Alkynes react with halogens in the same way as alkenes. The first addition of the halogen to the π bond produces a trans dihalide which is then converted into a tetrahalide:

The trans dihalide can be prepared by simply adding one equivalent of the halogen.
The stereochemistry of the alkyne Halogenation is explained by the formation of an abridged halonium ion intermediate as we have already seen in the halogenation of alkenes:



