Question: 49. Predict the major product ‘P’ in the following sequence of reactions for methylcyclopentene: (i) HBr, benzoyl peroxide (ii) KCN (iii) Na(Hg)/C2H5OH
(1) Cyclopentane ring with a carbon bearing both CH3 and CH2NH2 substituents
(2) Cyclopentane ring with adjacent carbons bearing CH3 and CH2NH2 substituents
(3) Cyclopentane ring with a carbon bearing both CH3 and NC substituents
(4) Cyclopentane ring with a carbon bearing NC and CH3 substituents in a different connectivity
Answer: Option (1)
Explanation:
In step (i), methylcyclopentene reacts with HBr in the presence of benzoyl peroxide.
Due to peroxide, the addition of HBr follows anti-Markovnikov mechanism (free-radical addition).
Therefore, Br attaches to the less substituted carbon of the double bond, giving a primary bromoalkane attached to the cyclopentane ring (bromomethyl group on the same carbon that also bears CH3).
In step (ii), KCN provides the nucleophile CN⁻, which undergoes \mathrm{S_N2} substitution on the primary alkyl bromide.
Thus, Br is replaced by CN, forming a nitrile group \mathrm{–CH_2CN} on the same carbon that already has the CH3 group attached to the cyclopentane ring.
In step (iii), Na(Hg)/C2H5OH reduces the nitrile group \mathrm{–CN} to a primary amine group \mathrm{–CH_2NH_2}.
Therefore, the final product has a cyclopentane ring with one carbon bearing both a CH3 group and a CH2NH2 group, which corresponds to Option (1).