See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Tautomers Analysis **Tautomers** are structural isomers that differ by the shift of a hydrogen atom and a double bond (typically keto-enol equilibrium). ## Examining Each Option: **(A) Salicylaldehyde vs. Salicylic aldehyde** - Left: $OH$ and $CHO$ groups (enol form) - Right: $C=O$ and $CHOH$ groups (keto form) - **These ARE tautomers** — hydrogen shifts between oxygen atoms with double bond migration **(B) Carboxylic acid vs. Lactone** - Left: Benzene ring with $COOH$ and $CHO$ - Right: Benzene ring fused to a cyclic ester (lactone) - **These ARE tautomers** — intramolecular hydrogen transfer and bond rearrangement **(C) Cyclic amide vs. Hydroxylamine** - Left: $N-CO$ (amide, keto form) - Right: $N-OH$ with $C-H$ (imidate, enol form) - **These ARE tautomers** — hydrogen and double bond shift between N and O **(D) Phenol vs. Cyclohexadienone** - Left: Aromatic ring with $OH$ (phenol) - Right: Non-aromatic ring with $C=O$ (cyclohexadienone) - **These are NOT tautomers** — They are resonance structures of the same molecule, not separate isomers. Phenol exists as a resonance hybrid; they don't interconvert as distinct species. **Answer: (D)** — Phenol and cyclohexadienone are resonance structures, not tautomers.