See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Tautomerism Explanation **Tautomerism** is the rapid interconversion between two structural isomers (tautomers) that differ in the position of a hydrogen atom and a double bond, typically involving keto-enol equilibrium. ## Analysis of Each Option: **(A) $CH_2=CH-CHO$** - Aldehyde group cannot form a stable enol tautomer because the resulting tautomer would have a vinyl alcohol ($CH_2=CH-OH$) adjacent to the double bond, which is unstable. - ✗ Does not show tautomerism **(B) $CH_3-CH=CH-CHO$** ✓ - The $\alpha$-hydrogen (on carbon next to $CHO$) can migrate to the carbonyl oxygen - Keto form: $CH_3-CH=CH-CHO$ - Enol form: $CH_3-CH=C(OH)-CH_2$ - The extended conjugation stabilizes both tautomers - **This is a classic example of keto-enol tautomerism** **(C) $CH_2=CH-CH_2OH$** - Already contains both a double bond and hydroxyl group (not in equilibrium) - This is a static structure, not tautomerism - ✗ No tautomeric shift possible **(D) $C_6H_5-\overset{O}{C}-H$** - Benzaldehyde lacks an $\alpha$-hydrogen needed for tautomeric shift - ✗ Cannot form enol tautomer **Answer: (B)** exhibits tautomerism because the $\alpha$-hydrogen to the carbonyl can readily migrate, establishing an equilibrium between the stable keto and conjugated enol forms.