Alkyne Reactions: Alkyne Halogenation using HX

Similar to alkenes, addition of a hydrohalic acid (HX) will result in the halogenation of an alkyne. Specifically, it will result in geminal dihalides, not vicinal:

The reaction proceeds through Markovnikov addition but if both alkyne carbons are equally substituted, a mixture of the two products is expected :

Geminal:

NOT Vicinal:

Mixture of 2 possible products

The reaction mechanism is depicted below:

In the first step, the electrons from the alkyne bond react with a proton on a nearby HBr molecule. Then a different bromine reacts with the carbon atom on the alkyne bond, breaking the H-Br bond and forming a Br-C bond.

The second step is a repeat of the first step. It is important to note that the halogen will be added to the same carbon as it was in Step 1.

HBr was used for these examples; however, HCl, HI, and HF would have the same results.

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