See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Solution: Ketone Racemization Mechanism **Key Principle:** Ketones racemize via enolization. The $\alpha$-carbon (adjacent to $C=O$) becomes a planar $sp^2$ hybridized enol intermediate, losing stereochemical information. Only ketones with a **stereogenic center at the $\alpha$-carbon** can racemize. ## Analysis of Each Option: **(A)** $CH_3CH(Cl)-CH_2-\underline{C}(=O)-CH_3$ - Stereocenter is at the carbon bearing $Cl$, **not adjacent to carbonyl** - $\alpha$-carbon is $CH_2$ (not stereogenic) - Racemization impossible ✗ **(B)** $CH_3-\underline{CH}(Cl)-\underline{C}(=O)-CH_3$ - Stereocenter at the $\alpha$-carbon (bearing $Cl$) - Enolization converts this $\alpha$-carbon to $sp^2$, creating a planar intermediate - Upon protonation, both $(R)$ and $(S)$ enantiomers form equally - **Racemizes** ✓ **(C)** $C_5H_5-\underline{C}(=O)-\underline{C}(CH_3)(Cl)-C_2H_5$ - Stereocenter at the carbon bearing $Cl$, not the $\alpha$-carbon - Enolization affects the carbonyl carbon only - No racemization occurs ✗ **(D)** All racemize — **False** (only B qualifies) --- **Answer: (B)** — The ketone racemizes because the stereogenic center lies directly at the $\alpha$-position to the carbonyl, allowing enolization to destroy stereochemistry.