|Chapter 9: Alkynes|
Halogenation of Alkynes
|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
The π electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cyclic bromonium ion as an intermediate.
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.
|© Dr. Ian Hunt, Department of Chemistry|