CARBONYL COMPOUNDS
In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketones, it is bonded to two carbon atoms. Aldehydes and ketones undergo nucleophilic addition reactions. A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridized orbitals of carbonyl carbon. The hybridization of carbon changes from sp2 to sp3 in the nucleophilic addition. Aldehydes give Fehling’s test positive while aromatic aldehydes do not respond to this test .
An organic compound ‘A’ with molecular formula C3H5N, undergoes Stephen reaction to produce another compound ‘B’. Compound ‘B’ gives orange precipitate with 2,4-DNP but it does not undergo Haloform reaction. However, the reaction of compound B with dilute alkali produces a compound ‘C’ which is β-hydroxy compound with increased number of carbon atoms.
Which functional group is present in compound ‘A’?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → –CN
The Stephen reaction is a reaction of nitriles. A nitrile is reduced by stannous chloride and HCl to an imine, which is hydrolysed to the corresponding aldehyde:
\( \displaystyle \ce{R-CN} \xrightarrow[\text{(ii) }\ce{H3O^+}]{\text{(i) }\ce{SnCl2 + HCl}} \ce{R-CHO} \)
Compound A, \( \ce{C3H5N} \), undergoes this reaction, so it must contain the cyano group. The formula fits propanenitrile, \( \ce{CH3CH2CN} \):
\( \displaystyle \ce{CH3CH2-C#N}\quad(\ce{C3H5N}) \)
So the functional group in A is –CN (nitrile).
Why the other options are incorrect
Option (1) –NH2 — Incorrect. A three-carbon primary amine is \( \ce{C3H9N} \) (propan-1-amine), and amines do not undergo the Stephen reaction.
Option (3) – NC — Incorrect. Ethyl isocyanide, \( \ce{CH3CH2NC} \), has the same formula \( \ce{C3H5N} \), but isocyanides do not give aldehydes on reduction; they give secondary amines.
Option (4) –NHCH3 — Incorrect. N-methylethanamine is \( \ce{C3H9N} \), a secondary amine, and does not undergo the Stephen reaction.
NCERT (2026-27 edition):
From Nitriles and Esters
