See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Wurtz reaction (Na/dry ether) couples two C-Br (or C-X) bonds by removing both halogens and forming a new C-C bond. Zinc (Zn) in dehalogenation removes two vicinal or geminal halogens to form a double bond or strained ring. Step 1 - Option (a): The starting material is a bridged bicyclic compound (resembling a diamondoid skeleton) with two Br atoms on bridgehead carbons. Treatment with Na/dry ether (intramolecular Wurtz) removes both Br atoms and forms a new C-C bond between the two bridgehead carbons, yielding adamantane. This is correct - the product shown (adamantane) is the expected major product. Step 2 - Option (b): The starting material is a spiro compound (two cyclobutane rings at a spiro carbon) bearing Br (wedge) and Cl (dash) on the spiro carbon. Na/dry ether promotes intramolecular coupling between the carbon bearing Br and the carbon bearing Cl (they are on the same carbon, so this is a geminal dihalide at the spiro center). Removal of both halogens from the same carbon and bond formation gives a bicyclo[1.1.0]butane (butterfly) structure. The product shown (two squares sharing an edge with a line indicating the new bond) is consistent with bicyclo[1.1.0]butane. This is a correct representation. Step 3 - Option (c): The starting material is a 1,2-dibromide (vicinal dibromide) on a cyclobutane ring that also has another set of Br atoms. Treatment with Zn causes reductive elimination of the vicinal Br-Br pair to give a cyclopropane ring fused to the cyclobutane (a bicyclobutane), retaining the remaining Br substituents. The product shown retains two Br atoms and shows a strained bicyclic ring system. This is plausible as the major product. Step 4 - Option (d): The starting material is 1,3-dibromocyclobutane (Br on C1 and C3 of cyclobutane, drawn as Br-(diamond)-Br). Na/dry ether should perform an intramolecular Wurtz reaction, removing both Br atoms and forming a new C1-C3 bond, yielding bicyclo[1.1.0]butane. However, in 1,3-dibromocyclobutane, C1 and C3 are already part of the ring; forming a bond between them would create a [1.1.0] bicyclic system. The product shown is a diamond with a double line, suggesting bicyclo[1.1.0]butane. The issue is that 1,3-dibromocyclobutane undergoing intramolecular Wurtz to give bicyclo[1.1.0]butane is geometrically very strained and is NOT the major product. The actual major product of Na with 1,3-dibromocyclobutane would involve intermolecular coupling or elimination, not the highly strained bicyclo[1.1.0]butane. Bicyclo[1.1.0]butane has extreme ring strain (~68 kcal/mol), making it not the expected major product of this reaction under standard Wurtz conditions. Therefore option (d) does NOT correctly represent the major product. Why other options don't fail: Options (a), (b), and (c) all involve geometrically feasible intramolecular reactions where the two reacting carbons are positioned appropriately and the products, while strained in some cases, are well-precedented. Option (d) is the exception because the claimed product (bicyclo[1.1.0]butane) is extremely strained and not the major product of Wurtz reaction of 1,3-dibromocyclobutane. Therefore, the correct answer is D.