See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The reaction of HBr with an exocyclic alkene (addition of HBr) proceeds via Markovnikov's rule, where HBr adds across the C=C double bond. The rate of addition depends on the stability of the carbocation intermediate formed at the more substituted (exocyclic) carbon after protonation of the double bond. Step 1 – Identify the structures: - (P): A six-membered ring containing oxygen (tetrahydropyran-like) with an exocyclic C=C double bond. The oxygen lone pairs can stabilize an adjacent carbocation via resonance (oxocarbenium ion formation). - (Q): Methylenecyclohexane – a plain cyclohexane ring with an exocyclic C=C double bond. No heteroatom stabilization; the carbocation is a simple tertiary carbocation. - (R): A six-membered carbocyclic ring with an exocyclic C=C double bond and an NH group at the ring carbon bearing the double bond. The nitrogen lone pair can stabilize an adjacent carbocation via resonance (iminium ion), which is even more stabilizing than oxygen. Step 2 – Assess carbocation stability after protonation: - Protonation of the exocyclic =CH2 gives a carbocation at the ring carbon adjacent to the heteroatom (or just ring carbon for Q). - For (R): N stabilizes carbocation better than O because nitrogen is less electronegative and donates electrons more effectively → most stable carbocation → fastest rate. - For (P): O stabilizes carbocation via lone pair donation (oxocarbenium) → moderately stable carbocation → intermediate rate. - For (Q): No heteroatom, only a tertiary carbocation → least stable → slowest rate. Step 3 – Rank in increasing order of rate: Increasing order means slowest to fastest: Q < P < R, i.e., R > P > Q. Step 4 – Match to options: Option (a) states R > P > Q, which matches increasing order Q < P < R. Step 5 – Why other options fail: - (b) R > Q > P would mean oxygen destabilizes more than no heteroatom, which is incorrect. - (c) and (d) reference a compound 'S' not present in the question, making them invalid. Therefore, the correct answer is A.