# 11.8: Addition of Halogen to Alkynes

## Reaction: Halogenation of Alkynes

Summary:

• Stereoslectivity: anti addition
• Reaction proceeds via cyclic halonium ion

### Addition of Br2

• The addition of Br2 to an alkyne is analogous to adding Br2 to an alkene.
• Once Br2 approaches the nucleophilic alkyne, it becomes polarized.
• The $$\pi$$ electrons, from the triple bond, can now attack the polarized bromine forming a C-Br bond and displacing the bromine ion.
• Now, you will get an intermediate electrophilic carbocation, which will immediately react with the bromine ion giving you the dibromo product.

First, you see the polarized Br2 being attacked by the $$\pi$$ electrons. Once you form the C-Br bond, the other bromine is released as a bromine ion. The intermediate here is a bromonium ion, which is electrophilic and reacts with the bromine ion giving you the dibromo product.