See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Keto-Enol Tautomerism Analysis **Key Requirement for Keto-Enol Tautomerism:** A compound must have an α-hydrogen (hydrogen on the carbon adjacent to the carbonyl group) to undergo keto-enol tautomerism. **Mechanism:** $\text{R-CO-CH}_2\text{-R'} \rightleftharpoons \text{R-C(OH)=CH-R'}$ --- **Analysis of Each Option:** **(A) Bicyclic ketone (bridged system):** - The carbonyl carbon is part of a bicyclic bridge with **no α-hydrogens** - The carbons adjacent to $C=O$ are quaternary (bridgehead carbons) - **Cannot undergo keto-enol tautomerism** ✓ **(B) Bicyclic ketone with $CH_3$ groups:** - Despite the bridge, the α-positions have $CH_3$ groups attached - Contains α-hydrogens → **can tautomerize** **(C) Cyclohexanone derivative:** - Has $CH_2$ groups adjacent to the ketone - Contains α-hydrogens → **can tautomerize** **(D) Cyclic diketone:** - The central $CH_2$ is α to both carbonyl groups - Contains α-hydrogens → **can tautomerize** --- **Answer: (A)** is correct because the bridged bicyclic structure has **no α-hydrogens** on carbons adjacent to the carbonyl, making keto-enol tautomerism impossible.