See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Enol content in keto-enol tautomerism depends on the stability of the enol form relative to the keto form. The enol form is stabilized by: (1) intramolecular hydrogen bonding in the enol form (forming a six-membered chelate ring), (2) extended conjugation of the C=C double bond with adjacent carbonyl or aromatic groups, and (3) the acidity of the alpha hydrogen (more acidic alpha-H leads to more enol). Step 1 - Identify the nature of each compound: (a) 1,3-dibenzoylmethane (dibenzoylmethane): C6H5-CO-CH2-CO-C6H5 — a 1,3-diketone with two phenyl groups. (b) Acetylacetone (2,4-pentanedione): CH3-CO-CH2-CO-CH3 — a simple 1,3-diketone. (c) 2-pentanone: CH3-CO-CH2-CH2-CH3 — a simple monoketone with no activating group on the other side. (d) Ethyl acetoacetate: CH3-CO-CH2-COOC2H5 — a beta-ketoester. Step 2 - Evaluate enol stability: - Options (c) is a simple monoketone; there is no second carbonyl to stabilize the enol through chelation or extra conjugation, so enol content is negligible (~very small %). - Option (b) acetylacetone is a well-known 1,3-diketone with ~80% enol content in neat liquid due to intramolecular H-bonding forming a six-membered ring and conjugation of the enol C=C with the adjacent C=O. - Option (d) ethyl acetoacetate is a beta-ketoester with ~8% enol content; the ester group is less electron-withdrawing than a ketone, so the enol is less stabilized than in acetylacetone. - Option (a) dibenzoylmethane: The enol form is C6H5-C(OH)=CH-CO-C6H5. In the enol form, the C=C double bond is conjugated with BOTH phenyl rings AND the remaining C=O group, giving extensive delocalization. Additionally, intramolecular hydrogen bonding still forms a six-membered chelate ring. The two phenyl groups provide additional resonance stabilization of the enol form beyond what two methyl groups can provide. This results in dibenzoylmethane existing predominantly (>95%) in the enol form in solution. Step 3 - Compare (a) vs (b): Both are 1,3-diketones capable of enol chelation, but in (a) each carbonyl is flanked by a phenyl group. The enol tautomer of (a) benefits from conjugation extending into both phenyl rings (cross-conjugated aromatic stabilization), making the enol significantly more stable than in acetylacetone (b). Experimentally, dibenzoylmethane has higher enol content (~100% in nonpolar solvents) compared to acetylacetone (~80%). Step 4 - Why other options fail: - (c) has negligible enol content due to lack of a second electron-withdrawing group. - (d) has lower enol content than (b) because an ester is a weaker activating group than a ketone. - (b) has high enol content but less than (a) because methyl groups do not provide additional conjugative stabilization to the enol. Therefore, the correct answer is A.