See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Step 1: Cyclopentanone reacts with HCN (nucleophilic addition) to give a cyanohydrin: 1-cyanocyclopentan-1-ol (compound A). The CN group adds to the carbonyl carbon, giving a tertiary alcohol with a -CN substituent at what was the carbonyl carbon of cyclopentanone. Step 2: A (the cyanohydrin, 1-hydroxycyclopentane-1-carbonitrile) is reduced with LiAlH4. LiAlH4 reduces the -CN group to -CH2NH2, and also the -OH remains (it is not reduced under these conditions since it is already an alcohol). So compound B is 1-(aminomethyl)cyclopentan-1-ol, i.e., the cyclopentane ring bearing both an -OH and a -CH2NH2 group at C1. Step 3: B (a primary amine, -CH2NH2) reacts with NaNO2/HCl (diazotization conditions for primary aliphatic amine). Primary aliphatic amines treated with NaNO2/HCl form unstable diazonium salts that immediately decompose. The -CH2NH2 group becomes -CH2N2+ which loses N2 to form a carbocation/carbenium ion (-CH2+). This primary carbocation rearranges: the ring expands as a C-C bond from the cyclopentane migrates to the adjacent carbocation center, giving a six-membered ring carbocation (cyclohexyl cation at the carbonyl equivalent position). The -OH group at the tertiary center assists: the whole process is a ring expansion. Specifically, the 1-(aminomethyl)cyclopentan-1-ol undergoes deamination with ring expansion: the cyclopentane ring expands to a cyclohexanone ring. The tertiary carbinolamine system loses N2 and the hydroxyl group, with ring expansion yielding cyclohexanone. Mechanism detail: After loss of N2 from -CH2N2+, the resulting -CH2+ (primary carbocation) is adjacent to the ring carbon bearing -OH. A 1,2-shift of the C1-C2 bond of the cyclopentane ring into the carbocation gives a six-membered ring with an oxocarbenium ion or the -OH at the former C1 becomes a ketone after loss of H+. The net result is ring expansion from cyclopentanone (5-membered) to cyclohexanone (6-membered). Why other options fail: - (b) 2-methylcyclopentan-1-one and (c) 3-methylcyclopentan-1-one: these would require methyl substitution without ring expansion, which is not the outcome of this sequence. - (d) 1-(hydroxymethyl)cyclopentan-1-ol: this would be compound B (after LiAlH4 reduction) if we consider -CN → -CH2OH, but LiAlH4 reduces CN to CH2NH2, not CH2OH; moreover, product C requires the NaNO2/HCl step which causes ring expansion. Therefore, the correct answer is A.