Halogenation – Alkene Electrophilic Addition Mechanism

As a beginner (or even advanced) organic chemistry student, you will be required to study a number of alkene reaction mechanisms. One of them, the halogenation mechanism, works in an interesting manner. Rather than memorizing the details and potentially messing up the product, I will explain the steps to you, to help you understand how to approach related problems

Halogenation, as the name implies is the addition of halogens or halides to an alkene. Halogens typically used include bromine and chlorine, though you may also see this reaction for fluorine and chlorine. In summary, the dihalide will break the pi bond and add to each of the two carbons in an anti conformation

The alkene starting material has a pi bond rich in electrons, sitting well above and below the trigonal planar sp2 carbons. This allows the pi bonds to be quite reactive, and therefor the nucleophile in this reaction. The halogen molecule is actually a neutral molecule made of non-polar covalent bonding. However, when they get close enough to the pi electrons, the closer halogen will become temporarily polarized with a partial positive charge. This makes the closer halogen the electrophile in this reaction

The electrophilic halogen will induce the nucleophilic pi bond to reach out thereby breaking the carbon to carbon double bond. The alkyl chain now has one carbon atom bound to a halogen, and the second carbon atom with a positive charge. The intial halogen bond is also broken in this attack, leaving one halogen bound to the carbon atom, and the second halogen remains as a halide in solution with a negative charge

Despite being attached to a carbon atom with a full octet, the halogen bound to carbon is attracted to the nearby carbon atom’s positive charge. It will therefor reach out and attack this carbocation with one of it’s lone electron pairs forming a bridge between the two carbon atoms and itself. Since the halogen now has two bonds and two lone pairs, it gets a formal charge of positive one.

Since the halogen is not very stable with a negative charge, it will pass some of the negativity to the attached carbon atoms. With a partial positive charge on carbon, these atoms are considered the electrophilic intermediates. The lone negative halide in solution is the nucleophile in this step. It will reach out and attack one of the partial positive carbon atoms, breaking the halide bridge so that each of the former sp2 carbons are now bound to a halogen

To read about this mechanism in greater detail, visit my page about the Alkene Halogenation Mechanism

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