||Chapter 9: Alkynes
Reaction of Alkynes
with HBr (radical)
Reaction type: Radical Addition
- 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
- In the presence of excess HBr, a second addition can occur
to the product alkene giving a vicinal or 1,2-dibromide (see above).
- 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 this radical mechanism (see above) and the heterolytic pathway (see below).