Q.20 The major product formed in the given reaction is
The major product of this reaction is the tert‑butyl‑substituted cyclopentyl aldehyde, i.e. option (D). The mechanism proceeds through diazotization of the amine followed by a Demjanov-type ring contraction to give the ring‑contracted aldehyde.
Introduction (SEO‑optimized)
When a β‑hydroxycyclohexylamine is treated with nitrous acid, the reaction does not stop at simple formation of an alcohol; instead, a Demjanov rearrangement can occur, giving a rearranged product through carbocation chemistry. Understanding how diazotization, nitrogen loss, and ring contraction combine explains why a tert‑butyl‑substituted cyclopentyl aldehyde emerges as the major product in this multiple‑choice problem. This makes the question a classic example for CSIR‑NET and other competitive exams testing advanced reactivity of amines.
Stepwise mechanism and reason for option (D)
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Diazotization of the primary aliphatic amine
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Primary aliphatic amines react with nitrous acid to give aliphatic diazonium salts, which are extremely unstable.
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For the given β‑hydroxycyclohexylamine, treatment with HNO₂ first forms the corresponding diazonium intermediate on the ring carbon bearing the –NH₂ group.
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Loss of N₂ and formation of carbocation
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Aliphatic diazonium ions rapidly expel nitrogen gas, generating a carbocation at the former amino‑bearing carbon.
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Because the adjacent carbon bears an OH group and the ring also carries a bulky tert‑butyl substituent, the system is predisposed to rearrangements that relieve strain and stabilize the cation.
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Demjanov rearrangement and ring contraction
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In the Demjanov rearrangement, a primary amine treated with nitrous acid forms a diazonium ion whose cationic intermediate undergoes 1,2‑shift(s), often accompanied by ring expansion or contraction, to give rearranged alcohols or carbonyl compounds after capture by nucleophiles.
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For substituted cyclohexyl systems, the cation can undergo a 1,2‑shift that contracts the six‑membered ring to a five‑membered ring (ring contraction) while migrating a carbon substituent and finally giving a cyclopentyl carbonyl derivative.
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Formation of aldehyde under aqueous acidic work‑up
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In this substrate, the β‑hydroxy group can participate in a semi‑pinacol–like rearrangement, leading to migration and formation of a carbonyl group at the side chain after ring contraction.
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Under the subsequent acidic work‑up with H₃O⁺, the rearranged cationic intermediate is trapped as an aldehyde, giving the tert‑butyl‑substituted cyclopentyl aldehyde shown in option (D).
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Thus, the mechanistic sequence is:
primary amine → diazonium salt → carbocation → Demjanov ring contraction/semi‑pinacol rearrangement → cyclopentyl aldehyde, making (D) the correct major product.
Detailed analysis of each option
Option (A): tert‑butyl cyclohexanol with amino group
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Primary aliphatic amines with HNO₂ generally give alcohols (R–OH) along with N₂ and H₂O, and this could suggest a simple replacement of –NH₂ by –OH on the ring.
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However, in strongly rearrangement‑prone systems like substituted cyclohexylamines, the initially formed carbocation undergoes 1,2‑shifts and ring contraction instead of cleanly capturing water; therefore, the unrearranged amino alcohol of option (A) is not the major product.
Option (B): tert‑butyl cyclohexanone (ring intact)
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A carbocation adjacent to an OH group often rearranges to give a carbonyl (ketone) via hydride or alkyl shift followed by loss of a proton or water, so cyclohexanone‑type products can be formed in some systems.
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In the present case, the driving force for ring contraction to a cyclopentyl system plus relief of steric strain from the tert‑butyl group makes the contracted aldehyde more favorable than a simple cyclohexanone, so option (B) is not the major product.
Option (C): tert‑butyl cyclohexyl aldehyde (side‑chain carbonyl, ring intact)
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Option (C) keeps the six‑membered ring intact and places an aldehyde on a side chain, corresponding to rearrangement without ring contraction.
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Experimental and theoretical studies on similar β‑substituted cyclohexyl diazonium systems show that 1,2‑shifts often proceed with ring contraction to a five‑membered ring, especially when a bulky substituent like tert‑butyl is present, making a cyclopentyl carbonyl compound more stable than the unrearranged cyclohexyl aldehyde.
Option (D): tert‑butyl‑substituted cyclopentyl aldehyde (correct)
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Option (D) shows a five‑membered ring bearing a tert‑butyl group and a terminal –CHO side chain, which matches the product expected from Demjanov rearrangement with ring contraction of the cyclohexane ring.
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This structure incorporates: loss of the amine (deamination), rearrangement of the carbocation, contraction of the ring from six to five members, and final formation of an aldehyde after aqueous acidic work‑up, consistent with known behavior of β‑hydroxycyclohexylamines under nitrous‑acid conditions.
Key takeaways for exam preparation
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Primary aliphatic amines + HNO₂: give unstable diazonium ions that decompose to carbocations, usually leading to alcohols but often to rearranged products (Demjanov rearrangement).
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Cyclic systems (cyclohexylamines): carbocations formed after N₂ loss can undergo ring expansion or contraction, especially when neighboring groups (like OH) promote semi‑pinacol rearrangement, frequently giving ring‑contracted carbonyl compounds.
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MCQ strategy: when a β‑hydroxy or other rearrangement‑activating group is present and the options include a ring‑contracted carbonyl product, that option is often the correct major product, as in this question where option (D) is chosen.
