Chapter 9: Alkynes

Reaction of Alkynes with Hydrogen Halides

Summary

• When treated with 1 equivalent of 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:
• The reaction proceeds via a termolecular mechanism, rate = k [HX]²[alkyne]
• H+ adds from one HX at the same time as X- adds to the other side from another HX.
• This avoids the unstable vinyl carbocation intermediate (note: no rearrangements are known which suggests no carbocation)
  
• The reaction is trans-stereoselective (concerted nature, see above)
  
• In the presence of excess HX, a second addition can occur to the product alkene giving a geminal dihalide

• For HBr, care must be taken to avoid the formation of radicals (e.g. the presence of peroxides or uv light) as the alternate radical addition occurs with opposite regiochemistry:

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

• Addition of HX to alkenes

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.
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.

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