Question

Question: Draw all constitutional isomers formed when X is treated with NBS + hv.

Short answer

Answer

Isomers formed from compound X

Step-by-step solution

Selective bromination of alkenes

Radical bromination is a selective substitution reaction. Therefore, bromination occurs at the weakest C-H bond in the compound.

In alkenes, the weakest C-H bond is in the allylic position of the unsaturated bond.

Therefore, if radical bromination is carried out in an alkene, substitution occurs at only the allylic position.

Bromination with NBS

NBS is a compound that can produce bromine radicals in the presence of light or peroxides.

These bromine radicals can act as initiators for allylic bromination of an alkene by producing allyl radicals.

The allyl radical is stabilized by resonance and can attack another bromine molecule to form allyl halide.

NBS can also produce the bromine molecules required for the reaction.

Allylic bromination of the given compound

The double bond in the given compound has three allylic positions. Therefore, three allyl radicals can be formed from compound X.

Each allyl radical of the compound has two resonance structures. Therefore, six allyl radicals will be formed in the reaction medium when the bromine radical attacks compound X.

Allyl halides are formed from each allyl radical, giving a mixture of six products related as constitutional isomers.

Products of the given reaction