See image — IUPAC and Nomenclature Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the structure: HOOC—CH=CH—COOH is a four-carbon chain with carboxylic acid groups at C1 and C4, and a double bond between C2 and C3. Step 2 - Apply IUPAC nomenclature rules: The longest carbon chain includes the carbonyl carbons of both —COOH groups, giving a total of 4 carbons. The parent chain is 'but' (4 carbons). The double bond is between C2 and C3, so the locant is 'but-2-ene'. Both terminal carbons (C1 and C4) bear the principal characteristic group —COOH (expressed as '—oic acid'). For a diacid, the suffix is '—dioic acid', giving 'but-2-enedioic acid'. Step 3 - Evaluate options: (a) 'But-2-ene-1,4-dicarboxylic acid' would imply —COOH groups are substituents on a 4-carbon chain, meaning the molecule would have 6 carbons total — incorrect for this structure. (b) 'But-2-ene-1,4-dioic acid' correctly names a 4-carbon chain (including both carboxyl carbons) with a double bond at position 2 and dioic acid suffix at positions 1 and 4 — this is correct. (c) 'Ethene dicarboxylic acid' treats the two carboxylic acids as substituents on a 2-carbon (ethene) chain, which is a semi-systematic/trivial-style name, not strict IUPAC. (d) 'Ethene dioic acid' similarly uses only 2 carbons as the parent chain — incorrect under IUPAC rules. Step 4 - Conclusion: The correct IUPAC name includes all four carbons in the parent chain with the suffix 'dioic acid', making option (b) correct. Therefore, the correct answer is B.