See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Concept: Pinacol rearrangement. When a vicinal diol (or in this case a geminal diol on a spiro carbon) is treated with acid and heat, it undergoes the pinacol rearrangement, involving loss of water to form a carbocation, followed by a 1,2-shift (alkyl or hydride migration) to give a carbonyl compound. Step 1 - Identify the substrate: The starting material is 1,1'-spirobi[cyclopentan]-1,1'-diol — a single carbon bearing two OH groups and serving as the spiro center connecting two cyclopentane rings. This is effectively a geminal diol at the spiro carbon (the carbon has two OH groups and is part of both cyclopentane rings). Step 2 - Pinacol rearrangement mechanism: Under acidic conditions, one OH is protonated and leaves as water, generating a tertiary carbocation at the spiro carbon. This carbocation is stabilized by the two cyclopentyl rings. Step 3 - 1,2-Carbon shift: A C-C bond from one of the cyclopentane rings migrates to the adjacent carbocation center. Since the carbocation is at the spiro carbon and a ring C-C bond migrates, this effectively expands one ring. The migration of a carbon from a five-membered ring into the carbocation produces a ring-expanded intermediate. Step 4 - Product formation: After the 1,2-shift, the remaining OH (or the oxocarbenium) loses a proton to give a ketone. The ring expansion of one cyclopentane ring by one carbon (due to migration) yields a six-membered ring (cyclohexanone portion) fused in a spiro arrangement with the remaining cyclopentane ring. This gives spiro[4.5]decan-6-one or equivalently a spiro compound of cyclopentane and cyclohexanone — which matches option (c): a spiro bicyclic structure with one cyclopentane ring and one ring bearing a ketone at the spiro junction region. Step 5 - Why other options fail: - (a) Dehydration to an alkene is not the pinacol product; simple dehydration would give an enol or alkene but not the thermodynamically preferred carbonyl via rearrangement. - (b) An epoxide would require different conditions and mechanism; not consistent with H+/Delta pinacol conditions. - (d) Shows a different ring size combination or connectivity that does not match the ring expansion from two cyclopentane rings. Therefore, the correct answer is C.