See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Concept: This reaction involves the thermal rearrangement (retro-ring opening / ring expansion) of a spiro compound upon heating. The starting material is spiro[4.5]decane, which has a spiro carbon connecting a cyclopentane ring and a cyclohexane ring. Step 1 - Identify the starting material: The structure shown is spiro[4.5]decane - a spiro junction between a five-membered ring (cyclopentane) and a six-membered ring (cyclohexane), sharing one quaternary spiro carbon. Step 2 - Reaction under heat (Delta): Heating spiro compounds can cause ring-opening and ring-expansion rearrangements. The spiro carbon is a quaternary carbon. Under thermal conditions, a C-C bond of the smaller ring (cyclopentane) can break homolytically or via a concerted process, generating a diradical or carbocationic intermediate. The cyclopentane ring opens to give a 1,5-diradical or the bond migrates. Step 3 - Mechanism: When the spiro[4.5]decane is heated, one of the C-C bonds in the cyclopentane ring breaks. This ring-opening of the five-membered ring with simultaneous formation of a new C-C bond effectively converts the spiro system into a fused bicyclic system. The cyclopentane (5-membered) ring opens and the carbon inserts into the cyclohexane ring, expanding it. A 5-membered + 6-membered spiro system rearranges to give two fused 6-membered rings (decalin skeleton) via a [1,2]-carbon shift or retro-spiro/ring expansion mechanism. Step 4 - Product identification: The ring expansion of spiro[4.5]decane under thermal conditions gives decalin (bicyclo[4.4.0]decane) - two fused six-membered rings. The major product under these thermodynamic/thermal conditions is the fully saturated decalin (two fused cyclohexane rings) which corresponds to option (c). The rearrangement proceeds via a carbocation or diradical where the ring expansion is complete and the double bond (if transiently formed) is reduced or the product is the saturated bicyclic. In the thermal rearrangement, the spiro carbon undergoes a 1,2-shift converting the [4.5] spiro into the [4.4.0] fused system (decalin) as the thermodynamically stable product. Step 5 - Why other options fail: - Option (a): A fused 5-6 bicyclic with a double bond would require incomplete rearrangement and is not the major product of full ring expansion. - Option (b): A decalin with a double bond at one position could be an intermediate but the fully saturated decalin is thermodynamically more stable and is the major product. - Option (d): Similarly, a partially unsaturated decalin is a minor product or intermediate. The major product is decalin (bicyclo[4.4.0]decane), which is option (c). Therefore, the correct answer is C.