See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Position Isomerism in Ketones **Understanding Position Isomerism:** Position isomers differ in the location of the functional group (or double bond) on the carbon chain. **Analyzing Ketones by Carbon Number:** **3 carbons ($C_3H_6O$):** Propanone - Only one possible structure: $CH_3-CO-CH_3$ - No position isomerism possible **4 carbons ($C_4H_8O$):** Butanone - Butan-2-one: $CH_3-CO-CH_2-CH_3$ (only possibility) - The carbonyl must be at position 2; position 1 or 3 would give the same molecule - No position isomerism **5 carbons ($C_5H_{10}O$):** Pentanone - **Pentan-2-one:** $CH_3-CO-CH_2-CH_2-CH_3$ - **Pentan-3-one:** $CH_3-CH_2-CO-CH_2-CH_3$ - These are **distinct isomers** ✓ The carbonyl can be at position 2 or 3 (position 4 or 5 is equivalent to position 2 or 3 by symmetry), creating true position isomerism. **Answer: (3) 5 carbons** The minimum carbon chain length for a ketone to show position isomerism is **5 carbons**, where the carbonyl group can be located at different positions along the chain.