See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: This problem involves an acid-catalyzed polyene cyclization (biomimetic cationic olefin cascade cyclization), similar to the biosynthesis of steroids from squalene oxide. Step 1 - Identify the substrate: The starting material is a polyene alcohol (resembling a monocyclic terpenoid with an allylic alcohol on a cyclohexene ring, bearing a geranyl/farnesyl-type side chain with multiple trisubstituted double bonds). This is structurally analogous to a biosynthetic steroid precursor. Step 2 - Acid-catalyzed cascade cyclization: Under H3O+ conditions, the tertiary allylic alcohol is protonated and the hydroxyl leaves (or the OH acts as initiator of a cationic cascade). The resulting carbocation triggers a concerted or stepwise cascade cyclization of the polyene chain. Each alkene in the chain attacks the adjacent carbocation in a Markovnikov sense, forming new C-C bonds and new rings sequentially. Step 3 - Ring formation: The cascade builds a tetracyclic framework: three six-membered rings (A, B, C rings) and one five-membered ring (D ring), exactly like the steroid carbon skeleton. The terminal alkene (isopropylidene end of the geranyl chain) ends up as a tertiary carbocation that is captured by water to give a tertiary alcohol -C(CH3)2OH. Step 4 - Identify the product: The product is a tetracyclic compound with an A/B/C/D ring system (steroid skeleton), retaining one double bond in the A ring (the original cyclohexene double bond that initiated or participated in the cyclization), with angular methyl groups at the B/C and C/D ring junctions, and a -C(CH3)2OH group on the D ring (cyclopentane). This matches option (a) exactly. Step 5 - Why other options fail: - Option (b) shows no double bond in the A ring; the original cyclohexene double bond should be retained in the product since it is not involved in the cyclization cascade (the OH departs and cyclization proceeds through the side-chain alkenes), so the A-ring double bond persists - ruling out (b). - Option (c) is a fully aromatic polycyclic hydrocarbon, which cannot result from this type of acid-catalyzed polyene cyclization of an aliphatic substrate - ruling out (c). - Option (d) is eliminated since option (a) correctly describes the product. Therefore, the correct answer is A.