anti-markovnikov addition

Question

In presence of peroxides, HBr addition to propene gives:

RankGuru · Accepted Answer

1. **Kharasch Effect**: In the presence of organic peroxides ($R-O-O-R$), $HBr$ adds to alkenes via a **free radical mechanism** instead of the standard carbocation pathway.
2. **Radical Generation**: Peroxides initiate the cleavage of $HBr$ to form a Bromine radical ($Br\cdot$).
3. **Addition Step**: The $Br\cdot$ radical adds to the alkene first. It prefers to add to the terminal carbon to leave behind the more stable **secondary radical** ($CH_3-\dot{C}H-CH_2Br$).
4. **Hydrogen Transfer**: This secondary radical then abstracts a hydrogen atom from another $HBr$ molecule.
5. **Regiochemistry**: The result is **Anti-Markovnikov addition**, where the Bromine ends up on the terminal carbon, producing **1-Bromopropane**.
6. **Note**: This effect is highly specific to $HBr$ (not $HCl$ or $HI$).

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anti-markovnikov addition

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