|Chapter 9: Alkynes|
Reaction of Alkynes with Hydrogen Halides
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
|MECHANISM FOR REACTION OF ALKYNES WITH HBr|
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.
In the presence of excess reagent, a second protonation occurs to generate the more stable carbocation.
Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the geminal dibromide.
|© Dr. Ian Hunt, Department of Chemistry, University of Calgary|