See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Tautomerism Analysis **Tautomerism** requires: (1) a labile hydrogen, (2) a π-system nearby to accept it, and (3) formation of a more stable isomer via keto-enol or similar equilibrium. ## Option (A): $C_2H_5NO_2$ ✓ This is **nitromethane** ($CH_3NO_2$) or a related compound. It contains: - An acidic C–H α to the electron-withdrawing $NO_2$ group - Can undergo keto-enol-like tautomerism to form $CH_2=N^+(\omegas)O^-$ (nitroso form) - Shows tautomerism between two resonance-stabilized forms ## Option (B): **Acetoacetic ester** ✓ Structure: $CH_3\text{-}CO\text{-}CH_2\text{-}COOC_2H_5$ - The α-CH₂ is acidic (flanked by C=O and ester) - Exhibits **keto-enol tautomerism**: $$\text{Keto form} \rightleftharpoons \text{Enol form}$$ - Enol form is stabilized by hydrogen bonding and conjugation - Classical example of tautomerism in organic chemistry ## Option (C): Hindered ketone ✗ The gem-dimethyl groups prevent enolization by steric hindrance (Bredt-like restriction). The α-hydrogens are inaccessible to the carbonyl π-system. **No tautomerism occurs.** ## Option (D): **Cyclohex-2-enone** ✓ - Contains α,β-unsaturated ketone - Can undergo **keto-enol tautomerism** with the enol form stabilized by extended conjugation (dienol) - The α-hydrogen is sufficiently acidic for proton transfer **Answer: A, B, D** — all contain mobile hydrogens adjacent to electron-withdrawing groups that allow resonance-stabilized tautomeric equilibria.