See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: We need to find all structural isomers of C7H16 (heptane isomers) that have a 5-carbon parent chain (pentane backbone), with the remaining 2 carbons attached as substituents (methyl groups) on that chain. Step 1: Identify the parent chain. A 5-membered parent chain means the longest chain chosen as parent is exactly 5 carbons (pentane). The remaining 2 carbons must be attached as substituents — either as two separate methyl groups or as one ethyl group. Step 2: Consider substituent combinations on a 5-carbon chain. The available positions on pentane for substitution are C1, C2, C3, C4, C5. Due to symmetry, unique positions are C1, C2, C3 (C4 mirrors C2, C5 mirrors C1). We need to place substituents such that the parent chain remains exactly 5 carbons (i.e., no longer chain of 5+ can be drawn through the substituents). Step 3: Two methyl groups as substituents (total extra carbons = 2). Place two methyl groups on the pentane chain: - 2,2-dimethylpentane: methyls at C2,C2 — valid, longest chain is 5 - 2,3-dimethylpentane: methyls at C2,C3 — valid, longest chain is 5 - 2,4-dimethylpentane: methyls at C2,C4 — valid, longest chain is 5 - 3,3-dimethylpentane: methyls at C3,C3 — valid, longest chain is 5 Note: 2,2 and 3,3 are distinct; 2,3 and 2,4 are distinct; placement at C1 would extend the chain making it 6 carbons, so not valid. Step 4: One ethyl group as substituent (total extra carbons = 2). Place one ethyl group on the pentane chain: - 3-ethylpentane: ethyl at C3 — valid, longest chain is 5 Note: ethyl at C2 would create a 6-carbon longest chain (C1-C2-C2branch gives chain of 6), so not valid as a 5-membered parent chain structure. Step 5: Count valid structures with 5-membered parent chain: 1. 2,2-dimethylpentane 2. 2,3-dimethylpentane 3. 2,4-dimethylpentane 4. 3,3-dimethylpentane 5. 3-ethylpentane This gives exactly 5 structures. Therefore, the correct answer is 5.