Return to Contents Chapter 6: Reactions of Alkenes: Addition Reactions Ch 6 contents

Halogenation of Alkenes

halogenation of alkenes

Reaction type: Electrophilic Addition

Summary

These 3D JSMOL images show the structure of the bromonium ion formed during the reaction of ethene with Br2. Note how the Br is attached to both the C atoms. 

QUESTION : What is the formal charge on the bromine atom ? ANSWER

When the Br- nucleophile attacks, it order to reach the C atoms, it will attack from the least hindered face, the side opposite to the large Br in the intermediate. 
Make sure you look at the spacefilling model on the right to visualise this.

Related reactions

MECHANISM FOR REACTION OF ALKENES WITH HALOGENS
Step 1:
The π electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cationic cyclic bromonium ion as an intermediate.
addition of Br2 to C=C
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