See image — Alcohols Phenols and Ethers Chemistry Question

💡 Solution & Explanation

Step 1 - Identify the starting material and product. The starting material is 1-hydroxyspiро[4.5]decane, which has a cyclopentane ring and a cyclohexane ring joined at a spiro carbon bearing the OH. The product is decalin (bicyclo[4.4.0]decane), which consists of two fused six-membered rings. Step 2 - Generate the carbocation. Protonation of OH by H2SO4 converts it to a good leaving group (water). Loss of water gives a tertiary carbocation at the spiro carbon. At this point the carbon bears a positive charge and is the junction of the cyclopentane ring and the cyclohexane ring. Step 3 - Determine what rearrangement converts a spiro[4.5] system into a fused bicyclo[4.4.0] (decalin) system. In the spiro carbocation, the spiro carbon is shared between a five-membered ring (cyclopentane, 4 carbons + spiro C) and a six-membered ring (cyclohexane, 5 carbons + spiro C). To obtain decalin (two fused six-membered rings), one C-C bond of the cyclopentane ring must migrate (ring expansion of the five-membered ring to a six-membered ring). This is a 1,2-carbon shift where one of the C-C bonds of the cyclopentane ring migrates to the adjacent carbocation, expanding the five-membered ring to a six-membered ring while the positive charge moves to a new carbon. The resulting bicyclic carbocation, after deprotonation, yields decalin. Step 4 - Identify which specific bond migrates. Bond (c) is the C-C bond of the cyclopentane ring on the upper side of the carbocation center. Migration of bond (c) means the carbon-carbon bond connecting the spiro carbon to the cyclopentane ring migrates so that the five-membered ring expands to a six-membered ring, creating the fused bicyclic (decalin) skeleton. This is a ring-expansion via a 1,2-shift of a C-C bond of the cyclopentane portion. Step 5 - Eliminate other options. Bond (a) is a C-C bond of the cyclohexane ring; its migration would not produce decalin but would give a different skeleton. Bond (b) is a C-H bond; a hydride shift would only move the carbocation to an adjacent carbon without changing the ring sizes, and would not yield decalin. Bond (d) is the other C-C bond of the cyclopentane ring on the lower side; migration of (d) would lead to a different connectivity than decalin. Therefore, the correct answer is C.