See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the starting material and product. Starting material: a cyclobutane bearing an acetyl group (–COCH3) at C1 and a quaternary carbon at C2 that carries a methyl group and a hydroxyl group (–C(CH3)(OH)–). This is a β-hydroxy ketone (aldol-type adduct) embedded in a four-membered ring. Product: 3-methylcyclohex-2-en-1-one, a six-membered α,β-unsaturated ketone. Step 2 – Recognize the retro-aldol pathway. The starting material is structurally an intramolecular aldol product. Under basic or acidic conditions, retro-aldol cleavage of the C1–C2 bond of the cyclobutane ring opens the ring. The β-hydroxy ketone (the –C(CH3)(OH)– adjacent to the –COCH3) undergoes retro-aldol to give an open-chain diketone or keto-aldehyde fragment. Specifically, retro-aldol of the cyclobutane β-hydroxy ketone cleaves the ring to generate a 1,6-dicarbonyl compound (a linear diketone): hexane-2,6-dione (acetonylacetone type) or more precisely the appropriate chain that can recyclize. Step 3 – Further condensation (intramolecular aldol condensation). The open-chain diketone produced by retro-aldol can undergo intramolecular aldol condensation. The enolate of one ketone attacks the other carbonyl, forming a six-membered ring β-hydroxy ketone, which then dehydrates to give the α,β-unsaturated cyclic ketone. The methyl substituents and regiochemistry of the condensation lead to 3-methylcyclohex-2-en-1-one as the thermodynamically favored product. Step 4 – Why other options fail. (a) Dehydration alone would give a cyclobutene derivative or eliminate water from the tertiary alcohol, not ring-open to a six-membered ring. Hydrolysis is irrelevant here. (c) Perkin condensation involves an anhydride and an aldehyde with an acid salt; Clemmensen reduces a carbonyl to CH2. Neither applies to this ring-expansion scenario. (d) Clemmensen followed by Perkin condensation similarly does not accomplish the ring opening and recyclization to give the observed product. Only retro-aldol (ring opening of the strained cyclobutane β-hydroxy ketone) followed by intramolecular aldol condensation (recyclization with dehydration) logically converts a cyclobutane compound into a six-membered α,β-unsaturated ketone with the correct substitution pattern. Therefore, the correct answer is B.