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Determine the number of carbon atoms in the principal chain (longest chain containing the principal characteristic group) in the IUPAC name of the following compound: CH3-CH(-CH2CH3)-CH(-COOH)-CH2-CH3

1. 3
2. 4
3. 5
4. 6

Correct answer: 5

Solution

The COOH carbon connects to: (i) a chain going CH2-CH3 (2 C) and (ii) a chain going CH(CH2CH3)-CH3 (through a branched carbon). The longest continuous chain through COOH: COOH-C-CH(CH2CH3)-CH3 = 4 C including COOH, or COOH-C-CH2-CH3 = 3 C. But we need to count the COOH carbon itself. The principal chain = COOH + CH2CH3 side + CH(CH2CH3)CH3 side if all are contiguous. The structure: CH3-CH(C2H5)-CH(COOH)-CH2-CH3. Chain: COOH-CH-CH2-CH3 = 4 atoms. Chain: COOH-CH-CH(C2H5)-... no. Longest chain including COOH: C1(OOH)-C2-C3-C4-C5 where C1=COOH carbon, C2=adjacent CH, then C3-C4-C5 going through the ethyl branch gives 5 carbons total.

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