Editing Halogen addition reaction

A '''halogen addition reaction''' is a simple [[organic reaction]] where a [[halogen]] molecule is added to the [[carbon–carbon double bond]] of an [[alkene]] [[functional group]].<ref>''Organic chemistry'' 4th Ed. Morrison & Boyd {{ISBN|0-205-05838-8}}</ref>

The general [[chemical formula]] of the halogen addition reaction is:

:C=C + X<sub>2</sub> → X−C−C−X

(X represents the halogens [[bromine]] or [[chlorine]], and in this case, a solvent could be [[dichloromethane|CH<sub>2</sub>Cl<sub>2</sub>]] or [[carbon tetrachloride|CCl<sub>4</sub>]]). The product is a [[Vicinal (chemistry)|vicinal]] dihalide.

This type of reaction is a [[halogenation]] and an [[electrophilic addition]].

==Reaction mechanism==
The [[reaction mechanism]] for an alkene bromination can be described as follows. In the first step of the reaction, a bromine molecule approaches the electron-rich alkene carbon–carbon double bond. The bromine atom closer to the bond takes on a partial positive charge as its [[electron]]s are repelled by the electrons of the double bond.

{|align="center" class="wikitable"
|valign=top| [[File:Alkene-bromine-addition-2D-skeletal.png|400px|center|Bromine addition to alkene reaction mechanism]]
|valign=top| [[Image:Bromonium-backside-attack-2D-orbitals.png|100px|A bromide ion attacks the C–Br σ* antibonding molecular orbital of a bromonium ion]]
|-
|valign=top|Bromine addition to alkene reaction mechanism
|valign=top width=200px|A bromide ion attacks the C–Br σ* antibonding molecular orbital of a bromonium ion
|-
|}

The atom is [[electrophile|electrophilic]] at this time and is attacked by the pi electrons of the alkene [carbon–carbon double bond]. It forms for an instant a single [[sigma bond]] to ''both'' of the carbon atoms involved. The bonding of bromine is special in this intermediate, due to its relatively large size compared to [[carbon]], the bromide [[ion]] is capable of interacting with both carbons which once shared the [[π-bond]], making a three-membered ring. The bromide ion acquires a positive formal charge. At this moment the halogen ion is called a "[[bromonium ion]]" or "chloronium ion", respectively.

When the first bromine atom attacks the carbon–carbon π-bond, it leaves behind one of its electrons with the other bromine that it was bonded to in Br<sub>2</sub>. That other atom is now a negative [[bromide]] anion and is attracted to the slight positive charge on the carbon atoms. It is blocked from [[nucleophilic attack]] on one side of the carbon chain by the first bromine atom and can only attack from the other side. As it attacks and forms a bond with one of the carbons, the bond between the first bromine atom and the other carbon atoms breaks, leaving each carbon atom with a halogen substituent.

In this way the two halogens add in an [[anti addition|''anti'' addition]] fashion, and when the alkene is part of a cycle the dibromide adopts the [[trans configuration|''trans'' configuration]]. For maximum overlap of the C–Br σ* [[antibonding molecular orbital]] (the [[LUMO]], shown to the right in red) and the nucleophile (X<sup>−</sup>) [[lone pair]] (the [[HOMO]], shown to the right below in green), X<sup>−</sup> must attack the bromonium ion from behind, at carbon.

This reaction mechanism was proposed by Roberts and Kimball in 1937.<ref>{{cite journal | doi = 10.1021/ja01284a507 | title = The Halogenation of Ethylenes | year = 1937 | last1 = Roberts | first1 = Irving | last2 = Kimball | first2 = George E. | journal = Journal of the American Chemical Society | volume = 59 | issue = 5 | pages = 947}}</ref> With it they explained the observed stereospecific ''trans''-additions in brominations of [[maleic acid]] and [[fumaric acid]]. Maleic acid with a ''cis''-double bond forms the dibromide as a mixture of [[enantiomer]]s:

:[[Image:MaleicAcidBromination.png|400px|Bromination of maleic acid]]

while the ''trans''-isomer fumaric acid forms a single [[meso compound|''meso'' compound]]:

:[[Image:FumaricAcidBromination.png|400px|Bromination of fumaric acid]]

The reaction is even stereospecific in alkenes with two bulky [[tert-butyl|''tert''-butyl]] groups in a ''cis'' position as in the compound ''cis''-di-''tert''-butylethylene.<ref>{{cite journal | doi = 10.1021/ja00972a030 | title = Polar Additions to Olefins. II. The Chlorination of Di-t-butylethylene | year = 1966 | last1 = Fahey | first1 = Robert C. | journal = Journal of the American Chemical Society | volume = 88 | issue = 20 | pages = 4681}}</ref> Despite the [[steric repulsion]] present in the chloronium ion, the only product formed is the ''anti''-adduct.

==β-Halocarbocations==
In an alternative reaction scheme depicted below the reactive intermediate is a β-bromocarbocation or β-bromocarbonium ion with one of the carbon atoms a genuine [[carbocation]].

:[[File:AlkeneAndBr2Reaction.png|400px|halogen addition reaction with carbocationic intermediate]]

For reactions taking place through this mechanism no [[stereospecificity]] is expected and indeed not found.

Roberts and Kimball in 1937 already accounted for the fact that brominations with the [[maleate]] ion resulted in ''cis''-addition driven by repulsion between the negatively charged carboxylic acid anions being stronger than halonium ion formation. In alkenes such as [[anethole]]s and [[stilbene]]s the substituents are able to stabilize the carbocation by donating electrons at the expense of the halonium ion.<ref>{{cite journal | doi = 10.1021/ar00171a006 | title = Bromonium ions or β-bromocarbocations in olefin bromination. A kinetic approach to product selectivities | year = 1990 | last1 = Ruasse | first1 = Marie Francoise | journal = Accounts of Chemical Research | volume = 23 | issue = 3 | pages = 87}}</ref>

Halonium ions can be identified by means of [[NMR spectroscopy]]. In 1967 the group of [[George A. Olah]] obtained NMR spectra of tetramethylethylenebromonium ions by dissolving 2,3-dibromo-2,3-dimethylbutane in [[magic acid]] at −60 °C.<ref>{{cite journal | doi = 10.1021/ja00994a031 | title = Stable carbonium ions. XLVIII. Halonium ion formation via neighboring halogen participation. Tetramethylethylene halonium ions | year = 1967 | last1 = Olah | first1 = George A. | last2 = Bollinger | first2 = J. Martin | journal = Journal of the American Chemical Society | volume = 89 | issue = 18 | pages = 4744}}</ref> The spectrum for the corresponding [[fluorine]] compound on the other hand was consistent with a rapidly [[chemical equilibrium|equilibrating]] pair of β-fluorocarbocations.

==See also==
* Example of bromination in the [[Auwers synthesis]]

==References==
{{reflist}}

[[Category:Electrophilic addition reactions]]
[[Category:Halogenation reactions]]