Monday, October 1, 2012

Halogenation of Alkenes

While studying alkene reaction mechanisms in organic chemistry, you are very likely to come across the halogenation mechanism.

This mechanism is different from the standard alkene addition reactions given the unique nature of the halogen bridge formed as an intermediate of this reactions

Which Molecules Are Involved in Halogenation of Alkenes
Halogenation of alkenes involves the addition of a dihalide onto a carbon to carbon double bound. Dihalides are molecules composed of two atoms of the same halogen bound to each other including fluorine, chlorine, bromine, and iodine.

How Do These Molecules Reaction
The carbons involved in the doubled bound alkene are sp2 hybridized, meaning trigonal planar or flat. The double bond resides in the overlapping 'p' orbital which sits well above and well below the flat molecule. Having the electrons stick out so much from the molcule allows them to attack a nearby molecule as desired

A dihalogen is composed of two equal electronegative atoms. While the bond between them is a non-polar bond, meaning there is equal sharing of electrons, there does exist the phenomenon of induced polarity which makes the molecule temporarily polar

The concept of induced polarity occurs when a negative group such as the pi electrons get too close the halogen molecule. The pi electrons temporarily repel the electrons of the closer halogen onto it's bound partner, making it temporarily partial positive, and the other atom partial negative.

This brief change in polarity induces enough of a partial charge to cause the pi electrons on the alkene to break out of their double bond and attack the halogen.
The bond between the two halides collpase as a result and break onto the second halogen leaving it as a negative ion in solution

The Halogen Bridge
After the pi bond breaks, one of the original attacking carbons will be bound to the halogen, and the second one will have a positive charge due to the loss of the pi bond.
The halogen, while neutral and happy to be bound to the carbon, is still an electronegative atom and will reach out with one lone pair of electrons to attack the positive neighboring carbocation

This second attack by the halogen leaves the atom bound to both carbons. Of the 8 original halide electrons, 4 are now involved in bonding, 4 are found as 2 lone pairs, and the resulting atom has a positive charge. This phenomenon is called a halogen bridge and is not very stable. Some of the negativity is passed onto the connected carbon atoms making them both partially positive

Attacking the Carbon-Halogen Bridge
The free negative halide in solution, which came from the original dihalide molecule is attracted to the partially positive carbon atoms. The halogen bridge sitting on one side of the molecule completely crowds that area and thus prevents the second halide from attacking from the same side.

Instead, the negative halide is forced to attack the partially positive carbons from the opposite side as the halogen bridge. This attack on the carbon atom breaks the halogen bridge by kicking the electrons off the attacked carbon and onto the first halogen atom.

Charge of the Final Product
Having a negative halogen attacking a positive carbon-halogen group cancels out all the charges yielding a neutral final product. This type of reaction is considered an 'anti' reaction given that the second group attacked from the opposite side as the first group.

Structure of the Final Product
The final product is an alkane with two halogen atoms bound to neighboring carbons. These former sp2 carbons are now sp3 hybridized, each is bound to four groups, and has a tetrahedral conformation

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