See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: Carbocation rearrangements (hydride or alkyl shifts) occur when a less stable carbocation can rearrange to form a more stable one. A carbocation will undergo rearrangement if there is a neighboring group (H or alkyl) whose migration leads to a more stable (or at least equally stable) carbocation. Analysis of each option: (a) Methylcyclohexyl carbocation: This is a tertiary carbocation on the ring carbon bearing the methyl group. A tertiary carbocation is already quite stable, and rearrangement would not lead to a more stable species. No driving force for rearrangement. (b) Cyclopentane ring with an exocyclic carbocation (a primary or secondary carbocation outside the ring, e.g., cyclopentyl-C+(CH3) type, or more precisely a carbocation on a carbon attached to the cyclopentane ring). If the exocyclic carbon bears the positive charge, a 1,2-alkyl shift (ring expansion) from cyclopentane to cyclohexane ring system is possible. Ring expansion from a 5-membered ring to a 6-membered ring is highly favorable because cyclohexyl carbocation (secondary, 6-membered) is more stable than the cyclopentylmethyl arrangement. This ring-expansion rearrangement is a well-known and favorable process, providing strong driving force for rearrangement. (c) Acylium ion (CH3-CH(CH3)-C≡O+): Acylium ions are stabilized by resonance with the C≡O triple bond. They are inherently stable and do not typically undergo rearrangement under normal conditions. (d) CH3-NH-C+(H)-CH(CH3)-CH3: The nitrogen lone pair stabilizes the adjacent carbocation through resonance (forming an iminium ion), making it very stable. No rearrangement is needed. The key reasoning for (b): A cyclopentylmethyl carbocation (or similar exocyclic carbocation on cyclopentane) undergoes ring expansion via a 1,2-carbon shift to give a cyclohexyl carbocation, which is more stable. This is a classic example of ring-expansion rearrangement driven by increased ring size stability. Therefore, the correct answer is B.