See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: Under basic aqueous conditions (HO(-)/H2O), alpha,beta-unsaturated carbonyl compounds (enones) can undergo base-catalyzed isomerization via proton transfers. The hydroxide deprotonates at an alpha or allylic position, generating a stabilized enolate/dienolate, which can then be reprotonated at a different position, shifting the position of the double bond or substituents. Step 1: Identify compound (A). Compound (A) is 3-ethylcyclopent-2-en-1-one — a cyclopentenone ring with the carbonyl at C1, double bond between C2 and C3, and an ethyl group at C3 (the beta-carbon). Molecular formula: C7H10O. Step 2: Understand the isomerization. Under HO(-)/H2O, the alpha carbon (C2, alpha to carbonyl) or the allylic/gamma position can be deprotonated. Deprotonation at C3 (bearing the ethyl) or at the allylic CH2 of the ethyl group leads to a cross-conjugated or extended enolate. Reprotonation at a different site can migrate the double bond. Step 3: Consider option (a): 2,3-dimethylcyclopent-2-en-1-one has molecular formula C7H10O — same as A. This is a structural isomer. Under base-catalyzed conditions, a 1,3-proton shift (moving the double bond from the 2,3-position and redistributing the ethyl into two methyls via retro-[1,3] / tautomerism pathway) would convert the ethyl substituent at C3 into methyl groups at C2 and C3. This type of isomerization is known for cyclopentenones under basic conditions. Step 4: Verify option (b): Option (b) appears to be essentially the same connectivity as (A) (3-ethylcyclopent-2-en-1-one) or a very close structural variant — not a useful isomer product under these conditions, and it is likely the same or a trivial isomer not formed preferentially. Step 5: Verify option (c): 4,5-dimethylcyclopent-2-en-1-one is also C7H10O, but formation would require migration of the double bond away from conjugation with the carbonyl, which is thermodynamically disfavored under basic conditions (conjugated enone is more stable). Step 6: Verify option (d): 2-cycloheptenone is a seven-membered ring — different ring size, not an isomer of C7H10O five-membered ring compound in the same sense (it is a constitutional isomer by molecular formula but ring expansion is not expected under simple base/water conditions). Step 7: The base-catalyzed isomerization of 3-ethylcyclopent-2-en-1-one under HO(-)/H2O gives the thermodynamically more substituted or differently conjugated enone. The conversion of the C3-ethyl group (–CH2CH3) into C2-methyl and C3-methyl groups via a base-mediated 1,3-proton shift / retro-Favorskii-type tautomerism yields 2,3-dimethylcyclopent-2-en-1-one, which is option (a). Therefore, the correct answer is A.