See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Keto-enol tautomerism and stabilization through intramolecular hydrogen bonding and conjugation in beta-dicarbonyl compounds. Step 1: Identify the compound. All four structures are tautomers of acetylacetone (2,4-pentanedione), CH3COCH2COCH3. Step 2: Evaluate each structure: (a) Diketo form: CH3-C(=O)-CH2-C(=O)-CH3. No conjugation between the two carbonyl groups. No intramolecular hydrogen bonding. Least stabilized among the tautomers shown. (b) Enol at C1 (terminal vinyl enol, =CH2 end): CH2=C(OH)-CH2-C(=O)-CH3. The enol double bond is not conjugated with the remaining ketone carbonyl (there is a CH2 spacer breaking conjugation). No intramolecular H-bond possible in a favorable 6-membered ring. (c) Enol at C4 (internal enol): CH3-C(OH)=CH-C(=O)-CH3. Here the hydroxyl enol is on C4, giving the structure CH3-C(OH)=CH-CO-CH3. The C=C double bond is conjugated with the C=O group (forming a cross-conjugated or extended pi system: HO-C=C-C=O). Additionally, this arrangement allows formation of a stable 6-membered intramolecular hydrogen bond between the OH and the carbonyl oxygen: O-H...O=C, closing a 6-membered chelate ring. This combination of conjugation and intramolecular H-bonding provides maximum stabilization. (d) Dienol form: CH2=C(OH)-CH=C(OH)-CH3. Both carbonyls are enolized. While there is extended conjugation, this form lacks a free carbonyl to accept an intramolecular hydrogen bond in the same effective 6-membered ring manner, and the terminal =CH2 reduces stabilization relative to (c). This tautomer is less stable than (c). Step 3: Why (c) is most stable. Structure (c) has: (i) conjugation of C=C with C=O (vinyl ketone-type extended pi system), (ii) a strong intramolecular hydrogen bond forming a 6-membered ring (chelate ring: C-C(OH)=CH-C=O with H bridging O to O), and (iii) resonance stabilization of the enol. This is the classic stabilized enol form of beta-diketones, well-known to predominate (>80%) in acetylacetone. Therefore, the correct answer is C.