Aldehydes and Ketones, having at least one methyl group linked to the carbonyl carbon atom (methyl Ketones), are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound is converted to haloform. This oxidation does not affect a carbon–carbon double bond, if present in a molecule.
Which of the following statements are true?
(A) \(CH_3CHO\) and \(CH_3CH_2CHO\) can be distinguished by Tollens’ test.
(B) \(CH_3CH_2CHO\) and \(CH_3COCH_3\) can be distinguished by Fehling’s test.
(C) \(CH_3CHO\), \(C_6H_5CHO\) can be distinguished by Fehling’s test.
(D) \(HCHO\), \(CH_3CHO\) can be distinguished by \(NaHCO_3\) test.
(E) \(HCOOH\), \(CH_3COOH\) can be distinguished by \(NaHCO_3\) test.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B), (C) only
Two compounds can be distinguished by a test only if one gives it and the other does not. Tollens’ reagent is reduced by all aldehydes, aliphatic and aromatic; Fehling’s solution is reduced by aliphatic aldehydes only; ketones give neither test. \( \ce{NaHCO3} \) releases \( \ce{CO2} \) with carboxylic acids only.
Tollens’: \( \ce{RCHO + 2[Ag(NH3)2]+ + 3OH- -> RCOO- + 2Ag v + 2H2O + 4NH3} \)
Fehling’s: \( \ce{RCHO + 2Cu^2+ + 5OH- -> RCOO- + Cu2O v + 3H2O} \) (red-brown precipitate)
(A) \( \ce{CH3CHO} \) and \( \ce{CH3CH2CHO} \) can be distinguished by Tollens’ test — Incorrect. Both are aldehydes and both give the silver mirror.
(B) \( \ce{CH3CH2CHO} \) and \( \ce{CH3COCH3} \) can be distinguished by Fehling’s test — Correct. Propanal gives the red-brown \( \ce{Cu2O} \) precipitate; acetone, a ketone, does not.
(C) \( \ce{CH3CHO} \), \( \ce{C6H5CHO} \) can be distinguished by Fehling’s test — Correct. Ethanal reduces Fehling’s solution; aromatic aldehydes do not.
(D) HCHO, \( \ce{CH3CHO} \) can be distinguished by \( \ce{NaHCO3} \) test — Incorrect. Neither is an acid; neither gives \( \ce{CO2} \).
(E) HCOOH, \( \ce{CH3COOH} \) can be distinguished by \( \ce{NaHCO3} \) test — Incorrect. Both are carboxylic acids and both give effervescence: \( \ce{RCOOH + NaHCO3 -> RCOONa + H2O + CO2 ^} \)
Note: It seems that there is a mistake in the original NTA paper, which prints “… than that of carbonyl compound is converted to haloform” when it should read “… than that of carbonyl compound. The methyl group is converted to haloform”.
NCERT (2026-27 edition):
Fehling’s Test
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.”