See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Tautomers Identification **Tautomers** are structural isomers that differ by the migration of a hydrogen atom and a double bond between atoms (usually C and O or C and N). ## Analysis of Each Option: **(A) $\ce{Ph-CH=CH-OH}$ (enol form) ↔ $\ce{Ph-CH2-CHO}$ (keto form)** - The enol form can tautomerize to the aldehyde form by: - H shifts from O to the adjacent carbon - Double bond shifts from C=C to C=O - These are **true tautomers** (keto-enol tautomerism) - ✓ **This is a tautomeric pair** **(B) $\ce{O=C6H4=O}$ (quinone-like structure)** - Shows only resonance structures of the same molecule - Not tautomers, but **resonance isomers** **(C) Two cyclic ketones with different ring sizes** - These are **constitutional isomers** with different carbon skeletons - No hydrogen migration between them **(D) Two different cyclic ketones** - Different structural isomers - No tautomeric relationship ## Answer: **(A)** is correct because cinnamyl alcohol (enol) and its tautomer phenylpropionaldehyde (keto) represent a classical **keto-enol tautomeric pair** involving hydrogen and double bond migration.