See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Acid-catalyzed ring expansion via carbocation rearrangement (semipinacol-type or cyclobutylcarbinyl to cyclopentyl rearrangement). Step 1: Identify the starting material. The starting material is spiro[3.5]nonan-6-one (a spiro compound where a cyclobutane ring and a cyclohexane ring share one carbon, with the ketone on the cyclohexane ring adjacent to the spiro carbon). Step 2: Protonation of the ketone by conc. H2SO4 generates an oxocarbenium ion (protonated ketone), or the acid facilitates formation of a carbocation alpha to the carbonyl. Step 3: The spiro carbon bearing the cyclobutane ring is alpha to the carbonyl. Under acidic conditions, protonation of the carbonyl oxygen creates an oxocarbenium ion. The adjacent C-C bond of the cyclobutane (a strained ring) migrates to relieve strain — this is a 1,2-alkyl shift (ring expansion of cyclobutane to cyclopentane). Step 4: The cyclobutane ring (4-membered) expands by migration of one C-C bond to give a cyclopentane ring, while the cyclohexane ring (6-membered) contracts to a cyclopentane ring. This converts spiro[3.5]nonane skeleton into a spiro[4.4]nonane skeleton. Step 5: After the 1,2-shift, the positive charge is quenched by deprotonation (or the oxocarbenium collapses), giving spiro[4.4]nonan-1-one — two cyclopentane rings spiro-fused with a ketone on one ring. Step 6: This matches option (b): spiro[4.4]nonan-1-one, where both rings are five-membered and the ketone is on one of the cyclopentane rings at C1. Why other options fail: - (a) shows ketone on one cyclopentane ring but the structural drawing indicates the ketone is at a different position (C2), inconsistent with the rearrangement product. - (c) and (d) involve double bonds, which would require elimination rather than a simple 1,2-shift rearrangement; no elimination is expected here. Therefore, the correct answer is B.