Chapter 9: Alkynes

Reaction of Alkynes with Hydrogen Halides

Summary

• When treated with HX alkynes form vinyl halides.
• Hydrogen halide reactivity order : HI > HBr > HCl > HF (paralleling acidity order).
• Regioselectivity predicted by Markovnikov's rule with the H adding to the C with the most H already present.

• Reaction proceeds via protonation to give the more stable carbocation intermediate.
• Not stereoselective since reaction proceeds via planar carbocation.
• In the presence of excess HX, a second addition can occur to the product alkene giving a geminal dihalide


Can you suggest a reason why this regioselectivity is observed ? 
What would be the product from the reaction of 2-butyne with excess HBr ? 

• For HBr, care must be taken to avoid the formation of radicals as the alternate radical addition occurs with opposite regiochemistry:

Why does this reaction have the opposite regiochemistry ? 

• Addition of HX to alkenes

MECHANISM FOR REACTION OF ALKYNES WITH HBr
Step 1: An acid/base reaction. Protonation of the alkyne to generate the more stable carbocation.  The p electrons act pairs as a Lewis base.
Step 2: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkenyl bromide.
Step 3: In the presence of excess reagent, a second protonation occurs to generate the more stable carbocation.
Step 4: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the geminal dibromide.