Chapter 9: Alkynes

Halogenation of Alkynes

Summary

• Overall transformation :  CºC to X-C=C-X (and potentially to X₂C-CX₂)
• Reagent : normally the halogen (e.g. Br₂) in an inert solvent like methylene chloride, CH₂Cl₂.
• Regioselectivity : not relevant since all the new bonds are the same, C-X.
• Reaction proceeds via cyclic halonium ion.
• Stereoselectivity : anti since the two C-X bonds form in separate steps one from X₂ the other X^-.

This CHIME image shows the structure of the bromonium ion formed during the reaction of ethyne with Br2.  Note how the Br  is attached to both the C atoms.  What is the charge on the bromine ?  When the Br- nucleophile attacks, it will attack from the least hindered face, the side opposite to the Br in this intermediate.  Make sure you look at the spacefilling model to show this.

MECHANISM FOR REACTION OF ALKENES WITH HALOGENS
Step 1: The p electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cyclic bromonium ion as an intermediate.
Step 2: Attack of the nucleophilic bromide from the side away from the bromonium center in an SN2 like fashion opens the cyclic bromonium ion to give overall anti addition.