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Reaction of Alkynes with Hydrogen Halides


addition of HX to alkynes
Reaction type: Electrophilic Addition

Summary

addition of HBr to 2-bromopropene to give 2,2-dibromopropane
addition of HBr to propyne under radical conditions

QUESTIONS

Under normal conditions for the addition of excess HBr :
(i)  with propyne, can you suggest a reason why the geminal regioselectivity is observed ? ANSWER

(ii) What would be the product from the reaction of 2-butyne with excess HBr ? ANSWER
(iii) Under radical conditions for the addition of 1 equivalent of HBr to propyne, why does this reaction have different regiochemistry ? ANSWER

Related Reactions
MECHANISM FOR REACTION OF ALKYNES WITH HBr


Step 1:

A concerted termolecular reaction...
This involves an acid/base reaction, protonation of the alkyne developing +ve charge on the more substituted carbon. The π electrons act pairs as a Lewis base.
The other part is attack of the nucleophilic bromide ion on the more electrophilic carbocation creates the alkenyl bromide.
addition of HBr to an alkyne
Step 2:
In the presence of excess reagent, a second protonation occurs to generate the more stable carbocation.



Step 3:
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the geminal dibromide.





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  Dr. Ian Hunt, Department of Chemistry, University of Calgary