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.
Choose the reagent that converts compound ‘A’ to ‘B’.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → SnCl2 + HCl
Compound A is propanenitrile and B is propanal. The change is the Stephen reaction: the nitrile is reduced by stannous chloride in the presence of hydrochloric acid to an imine, and the imine is hydrolysed by water to the aldehyde:
\( \displaystyle \ce{CH3CH2CN} \xrightarrow{\ce{SnCl2 + HCl}} \ce{CH3CH2CH=NH} \xrightarrow{\ce{H3O^+}} \ce{CH3CH2CHO} \)
\( \ce{SnCl2} \)/HCl is a mild reducing agent, so the reduction stops at the imine stage and the aldehyde is obtained after hydrolysis. So the reagent is SnCl2 + HCl.
Why the other options are incorrect
Option (2) FeCl2 + HCl — Incorrect. FeCl2 is too weak a reducing agent to reduce a nitrile; the reagent is stannous chloride, SnCl2, with HCl.
Option (3) LiAlH4 — Incorrect. It is a strong reducing agent and reduces the nitrile fully to the primary amine.
\( \displaystyle \ce{CH3CH2-C#N} \xrightarrow[\text{(ii) }\ce{H2O}]{\text{(i) }\ce{LiAlH4}} \ce{CH3CH2CH2NH2} \)
Option (4) CrO2Cl2 — Incorrect. Chromyl chloride is the oxidising agent of the Etard reaction (toluene to benzaldehyde); it does not reduce a nitrile.
\( \displaystyle \ce{C6H5CH3} \xrightarrow{\ce{CrO2Cl2}\text{, }\ce{CS2}} \ce{C6H5CH(OCrOHCl2)2} \xrightarrow{\ce{H3O+}} \ce{C6H5CHO} \)
NCERT (2026-27 edition):
From Nitriles and Esters
