Question

Write a short note on the allylic substitution.

Answer 1

Allylic substitution is a type of substitution reaction that takes place at the allylic position, the carbon atom adjacent to a carbon-carbon double bond (the alkene group) in an organic molecule.

In this reaction, a substituent on the allylic carbon is replaced by another atom or group of atoms (nucleophile or electrophile). Because the allylic position is stabilized by resonance, the reaction often proceeds via an allylic carbocation or allylic radical intermediate, which provides resonance stabilization and makes the substitution easier compared to substitution on a saturated carbon.

For example,

\[\ce{CH2 = CH - CH3 + Cl2 ->[773 K][] \underset{Allyl chloride}{CH2 = CH - CH2Cl} + HCl}\]

The allylic substitution can also be carried out with N-bromosuccinimide (NBS) or sulphuryl chloride (SO₂Cl₂).