X2 Addition

Much like alkenes, alkyne will also undergo addition to alkyne.

• 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.

A few mechanism have been considered and come into play depending upon the structure of the alkyne.

Aryl alkynes tend to proceed through a vinylcation intermediate since a mixture of syn and anti addition poducts are observed.  A vinyl carbocation would be stabilized in such cases by the adjacent aryl group.

The bromide anion can attack either side of the vinyl cation resulting in both syn and anti-products.

Alkyl alkynes reactivity depends upon whether they are internal or terminal alkynes.  Internal alkynes tend to undergo anti-addition, which is consistent with a termolecular or bridged halonium ion.  Terminal alkynes likely proceed through a vinyl cation intermediate.

In a termolecular process, two Br₂ molecules react simultaneously with the alkyne.

Termolecular Process

Bridged Halonium ion