Consider the following statements about simple chemical tests.
(A) Propanal and propanone can be distinguished by Tollens’ test.
(B) Benzaldehyde and acetophenone can be distinguished by Fehling’s test.
(C) Ethanal and propanal can be distinguished by the iodoform test.
(D) Acetophenone and benzophenone can be distinguished by Tollens’ test.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (B) and (D) only
A test can tell two compounds apart only if one gives it and the other does not. Tollens’ and Fehling’s reagents are mild oxidising agents that react with aldehydes, not ketones; the iodoform test needs a CH3CO group.
(A) Propanal and propanone can be distinguished by Tollens’ test. — Correct statement. Propanal, an aldehyde, gives a silver mirror; propanone, a ketone, does not:
\( \displaystyle \ce{CH3CH2CHO + 2[Ag(NH3)2]^+ + 3OH^- -> CH3CH2COO^- + 2Ag v + 2H2O + 4NH3} \)
(B) Benzaldehyde and acetophenone can be distinguished by Fehling’s test. — Incorrect statement. Aromatic aldehydes do not respond to Fehling’s test, and acetophenone is a ketone, so both are negative. Tollens’ test would tell them apart.
(C) Ethanal and propanal can be distinguished by the iodoform test. — Correct statement. Ethanal (CH3CHO) has a CH3CO group and gives yellow iodoform; propanal does not:
\( \displaystyle \ce{CH3CHO + 3I2 + 4NaOH -> CHI3 v + HCOONa + 3NaI + 3H2O} \)
(D) Acetophenone and benzophenone can be distinguished by Tollens’ test. — Incorrect statement. Both are ketones, so neither gives a silver mirror. The iodoform test tells them apart, because only acetophenone (C6H5COCH3) has a CH3CO group.
NCERT (2026-27 edition):
8.4 Chemical Reactions – “The mild oxidising agents given below are used to distinguish aldehydes from ketones:”
“(i) 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.”
“(ii) Fehling’s 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.”
“Iodoform reaction with sodium hypoiodite is also used for detection of CH3CO group or CH3CH(OH) group which produces CH3CO group on oxidation.”