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.
Fehling’s reagent is an aqueous solution of
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → copper sulphate and alkaline sodium potassium tartarate.
The alkali is needed for the oxidation of the aldehyde. The tartarate ion forms a complex with \( \ce{Cu^{2+}} \), so that copper(II) hydroxide does not precipitate in the alkaline solution. On heating with an aliphatic aldehyde, a reddish brown precipitate of \( \ce{Cu2O} \) forms:
\( \displaystyle \ce{R-CHO + 2Cu^2+ + 5OH^-} \xrightarrow{\text{heat}} \ce{RCOO^- + Cu2O v + 3H2O} \)
Why the other options are incorrect
Option (1) copper sulphate and sodium potassium tartarate — Incorrect. Without the alkali, the aldehyde is not oxidised and no \( \ce{Cu2O} \) is formed.
Option (3) copper sulphate and acidic sodium potassium tartarate — Incorrect. The test works only in alkaline medium; \( \ce{Cu2O} \) is not precipitated from acidic solution.
Option (4) copper sulphate — Incorrect. It lacks both the alkali and the tartarate; with alkali alone, \( \ce{Cu^{2+}} \) would precipitate as \( \ce{Cu(OH)2} \).
NCERT (2026-27 edition):
Fehling’s Test
