Samples of benzaldehyde and acetaldehyde can be distinguished by using
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Mixture of Fehling solution A + B
Fehling’s reagent is made by mixing Fehling solution A (aqueous copper sulphate) and Fehling solution B (alkaline sodium potassium tartrate). On warming, aliphatic aldehydes reduce the \( \ce{Cu^{2+}} \) complex to a red-brown precipitate of Cu2O, but aromatic aldehydes do not respond to this test.
\( \ce{CH3CHO + 2Cu^2+ + 5OH-} \xrightarrow{\Delta} \ce{CH3COO- + Cu2O v + 3H2O} \)
So acetaldehyde gives the red-brown precipitate and benzaldehyde does not; the two samples are told apart.
Why the other options are incorrect
Option (1) 2,4 Dinitrophenylhydrazine — Incorrect. It reacts with the C=O group of all aldehydes and ketones to give a yellow-orange 2,4-DNP derivative, so both samples respond.
Option (2) Tollen’s reagent — Incorrect. Both aliphatic and aromatic aldehydes reduce ammoniacal silver nitrate and give a silver mirror.
Option (4) Zn-Hg/HCl — Incorrect. This is the Clemmensen reagent; it reduces the C=O of both compounds to \( \ce{CH2} \) (to ethane and toluene) with no visible difference.
NCERT (2026-27 edition):
Fehling’s Test
