91Ó°ÊÓ

NBS is the abbreviation for N-bromosuccinamide.

When an alkene reacts with NBS, selective halogenation occurs at the allylic position to the double bond.

It is because allylic C-H bonds are weaker than any SP³ hybridized C-H bond.

The reaction requires the presence of peroxides or light.

The reaction is initiated when the NBS molecule produces bromine radical in the presence of light or peroxide.

The bromine radical abstracts the allylic hydrogen from the alkene, forming an allyl radical.

The allyl radical attacks a bromine molecule to form allyl halide and bromine radical. This cycle continues till the reaction is complete.

The allyl radical formed can undergo resonance to give more products.

a. NBS causes bromination at the allylic position. Bromination at both allylic positions gives the same product.

Reaction of NBS with a

b. Bromination occurs at allylic positions of both sides of the double bond, giving four products two from each allyl radical.

Reaction of NBS with b

c. Due to resonance in allyl radical, four products are formed from the possible two allyl radicals.

Reaction of NBS with c