See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Concept: Keto-enol tautomerism requires at least one alpha-hydrogen (a hydrogen on a carbon adjacent to the carbonyl). The degree of enol content depends on factors such as intramolecular hydrogen bonding, conjugation/aromaticity of the enol form, ring strain, and electronic effects. Step 2 - Compound (a): The structure shown is a bridged bicyclic ketone (resembling fenchone or a gem-dimethyl norcamphor). In this molecule, the alpha-carbons to the ketone are bridgehead carbons, which cannot lose a hydrogen to form an enol (Bredt's rule violation). Therefore, keto-enol tautomerism is NOT possible. This matches (s). Step 3 - Compound (b): Cyclobutane-1,2,3-trione has three carbonyl groups on a four-membered ring. The enol form is highly stabilized because it leads to an aromatic or highly conjugated system (the enol of a 1,2,3-triketone on a four-membered ring can form a fully conjugated/aromatic structure). In fact, the enol content is essentially 100% because the enol form is overwhelmingly more stable (analogous to how squaric acid derivatives are highly stabilized by resonance/pseudo-aromaticity). This matches (p) 100%. Step 4 - Compound (c): Acetylacetone (pentan-2,4-dione), CH3-C(=O)-CH2-C(=O)-CH3, is a classic beta-diketone. Its enol form is stabilized by intramolecular hydrogen bonding and conjugation across the C=C-C=O system. In the pure liquid state, acetylacetone exists approximately 76-80% in the enol form. This matches (q) 76%. Step 5 - Compound (d): Ethyl acetoacetate, CH3-C(=O)-CH2-C(=O)-OEt, is a beta-ketoester. The enol form is stabilized by intramolecular hydrogen bonding, but the ester group is less activating than a ketone because the lone pairs on oxygen partially donate into the ester carbonyl, reducing its electron-withdrawing character. Therefore, the enol content is lower than in acetylacetone, approximately 8% in the pure liquid. This matches (r) 8%. Step 6 - Summary of matching: - (a) bridged bicyclic ketone with no accessible alpha-H → keto-enol not possible → (s) - (b) cyclobutane-1,2,3-trione → 100% enol due to aromatic/pseudo-aromatic stabilization → (p) - (c) acetylacetone → 76% enol due to strong intramolecular H-bond and conjugation → (q) - (d) ethyl acetoacetate → 8% enol due to weaker activation by ester group → (r) Therefore, the correct answer is a-s; b-p; c-q; d-r.