HX Addition to Alkynes
Addition of HX to Alkynes - Multiple Addition
The reaction between a hydrogen halide (HX) and an alkyne yields a vinyl halide with Markovnikov orientation. A subsequent addition of HX to the product can occur because the vinyl halide still contains a double bond. This addition yields an alkyl dihalide and the formation cannot easily be prevented when HX is added to an alkyne, as the intermediate carbenium ion is stabilized by a +M effect of the halogen substituent.
The HX addition to a vinyl halide follows a mechanism analogous to the HX addition to an alkene, that is, protonation of the double bond and subsequent addition of the halide anion to the intermediate carbenium ion. This reaction yields a geminal alkyl dihalide with Markovnikov orientation.
The protonation of the vinyl halide occurs at the non-halogenated sp2 carbon atom because then the positive charge at the halogenated sp2 carbon atom can be stabilized by the +M effect of the halogen substituent. This resonance stabilization (3p-2p overlap) is not possible if the halogenated carbon atom is protonated.
The resonance stabilization of the intermediate carbenium-halonium ion hybride can be shown by comparison of the carbon-chlorine bond length of propyl chloride and 2-chloropropene.