Which reagent can be employed to distinguish Benzaldehyde and Propanal?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Fehling’s Reagent
To tell two compounds apart, the reagent must give a visible change with one and not with the other. Both compounds here are aldehydes, so most aldehyde tests react with both. Fehling’s solution is the exception: it is a weaker oxidising agent that is reduced by aliphatic aldehydes but not by aromatic aldehydes.
Propanal, an aliphatic aldehyde, gives a reddish brown precipitate of copper(I) oxide:
\( \displaystyle \ce{CH3CH2CHO + 2Cu^2+ + 5OH^- -> CH3CH2COO^- + Cu2O v + 3H2O} \)
Benzaldehyde, \( \ce{C6H5CHO} \), gives no precipitate. So Fehling’s reagent distinguishes the two.
Why the other options are incorrect
Option (1) 2,4-DNP reagent — Incorrect. It reacts with the C=O group of every aldehyde and ketone, giving a yellow-orange 2,4-dinitrophenylhydrazone with both compounds.
Option (3) Tollen’s Reagent — Incorrect. Tollens’ reagent is a stronger oxidant that is reduced by aliphatic and aromatic aldehydes alike, so both give a silver mirror.
Option (4) Baeyer’s Reagent — Incorrect. Cold, dilute alkaline \( \ce{KMnO4} \) is a test for C=C unsaturation; neither compound has a C=C bond outside the benzene ring, so it does not separate them.
NCERT (2026-27 edition):
Fehling’s Test
