|Chapter 6: Reactions of Alkenes: Addition Reactions|
Halogenation of Alkenes
|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
|MECHANISM FOR REACTION OF ALKENES WITH HALOGENS|
The π electrons act as a nucleophile, attacking the bromine, displacing a bromide ion but forming a cationic 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|