Addition of HBr twice to alkynes to give geminal dibromides

by James

Description: Addition of two equivalents of hydrogen bromide (HBr) to alkynes results in geminal dibromides. [private_ReactionGuide]

Notes: The reaction proceeds with Markovnikoff selectivity (Br adds to more substituted carbon of the alkyne). “Geminal” means that the two bromides are attached to the same carbon.

Also note that this reaction is identical in all respects to the double addition of HCl and HI to alkynes.

Examples: 

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

Mechanism: Attack of the alkyne π bond on H–Br leads to the vinyl carbocation at the more substituted position (Markovnikoff selectivity) (Step 1, arrows A and B) providing a vinyl carbocation. Attack of bromide ion on the carbocation (Step 2, arrow C) provides the vinyl bromide. When a second equivalent of HBr is present, the process then repeats itself! Protonation of the alkene (Step 3, arrows D and E) give a carbocation, which is then attacked by bromide (Step 4, arrow F)

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Ian Hunt

Hmmm… a couple of concerns here…. first vinyl carbocations are quite unstable and I think they should (if formed) rearrange to allyl cations which would be much more stable. I recall reading that there are no known examples of rearrangements…. thus the implication would be that the vinyl cation is not formed. Many texts tend to show a concerted process with 2 HX involved (which is supported by termolecular kinetics.

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