See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1 – Identify the final product. The final product is octahydronaphthalene (a bicyclic compound with two fused six-membered rings and one double bond at the ring junction), also called octalin or Δ1,9-2-octalin. It has the molecular formula C10H16 (with one double bond). More precisely the structure shown is bicyclo[4.4.0]dec-1(6)-ene, a decalin with one double bond between the bridgehead carbons. Step 2 – Work backwards from B to A. B is produced from A by treatment with LiAlH4 (reduction), and B then undergoes acid-catalyzed dehydration (H+, heat) to give the bicyclic alkene. The dehydration implies B is an alcohol. So B is a bicyclic alcohol whose dehydration gives the fused 6-6 bicyclic alkene. Step 3 – Identify B. For the product to be a fused bicyclic 6-6 ring system (decalin skeleton with a double bond), B must be a spiro or fused bicyclic alcohol that rearranges/dehydrates. A spiro alcohol where one ring is cyclopentane and the other is cyclohexane (spiro[4.5]decan-1-ol) can undergo ring expansion/dehydration via carbocation rearrangement: protonation of the OH gives a tertiary carbocation at the spiro carbon; ring expansion of the cyclopentane ring by a 1,2-carbon shift converts the 5-membered ring into a 6-membered ring, giving a bicyclic (6-6 fused) carbocation, which then loses a proton to give the observed decalin alkene. Step 4 – Identify A. LiAlH4 reduces a ketone to an alcohol. So A must be the ketone corresponding to B. If B is spiro[4.5]decan-1-ol (cyclohexane spiro-joined to cyclopentanol), then A is spiro[4.5]decan-1-one (the ketone: cyclopentanone ring spiro-fused to cyclohexane). This matches option (d): a cyclohexane ring spiro-fused at C1 of a cyclopentanone. Step 5 – Verify the rearrangement. Spiro[4.5]decan-1-ol (from reduction of spiro[4.5]decan-1-one) under acidic conditions: the tertiary spiro carbocation forms; the cyclopentane C-C bond migrates (ring expansion) to give a bicyclo[4.4.0] (decalin) carbocation; elimination gives Δ1(9)-octalin, matching the shown product. Step 6 – Eliminate other options. (a) 1-acetylcyclohexane is a monocyclic substrate and cannot directly give a bicyclic product without additional ring formation. (b) spiro[4.4]nonan-1-one has two cyclopentane rings; ring expansion would give a 5+6 fused system but the carbon count (9 carbons total) is insufficient for decalin (10 carbons). (c) has a cyclobutane ring; ring expansion from cyclobutane under these conditions and carbon count do not match the decalin skeleton. Therefore, the correct answer is D.