Similar to alkenes, alkynes add hydrogen halides (HBr, HCl, HI) to the triple bond to form alkenyl halides.
The reaction mechanism occurs through a carbocation, formed on the most substituted carbon of the triple bond. Therefore, it is a regioselective reaction that follows the Markovnikov rule, adding the halogen to the most substituted carbon of the alkyne.
![Addition of hydrogen halides to alkynes Adición de haluros de hidrógeno a alquinos](/images/stories/organica-i/alkynes/adicion-haluros-hidrogeno/adicion-haluros-hidrogeno01.png)
Stage 1. Electrophilic addition. The proton adds to the triple bond, joining the least substituted carbon.
![Addition of hydrogen halides to alkynes Adición de haluros de hidrógeno a alquinos](/images/stories/organica-i/alkynes/adicion-haluros-hidrogeno/adicion-haluros-hidrogeno02.png)
Stage 2. The halogen captures the carbocation formed in the previous step.
![Addition of hydrogen halides to alkynes Adición de haluros de hidrógeno a alquinos](/images/stories/organica-i/alkynes/adicion-haluros-hidrogeno/adicion-haluros-hidrogeno03.png)
Given the great instability of the carbocation formed (alkenyl cation) the previous mechanism is questioned. Another alternative mechanism is the concerted one (a single stage, without the formation of intermediates).
![Addition of hydrogen halides to alkynes Adición de haluros de hidrógeno a alquinos](/images/stories/organica-i/alkynes/adicion-haluros-hidrogeno/adicion-haluros-hidrogeno04.png)
In the presence of two equivalents of the hydrogen halide, successive additions to the alkyne occur, forming geminal dihaloalkanes.
![Addition of hydrogen halides to alkynes Adición de haluros de hidrógeno a alquinos](/images/stories/organica-i/alkynes/adicion-haluros-hidrogeno/adicion-haluros-hidrogeno05.png)