Addition of HBr once to alkynes to give alkenyl bromides

by James

Description: Addition of one equivalent of hydrogen bromide (HBr) to alkynes produces vinyl bromides.
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Notes: This reaction proceeds with Markovnikoff selectivity. The reaction is identical in all respects to the addition of HCl or HI to alkynes except for the identity of the halogen.

Examples: 

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

Mechanism: Attack of the alkyne π bond on H–Br leads to formation of the vinyl carbocation at the more substituted position (Markovnikoff selectivity) (Step 1, arrows A and B). Attack of the bromide ion on the carbocation (Step 2, arrow C) provides the vinyl bromide.

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