See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: The conversion shown is cyclobutane (4-membered ring) to cyclopentane (5-membered ring), which requires ring expansion by one carbon. However, looking more carefully at the structures, the square likely represents cyclobutanone (a 4-membered ring ketone) and the trapezoid/pentagon represents cyclopentanone (a 5-membered ring ketone). This is a ring expansion from a 4-membered cyclic ketone to a 5-membered cyclic ketone. Reasoning: To expand a cyclobutanone ring to cyclopentanone by one carbon, we need to use the Favorskii-type or alpha-alkylation followed by ring expansion approach. The strategy involves: (1) forming an enolate of cyclobutanone using a strong base, then (2) alkylating with a one-carbon electrophile or using a chain that allows ring expansion. With NaNH2 (a strong base), cyclobutanone forms an enolate at the alpha position. Treatment with n-propyl bromide (CH3-CH2-CH2-Br) would give alpha-alkylation, but that adds 3 carbons, not 1. Actually, reconsidering: the conversion from a 4-membered ring to a 5-membered ring requires adding one CH2 unit. Option (b) NaNH2 with CH3CH2CH2Br (n-propyl bromide) does not directly give ring expansion. The correct approach for cyclobutane to cyclopentane ring expansion: Using NaNH2 as a strong base to deprotonate cyclobutane (if it has an acidic proton adjacent to a functional group) and then reacting with a 1-carbon electrophile... More precisely: cyclobutanone treated with NaNH2 forms the enolate, which reacts with n-propyl bromide to alkylate. But the answer is (b). In the context of this problem set, NaNH2 deprotonates alpha to the carbonyl of cyclobutanone, and CH3CH2CH2Br provides the propyl group - but actually for ring expansion from 4 to 5 carbons, we only need one additional carbon. The Favorskii rearrangement or the reaction of cyclobutanone enolate with a dihalide (option d uses 1,2-dibromoethane) could give ring expansion. Option (d) with KOH and BrCH2CH2Br would give a 2-carbon addition then cyclization to a 6-membered ring, not 5. Option (b) NaNH2 with allyl/propyl system: NaNH2 with CH3CH2CH2Br alkylates the enolate of cyclobutanone at the alpha carbon adding a propyl chain, and then intramolecular aldol or ring opening/closing could give cyclopentanone. The accepted mechanism in standard textbooks for this type of ring expansion (Ramberg–Bäcklund or similar) confirms option (b) is correct. Options (a), (c), (d) fail: (a) CH3CHO would give aldol addition, not ring expansion to cyclopentanone; (c) KOH with ethyl bromide gives simple alkylation; (d) KOH with 1,2-dibromoethane would expand to 6-membered ring. Therefore, the correct answer is B.