View source for Chemistry:Halogen addition reaction

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

:C=C + X<sub>2</sub> → X−C−C−X

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

This type of reaction is a [[Chemistry:Halogenation|halogenation]] and an [[Chemistry:Electrophilic addition|electrophilic addition]].

==Reaction mechanism==
The [[Physics:Reaction mechanism|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 [[Physics:Electron|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 [[Chemistry: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 [[Chemistry:Sigma bond|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 [[Chemistry:Carbon|carbon]], the bromide [[Physics:Ion|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 [[Chemistry:Bromide|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 fashion, and when the alkene is part of a cycle the dibromide adopts the ''trans'' configuration. For maximum overlap of the C–Br σ* [[Chemistry:Antibonding molecular orbital|antibonding molecular orbital]] (the LUMO, shown to the right in red) and the nucleophile (X<sup>−</sup>) [[Chemistry:Lone pair|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 [[Chemistry:Maleic acid|maleic acid]] and [[Chemistry:Fumaric acid|fumaric acid]]. Maleic acid with a ''cis''-double bond forms the dibromide as a mixture of [[Chemistry:Enantiomer|enantiomer]]s:

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

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

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

The reaction is even stereospecific in alkenes with two bulky ''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 [[Chemistry:Carbocation|carbocation]].

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

For reactions taking place through this mechanism no [[Chemistry:Stereospecificity|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 [[Chemistry:Anethole|anethole]]s and stilbenes 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 [[Chemistry:Magic acid|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 [[Chemistry:Fluorine|fluorine]] compound on the other hand was consistent with a rapidly [[Chemistry:Chemical equilibrium|equilibrating]] pair of β-fluorocarbocations.

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

==References==
{{reflist}}

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

{{Sourceattribution|Halogen addition reaction}}