Ch 9: Alkynes + X2

Chapter 9: Alkynes

Halogenation of Alkynes

Reaction type: Electrophilic Addition

Summary

Overall transformation : C≡C to X-C=C-X (and potentially to X₂C-CX₂)
Reagent : normally the halogen (e.g. Br₂) in an inert solvent like methylene chloride, CH₂Cl₂.
In the presence of 1 equivalent of the halogen, a 1,2-dihaloalkene X-C=C-X is formed, but in the presence of an excess, the C=C reacts to form a 1,1,2,2-tetrahalide X₂C-CX₂.
Regioselectivity : not relevant since all the new bonds are the same, C-X.
Reaction proceeds via cyclic halonium ion.
Stereoselectivity : anti since the two C-X bonds form in separate steps one from X₂ the other X^-.

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.
Step 2: Attack of the nucleophilic bromide from the side away from the bromonium center in an S_N2 like fashion opens the cyclic bromonium ion to give overall anti addition.