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Addition of HCl once to alkynes to give alkenyl chlorides

Description: Addition of one equivalent of hydrogen chloride (HCl) to alkynes results in vinyl chlorides (aka alkenyl chlorides) [private_ReactionGuide]

Notes: This reaction proceeds with Markovnkoff selectivity (the chlorine attaches itself to the more substituted carbon)


Notes: Note how in all cases the chloride adds to the more substituted carbon (Markovnikoff selectivity). However if both ends of the acetylene are attached to carbon, mixtures can be obtained.

Mechanism: Attack of the alkyne PI bond upon H–Cl leads to the vinyl carbocation being formed at the more substituted position (Markovnikoff selectivity) (Step 1, arrows A and B). Attack of chloride ion at the carbocation (Step 2, arrow C) provides the vinyl chloride.