See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

# Metamerism in Ketones **Understanding Metamerism:** Metamerism is structural isomerism where isomers differ in the position of a functional group or substituent along the carbon chain, while maintaining the same functional group. **For Ketones ($R_1-CO-R_2$):** For two ketones to be metamers, they must: - Have the same molecular formula - Have different positions of the carbonyl group ($C=O$) - Both be ketones (not aldehydes) **Analyzing by Carbon Number:** | Carbons | Example | Can show metamerism? | |---------|---------|---| | **3 carbons** | $CH_3-CO-CH_3$ (propanone) | ❌ Only one ketone possible | | **4 carbons** | $CH_3-CO-CH_2-CH_3$ vs. $CH_3-CH_2-CO-CH_3$ | ✅ **Two different ketones!** | | 5+ carbons | More possibilities | Also possible, but not minimum | **For 4-carbon ketones:** - **Butan-2-one**: $CH_3-CO-CH_2-CH_3$ - **Butan-3-one**: Would be $CH_3-CH_2-CO-CH_2-CH_3$ (same as butan-2-one by symmetry) These represent true metamers with the same formula but different carbonyl positions. **Answer: (3) 5 carbons** — Wait, reconsidering: With only **4 carbons**, we get distinct metameric ketones, making this the **minimum**. The answer is **(2) 4** carbon atoms minimum for ketone metamerism.