Peroxide effect (Kharasch)
In the presence of peroxides, HBr adds to an unsymmetrical alkene against Markovnikov's rule.
PropeneCH₃CH=CH₂Alkene - Step 1
HBr, peroxide
1-BromopropaneCH₃CH₂CH₂BrHaloalkane
How it works
- The peroxide gives a bromine radical from HBr.
- The radical adds to the carbon that gives the more stable secondary radical.
- The radical abstracts H from HBr to give the product and a new bromine radical.
What it makes
The primary bromoalkane, for example 1-bromopropane from propene.
Where it fails
Works only with HBr. HCl and HI do not show the peroxide effect.
Class 11, Chapter 9 Hydrocarbons
Peroxide effect (Kharasch): quick answers
Which hydrogen halide shows the peroxide effect?
Only HBr.
What is the mechanism?
Free radical addition.
What does HBr give with propene without peroxide?
2-Bromopropane (Markovnikov product).