See image — IUPAC and Nomenclature Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the parent structure: The structure shows a five-membered carbocyclic ring with two double bonds, making it a cyclopentadiene (cyclopenta-1,3-diene). The ring contains four sp2 carbons (C1–C4) involved in conjugated double bonds at positions 1,2 and 3,4, and one sp3 carbon (C5). Step 2 - Identify the substituent: Attached to C5 (the sp3 carbon, which is the methylene bridge of the diene) is a -C≡CH group, which is an ethynyl group (a terminal alkyne). Step 3 - Assign the locant: The sp3 carbon bearing the substituent is C5 in the cyclopenta-1,3-diene numbering system. Numbering starts at one end of the conjugated diene system to give the double bonds the lowest possible locants (1,3), placing the substituent-bearing carbon at position 5. Step 4 - Construct the IUPAC name: Parent chain = cyclopenta-1,3-diene; substituent at C5 = ethynyl; full name = 5-ethynylcyclopenta-1,3-diene. Step 5 - Why no other name applies: The compound is not a spiro or bicyclic system, so no bridged nomenclature is needed. The ethynyl group is a substituent, not part of the ring, so it is cited as a prefix. There are no stereocenters to specify. Therefore, the correct answer is 5-ethynylcyclopenta-1,3-diene.