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Halogenation of Alkynes

halogenation of alkynes
Reaction type: Electrophilic Addition

Summary

This JSMOL 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. 

QUESTION
What is the charge on the bromine ?ANSWER

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 ALKYNES WITH HALOGENS
Step 1:
The π electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cyclic bromonium ion as an intermediate.
addition of Br2 to an alkyne
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.

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary