See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Acid-catalyzed rearrangement of spiro cyclohexadienones (dienone-phenol rearrangement). Step 1 - Identify the starting material: The compound is spiro[4.5]deca-6,9-dien-8-one, which is a cyclopentane ring joined at a spiro carbon to a 2,5-cyclohexadien-1-one (para-quinol type). The spiro carbon is at C1 of cyclopentane and C1 of the cyclohexadienone. Step 2 - Acid-catalyzed mechanism (dienone-phenol rearrangement with ring expansion): Under H+ conditions, the carbonyl is protonated, generating a carbocation at the carbonyl carbon. A 1,2-carbon shift (migration of one of the C-C bonds of the cyclopentane ring to the adjacent carbocation center) occurs. This is the classic spiro dienone rearrangement. Step 3 - Ring expansion: The cyclopentane C-C bond migrates into the six-membered ring. This converts the five-membered ring + six-membered ring spiro system into a fused bicyclic system. The migration of a cyclopentane carbon into the ring expands the cyclopentane to become part of a six-membered ring, giving a bicyclo[4.4.0] (decalin-type) framework with an aromatic ring after tautomerization/aromatization. Step 4 - Product identification: After the 1,2-shift and rearrangement, the product is a bicyclic compound where the six-membered dienone ring has aromatized to a phenol ring (OH on the ring), and the cyclopentane has ring-expanded to a fused cyclohexane ring. This gives 5,6,7,8-tetrahydro-2-naphthol (or equivalently octahydronaphthalene with an OH on the aromatic ring), which matches option (b): a naphthalene skeleton with one aromatic ring bearing HO and one saturated cyclohexane ring fused to it. Step 5 - Why other options fail: - Option (a): OH at C1 (ring junction, non-aromatic) does not arise; aromatization drives the OH onto the ring as a phenol. - Option (c): 4-cyclopentylphenol would require ring opening rather than ring expansion; not the major pathway under these conditions. - Option (d): The cyclohexadienol (non-aromatic) form is not the final product; aromatization to phenol is thermodynamically favored. Therefore, the correct answer is B.