See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Keto-Enol Tautomerism Analysis **Keto-enol tautomerism** requires an **α-hydrogen** (hydrogen on the carbon adjacent to the carbonyl group). **Checking each option:** **(A) $C_6H_5CHO$ (Benzaldehyde)** - Structure: $C_6H_5-CHO$ - The carbonyl carbon is bonded directly to the benzene ring - **No α-hydrogen available** (the adjacent carbon is the aromatic ring itself) - Wait — actually, benzaldehyde has **no α-hydrogen**, so it should NOT show keto-enol tautomerism Let me reconsider: The question asks where keto-enol tautomerism **is observed**. **(A) $C_6H_5CHO$** - Aldehyde with aromatic ring; no α-H → No tautomerism **(B) $C_6H_5COCH_3$** - Has $CH_3$ group next to C=O; **α-hydrogen present** ✓ $$C_6H_5-CO-CH_3 \rightleftharpoons C_6H_5-C(OH)=CH_2$$ **(C) $C_6H_5COC_6H_5$** - Both groups aromatic; no α-H on aliphatic carbon → No tautomerism **(D) $C_6H_5COCH_2COCH_3$** - While α-hydrogens exist, the conjugated system makes tautomerism less typical **The correct answer is (B)**, not (A). Option B ($C_6H_5COCH_3$) clearly shows keto-enol tautomerism because the methyl group provides accessible α-hydrogens adjacent to the carbonyl carbon. *(Note: If the answer key states A, please verify the structural formula—there may be a transcription difference.)*