Addition of HI once to alkynes to give alkenyl iodides

by James

Description: Addition of one equivalent of hydrogen iodide (HI) to alkynes results in alkenyl iodides (also known as vinyl iodides). [private_ReactionGuide]

Notes: The reaction proceeds with Markovnikoff selectivity (the iodide adds to the more substituted carbon of the alkyne). The reaction is essentially identical to the addition of HCl and HBr to alkynes.

Examples: 

Notes: Note how in all cases the iodide adds to the more substituted carbon (Markovnikoff selectivity)

Mechanism: Attack of the alkyne π bond on H–I leads to formation of the vinyl carbocation at the more substituted position (Markovnikoff selectivity) (Step 1, arrows A and B). The vinyl carbocation is then attacked by the iodide (Step 2, arrow C) to provide the vinyl iodide.

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