See image — Amines Chemistry Question

💡 Solution & Explanation

Concept: When a primary aliphatic amine reacts with nitrous acid (HNO2), it forms a diazonium ion intermediate. For aliphatic diazonium ions, the N2 is immediately lost, generating a carbocation. However, if the geometry is favorable (anti-periplanar arrangement), a neighboring group or bond migration (1,2-shift) can occur simultaneously with loss of N2. Step 1: Identify the substrate. The starting material is trans-2-aminocyclohexan-1-ol (the two substituents OH and NH2 are trans to each other on the cyclohexane ring, as indicated by H and OH on wedge and NH2 and H on dash at adjacent carbons). Step 2: Reaction with HNO2. The NH2 group is converted to a diazonium ion (–N2+) in situ. Step 3: Since the OH and N2+ groups are trans (anti-periplanar) to each other on the cyclohexane ring, the C–C bond between C1 and C2 is anti-periplanar to the leaving N2+ group. This sets up a semipinacol-type (or Whitmore) 1,2-carbon shift: as N2 departs from C2, the C1–C2 bond migrates with its electrons to C2, forming a ring-contracted carbocation at C1. Step 4: The ring contraction converts the six-membered ring into a five-membered ring (cyclopentane ring) with a carbocation at the former C1 position, which also bears the OH group. This gives a cyclopentane ring with an oxocarbenium or aldehyde-forming intermediate: the OH at the migrating carbon is now at the cationic center, and loss of a proton (or the oxocarbenium collapses) gives cyclopentane carboxaldehyde. Step 5: The product is cyclopentanecarboxaldehyde — a cyclopentane ring with a –CHO substituent, formed by ring contraction from cyclohexane. Why other options fail: - (b) cyclohexane-1,2-diol: would require simple substitution of NH2 by OH with retention of ring size, but aliphatic diazonium ions under these conditions undergo rearrangement, not simple substitution. - (c) 2-aminocyclohexene: would require elimination, not the observed rearrangement product. - (d) cyclohex-2-enol: also an elimination product retaining six-membered ring, inconsistent with ring contraction. Therefore, the correct answer is A.