Ch 6: Alkene + X2

Chapter 6: Reactions of Alkenes: Addition Reactions

Halogenation of Alkenes

halogenation of alkenes

Reaction type: Electrophilic Addition

Summary

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

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

Reaction of alkenes with hydrogen halides, HX

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.
Step 2: Attack of the nucleophilic bromide from the side away from the bromonium center opens the cyclic bromonium ion to give overall anti addition.