Certain Aldehydes, ketone and certain alcohol undergo iodoform test. Certain Aldehydes ketones undergo aldol condensation. While certain aldehydes undergo Cannizzaro’s reaction. Aldehydes respond to Tollens’ reagent and Fehling’s reagent test.
Which amongst the following will respond to both Tollen's reagent test and Fehling reagent test ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CH3CHO
Both tests detect easily oxidised aldehydes, but aromatic aldehydes give only the Tollens’ test. Ethanal, an aliphatic aldehyde, responds to both:
\( \displaystyle \ce{CH3CHO + 2[Ag(NH3)2]+ + 3OH- -> CH3COO- + 2Ag v + 4NH3 + 2H2O} \)
\( \displaystyle \ce{CH3CHO + 2Cu^2+ + 5OH- -> CH3COO- + Cu2O v + 3H2O} \)
Why the other options are incorrect
Option (1) C6H5CHO — Incorrect. Benzaldehyde gives the Tollens’ test but not the Fehling’s test.
Option (3) CH3COCH3 — Incorrect. Acetone is a ketone and gives neither test.
Option (4) C6H5COCH3 — Incorrect. Acetophenone is a ketone and gives neither test.
NCERT (2026-27 edition):
Tollens’ Test – “On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.”
Fehling’s Test – “Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.”