Halogen addition reaction
240971
142078114
2007-07-02T20:00:34Z
V8rik
195918
/* β-Halocarbocations */ NMR
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 0205058388</ref>.
Here is the general [[chemical formula]] of the halogen addition reaction:
:C=C + X<sub>2</sub> → X-C-C-X
(X represents the [[halogen]]s [[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 (''scheme 1''). 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.
[[Image:Alkene-bromine-addition-2D-skeletal.png|500px|center|Scheme 1. Bromine addition to alkene reaction mechanism]]
[[Image:Bromonium-backside-attack-2D-orbitals.png|100px|thumb|right|A bromide ion attacks the C-Br σ* antibonding molecular orbital of a bromonium ion]]
The atom is [[electrophile|electrophilic]] at this time and attacks the negatively charged, high energy [[pi bond]] portion of the [[alkene]]'s [[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 to latching onto both carbons which once formed the [[pi bond|π-bond]], making a three-membered ring. The bromide ion acquires a positive charge, which it shares slightly with the two carbon atoms. At this moment the halogen [[ion]] is called a "[[bromonium ion]]" or "[[chloronium ion]]", respectively.
When the first bromine atom attacks the carbon-carbon pi-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 on 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* [[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 mechamism was proposed by Roberts and Kimball in 1937 <ref>''The Halogenation of Ethylenes'' Irving Roberts and George E. Kimball [[J. Am. Chem. Soc.]]; '''1937'''; 59(5) pp 947 - 948; {{DOI|10.1021/ja01284a507}}</ref>. With it they explained the observed 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 reaction is completely [[stereoselective]] and [[stereospecific]] forming a single [[meso compound]]:
:[[Image:FumaricAcidBromination.png|400px|Bromination of fumaric acid]]
but both by anti-addition. The reaction is even stereoselective in alkenes with two bulky [[tert-butyl]] groups in a cis-position as in the compound ''cis-di-tert-butylethylene'' <ref>''Polar Additions to Olefins. II. The Chlorination of Di-t-butylethylene'' Robert C. Fahey [[J. Am. Chem. Soc.]]; '''1966'''; 88(20) pp 4681 - 4684; {{DOI|10.1021/ja00972a030}}</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]].
:[[Image:AlkeneAndBr2Reaction.png|400px|halogen addition reaction with carbocationic intermediate]]
For reactions taking place through this mechanism no [[stereoselectivity]] 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>''Bromonium ions or beta-bromocarbocations in olefin bromination. A kinetic approach to product selectivities'' Marie Francoise Ruasse [[Acc. Chem. Res.]]; '''1990'''; 23(3) pp 87 - 93; {{DOI|10.1021/ar00171a006}}</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>Stable carbonium ions. XLVIII. Halonium ion formation via neighboring halogen participation. Tetramethylethylene halonium ions George A. Olah and J. Martin Bollinger [[J. Am. Chem. Soc.]]; '''1967'''; 89(18) pp 4744 - 4752; {{DOI|10.1021/ja00994a031}}</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:Addition reactions]]
[[de:Elektrophile Addition]]