Alkene Halogenation


Alkenes can undergo halogenation when reacted with Cl2 or Br2. Alkene halogenation is an electrophilic addition in which a carbon-carbon double bond reacts with a halogen diatomic, dissolving the π bond and adding a halogen atom to each carbon.

Halogenation is related to halohydrin formation: both reactions share a bridged halonium ion intermediate. Halohydrin formation will occur instead of halogenation when there is considerably more H2O than halogen anion during the reaction.


A typical halogenation looks like this:


The (rather odd) mechanism follows. First the π bond of the alkene acts as a nucleophile, attacking one of the bromine atoms, which serves as an electrophile. In the carbocation thus created, the bromine is adjacent to a carbocation within the same molecule. Because of this, the bromine acts as a nucleophile and attacks the adjacent carbocation (this is the only case (that we shall study) in which a bromine acts as a nucleophile). The bridged ion thus created is called a bridged halonium ion. In this case we call the intermediate a "bromonium ion". The bromide produced from the Br2 at the beginning of the reaction now attacks one of the bridge carbons, opening the ring and creating the final product.



The addition of the bromide is anti-addition; the bromide adds on the side opposite the bromonium ion. Because the addition can occur on either side of the bridge, diastereomers of the final product will be produced. However, one or the other diastereomer may be favored by the regiochemistry of the reaction. An illustration of the stereochemistry follows.



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