See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Step 1 — Determine the molecular formula of 3-ethyl-2-methylpentane. 3-ethyl-2-methylpentane: pentane backbone (5 C) + ethyl group at C3 (2 C) + methyl group at C2 (1 C) = 8 carbons total. Counting hydrogens: C8H18. This is an octane isomer. Step 2 — Check option (a). Option (a) shows a straight zigzag chain that, by counting the vertices and endpoints, gives 8 carbons (n-octane, C8H18). This has the same molecular formula as 3-ethyl-2-methylpentane. It IS an isomer. Step 3 — Check option (c). Option (c) shows a branched chain with a downward methyl group and adjacent substituents. Counting carbons: the main chain plus branches totals 8 carbons (C8H18). This is consistent with 2,3-dimethylhexane or a similar C8H18 isomer. It IS an isomer. Step 4 — Check option (b). Option (b) shows a structure with two methyl branches on a shorter chain. Counting carefully: the structure drawn appears to be 2,4-dimethylpentane — a 5-carbon chain with methyl groups at C2 and C4, giving only 7 carbons total (C7H16). This has a DIFFERENT molecular formula from 3-ethyl-2-methylpentane (C8H18). Therefore option (b) is NOT an isomer of 3-ethyl-2-methylpentane. Step 5 — Conclusion. Because option (b) has a different molecular formula (C7H16 vs C8H18), it cannot be an isomer of 3-ethyl-2-methylpentane. Therefore, the correct answer is B.