thumb|Hydrodefluorination of fluorinated alkenes

==Further reading==
*Gotpagar, J.; Grulke, E.; Bhattacharyya, D.; Reductive dehalogenation of trichloroethylene: kinetic models and *Hetflejš, J.; Czakkoova, M.; Rericha, R.; Vcelak, J. Catalyzed dehalogenation of delor 103 by sodium hydridoaluminate. Chemosphere 2001, 44, 1521.
*Kagoshima, H.; Hashimoto, Y.; Oguro, D.; Kutsuna, T.; Saigo, K. Trophenylphosphine/germanium (IV) chloride combination: A new agent for the reduction of α-bromo carboxylic acid derivatives. Tetrahedron, 1998, 39, 1203-1206

==References==
{{reflist}}

Category:Halogenation reactions
Category:Organic reactions
Category:Inorganic reactions
Category:Halogens

---
Adapted from the Wikipedia article [Dehalogenation](https://en.wikipedia.org/wiki/Dehalogenation) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Dehalogenation?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.