Organic Compounds Containing Nitrogen: Chemistry | JEE Main
Cyclohexylamine when treated with nitrous acid yields (P). On treating (P) with PCC results in (Q). When (Q) is heated with dil. NaOH we get (R) The final product (R) is :
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Hint 1 of 3
What are the identities of intermediate (P) and compound (Q)?
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Correct answer
Cyclohexylamine reacts with nitrous acid to give cyclohexanol (P), which is oxidized by PCC to cyclohexanone (Q); heating with dilute NaOH induces self-aldol condensation followed by dehydration to yield 2-(cyclohex-1-en-1-yl)cyclohexan-1-one (R).
Option analysis
Why each option works or fails
A ·
Believing that dehydration after aldol addition yields an exocyclic double bond directly conjugated with the carbonyl inside the ring, rather than the more thermodynamically stable endocyclic double bond. Elimination yields the more stable, conjugated α,β-unsaturated enone where the double bond is endocyclic in the adjacent ring: 2-(cyclohex-1-en-1-yl)cyclohexan-1-one.
B ·
None. This is the correct structure representing 2-(cyclohex-1-en-1-yl)cyclohexan-1-one, formed by aldol condensation and dehydration of cyclohexanone. Diazotization gives cyclohexanol, PCC oxidizes it to cyclohexanone, and dilute NaOH heats it to produce the conjugated self-aldol condensation product.
C ·
Assuming that aldol reaction stops at the β-hydroxy ketone stage without dehydration upon heating. Heating with dilute base promotes the dehydration of the β-hydroxy ketone intermediate to form an α,β-unsaturated enone.
D ·
Believing that the amine undergoes deamination to directly form cyclohexene which couples under basic conditions, or confusing the connectivity of the two six-membered rings. Identify the functional groups sequentially: primary amine → alcohol → ketone → enone via self-aldol condensation.
Reviewed route
Solution
StepWorking
01identify
A 3-step sequence: (1) Primary aliphatic amine + HNO2 (diazotization to unstable diazonium salt, releasing N2 to form carbocation, trapped by H2O to give an alcohol); (2) PCC oxidation of secondary alcohol to ketone; (3) Aldol condensation of cyclic ketone using dil. NaOH with heat to give an α,β-unsaturated ketone.
02mechanism
Step 1: Cyclohexylamine (C6H11NH2) reacts with HNO2 to yield cyclohexanol (P) via loss of N2 and nucleophilic attack of H2O on the cyclohexyl carbocation.
03mechanism
Step 2: Cyclohexanol (P) is oxidized selectively by Pyridinium Chlorochromate (PCC) to cyclohexanone (Q).
04mechanism
Step 3: Two molecules of cyclohexanone undergo self-aldol condensation in the presence of dil. NaOH and heat. Deprotonation at the α-position generates an enolate, which attacks the carbonyl carbon of a second cyclohexanone molecule. Dehydration upon heating yields 2-(cyclohex-1-en-1-yl)cyclohexan-1-one (2-(1-cyclohexenyl)cyclohexanone), with the double bond conjugated to the carbonyl group.
05product
The product (R) is an α,β-unsaturated ketone. It consists of two six-membered rings joined by a C-C bond. The exocyclic double bond is in conjugation with the carbonyl group (Option 1).
✓verify
Total carbons: 6+6=12. Conjugation between C=C and C=O stabilizes the α,β-unsaturated enone, ruling out isolated double bond isomers.
Hints that build this answer step by step
What are the identities of intermediate (P) and compound (Q)?
(P) is cyclohexanol and (Q) is cyclohexanone
What reaction occurs when cyclohexanone (Q) is heated with dilute NaOH?
Self-aldol condensation with dehydration to give an α,β-unsaturated ketone
Which double-bond regioisomer is formed upon dehydration of the aldol adduct?
The endocyclic double bond in the non-carbonyl ring: 2-(cyclohex-1-en-1-yl)cyclohexan-1-one
Why is the double bond between the two rings or inside the second ring conjugated to the carbonyl?
Dehydration of aldol adducts (β-hydroxy ketones) specifically removes the α-hydrogen and β-OH to yield conjugated α,β-unsaturated carbonyl compounds due to extended resonance stabilization.