See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: A hydride shift is favorable when it relieves ring strain or converts a less stable carbocation into a more stable one. The driving force is both the stability gain of the resulting carbocation and any relief of angle strain. Step 1 - Analyze option (a): Structure (a) shows a cyclobutane ring with a carbocation at a ring carbon adjacent to a carbon bearing a methyl group (and a hydrogen). A 1,2-hydride shift from the adjacent carbon to the carbocation would open or rearrange the ring. Specifically, if the carbocation is at the ring junction and the adjacent carbon has a hydride, the shift converts the cyclobutane-ring carbocation into a ring-expanded carbocation or a more stable tertiary/open-chain carbocation, simultaneously relieving the significant ring strain (~26 kcal/mol) of cyclobutane. This dual driving force (ring strain relief + carbocation stabilization) makes the hydride shift highly favorable. Step 2 - Analyze option (b): Structure (b) is a neopentyl-type tertiary carbocation. The carbocation is already tertiary, so there is less thermodynamic driving force to shift a hydride (the resulting carbocation would not be significantly more stable). Ring strain relief is not a factor here. Step 3 - Analyze option (c): Structure (c) is an open-chain secondary carbocation. A 1,2-hydride shift would merely give another secondary carbocation (no gain in stability) or at best a similar secondary carbocation. No ring strain relief is involved, so the shift is not particularly favorable. Step 4 - Analyze option (d): Structure (d) shows a cyclobutane ring with an exocyclic secondary carbocation. While ring strain could in principle assist a rearrangement, the geometry and the fact that the carbocation is exocyclic (not at a ring carbon bearing a hydride that would lead to ring opening/expansion in a productive way) makes the shift less favorable compared to (a). Step 5 - Conclusion: Option (a) benefits from both ring strain relief of cyclobutane and conversion to a more stable carbocation upon hydride shift, making it the most favorable case. Therefore, the correct answer is A.