Chapter 9: Alkynes

Reaction of Alkynes with HBr (radical)

Summary

• When treated with 1 equivalent of HBr in the presence of oxygen or peroxides or uv light, an alkyne forms a vinyl bromide.
• However, under these conditions which promote the radical addition, the regioselectivity is anti Markovnikov

• Peroxides (or uv light) facilitate the formation of a bromine radical e.g. RO^.+ HBr -> ROH + Br ^.
• Reaction proceeds via the more stable radical intermediate.
  
• In the presence of excess HBr, a second addition can occur to the product alkene giving a vicinal or 1,2-dibromide:

• The major product from the reaction of excess HBr in the presence of peroxides to terminal alkynes are the 1,2-dibromoalkanes.
• Note the difference in regiochemistry between the radical addition (discussed here, see above) and the "normal" heterolytic pathway (see below).

• Addition of HBr to alkenes under radical conditions
• Addition of HX to alkynes

(c) Dr. Ian Hunt, Department of Chemistry