See image — IUPAC and Nomenclature Chemistry Question

💡 Solution & Explanation

Concept: IUPAC nomenclature of unsaturated carboxylic acids requires identifying the longest carbon chain containing both the carboxylic acid group and the double bond, numbering from the end closest to the principal characteristic group (–COOH), and citing the double bond position as a locant. Step 1: Identify the structure. The compound is CH3–CH=CH–COOH. It has 4 carbons in the chain: C1 (COOH carbon), C2, C3, C4 (CH3). Step 2: Assign the parent chain. The longest chain containing –COOH and the C=C is 4 carbons → parent name: butanoic acid → but-…-oic acid. Step 3: Number the chain. The carboxylic acid carbon gets the lowest locant, so it is C1. The chain is: C1(COOH)–C2=C3–C4(CH3). Step 4: Locate the double bond. The double bond is between C2 and C3, so it is named but-2-en-1-oic acid (the '1' refers to the C1 position of the acid, which by convention is indicated by 'oic acid' itself, so the name is But-2-en-1-oic acid — the double bond starts at C2, and the acid group is at C1). Step 5: Evaluate options. (a) But-1-en-4-oic acid — wrong numbering; would place double bond at C1 and acid at C4, which contradicts the rule that –COOH gets C1. (b) 1-hydroxybut-2-en-1-one — this would be a ketone/aldehyde with hydroxyl, not a carboxylic acid; incorrect. (c) But-2-en-1-oic acid — double bond between C2 and C3, carboxylic acid at C1; this correctly describes CH3–CH=CH–COOH. Correct. (d) But-2-en-4-oic acid — placing the acid at C4 is incorrect numbering. Therefore, the correct answer is C.