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Chemical distinction of aldehydes and ketones: NEET MCQs with answers

50 questions on this topic of Organic Compounds Containing Oxygen (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following reducing sugars or related carbonyl compounds reduces Fehling's solution despite having a keto group, due to base-catalyzed enediol rearrangement?

  1. Fructose
  2. Acetophenone
  3. Benzophenone
  4. Pentan-3-one
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Answer: (A) Fructose

α-Hydroxy ketones like fructose undergo keto-enol tautomerism (enediol rearrangement) in alkaline medium to form aldoses, which can readily reduce Fehling's solution. Simple ketones like acetophenone, benzophenone, and pentan-3-one lack the α-hydroxy group and do not reduce Fehling's solution.

medium

2. Given below are two statements: Assertion (A): Benzaldehyde and cyclohexanone can be distinguished by carrying out Tollens' test. Reason (R): Benzaldehyde is an aromatic aldehyde and reduces ammoniacal silver nitrate solution, whereas cyclohexanone is a ketone and does not reduce it. In light of the above statements, choose the most appropriate answer:

  1. Both (A) and (R) are true and (R) is the correct explanation of (A).
  2. Both (A) and (R) are true but (R) is not the correct explanation of (A).
  3. (A) is true but (R) is false.
  4. (A) is false but (R) is true.
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Answer: (A) Both (A) and (R) are true and (R) is the correct explanation of (A).

Tollens' reagent oxidises aldehydes (both aliphatic and aromatic like benzaldehyde) to the corresponding carboxylate ions with the deposition of silver. Ketones like cyclohexanone cannot be easily oxidised by weak oxidising agents such as Tollens' reagent. Hence, (A) is true and (R) correctly explains it.

hard

3. An unknown compound 'P' (C₈H₈O) reacts with 2,4-DNP to form a red-orange derivative. It neither reduces Tollens' reagent nor reduces Fehling's reagent. However, treatment of 'P' with I₂ and aqueous NaOH gives a yellow precipitate of 'Q' and acidification of the filtrate gives benzoic acid. What is compound 'P'?

  1. Phenylacetaldehyde
  2. 4-Methylbenzaldehyde
  3. Acetophenone
  4. Propiophenone
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Answer: (C) Acetophenone

Compound 'P' is acetophenone (C₆H₅COCH₃). Being a ketone, it fails Tollens' and Fehling's tests. Having a CH₃C=O group attached to a phenyl ring, it undergoes the haloform reaction to yield iodoform 'Q' (CHI₃) and sodium benzoate, which upon acidification gives benzoic acid.

medium

4. In the Tollens' test for aldehydes, the aldehyde is oxidized to a carboxylate ion while silver ions are reduced to metallic silver. The balanced half-reaction for the reduction of Tollens' reagent is:

  1. Ag⁺ + OH⁻ + e⁻ → AgOH
  2. [Ag(NH₃)₂]²⁺ + 2e⁻ → Ag + 2NH₃
  3. [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃
  4. [Ag(CN)₂]⁻ + e⁻ → Ag + 2CN⁻
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Answer: (C) [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃

Tollens' reagent contains the diamminesilver(I) complex ion, [Ag(NH₃)₂]⁺. During the test, Ag(I) gains one electron to form metallic silver: [Ag(NH₃)₂]⁺ + e⁻ → Ag + 2NH₃. This forms the characteristic silver mirror on clean glass walls.

easy

5. Which of the following reagents will give a positive test with both pentan-2-one and pentan-3-one?

  1. Tollens' reagent
  2. Fehling's solution
  3. I₂ / NaOH
  4. 2,4-Dinitrophenylhydrazine
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Answer: (D) 2,4-Dinitrophenylhydrazine

Both pentan-2-one and pentan-3-one are carbonyl compounds (ketones). 2,4-Dinitrophenylhydrazine (Brady's reagent) reacts with all aldehydes and ketones to form crystalline yellow-orange 2,4-dinitrophenylhydrazone derivatives. Ketones do not reduce Tollens' or Fehling's solutions, and pentan-3-one does not respond to the iodoform test.

hard

6. An unknown compound 'X' (C₇H₆O) reacts with hydroxylamine to form an oxime. It reduces Tollens' reagent but does not reduce Fehling's solution. When 'X' is treated with concentrated NaOH solution, it undergoes disproportionation. Compound 'X' is:

  1. o-Cresol
  2. Benzaldehyde
  3. Acetophenone
  4. Benzyl alcohol
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Answer: (B) Benzaldehyde

Compound 'X' forms an oxime, so it is a carbonyl compound. It reduces Tollens' reagent (silver mirror) but fails to reduce Fehling's solution, which is characteristic of aromatic aldehydes. Disproportionation with concentrated NaOH is the Cannizzaro reaction, confirming that 'X' is benzaldehyde (C₆H₅CHO).

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7. Given below are two statements: Statement I: Both aliphatic and aromatic aldehydes restore the pink color of Schiff's reagent. Statement II: Ketones generally fail to restore the pink color of Schiff's reagent under mild conditions. In light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct.
  2. Statement I is correct but Statement II is incorrect.
  3. Statement I is incorrect but Statement II is correct.
  4. Both Statement I and Statement II are incorrect.
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Answer: (A) Both Statement I and Statement II are correct.

Schiff's reagent is a dilute solution of rosaniline hydrochloride decolored by sulfur dioxide. Both aliphatic and aromatic aldehydes restore its characteristic pink or magenta color rapidly. Ketones generally do not respond or react extremely slowly, making it an effective distinguishing test.

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8. Benedict's solution is commonly used to detect aldehydes. Chemically, Benedict's reagent consists of an alkaline solution of copper(II) sulfate complexed with:

  1. Tartrate ions
  2. Oxalate ions
  3. Citrate ions
  4. EDTA
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Answer: (C) Citrate ions

Benedict's reagent is an alkaline solution containing copper(II) sulfate, sodium carbonate, and sodium citrate. Citrate ions act as the chelating agent to keep cupric ions in solution, preventing premature precipitation of Cu(OH)₂.

medium

9. Consider the following chemical equation for the reduction of Tollens' reagent: R-CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → R-COO⁻ + 2Ag↓ + 4NH₃ + 2H₂O In this redox reaction, how many moles of electrons are transferred per mole of aldehyde oxidized?

  1. 1
  2. 3
  3. 2
  4. 4
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Answer: (C) 2

In the reaction, each silver ion Ag⁺ (oxidation state +1) is reduced to metallic silver Ag⁰ (oxidation state 0). Since 2 moles of [Ag(NH₃)₂]⁺ are reduced per mole of aldehyde oxidized to carboxylate ion, the total number of moles of electrons transferred is 2 × 1 = 2 moles.

easy

10. Aliphatic aldehydes give a red-brown precipitate when heated with:

  1. 2,4-DNP
  2. Sodium hydrogencarbonate
  3. Neutral FeCl₃
  4. Fehling's solution
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Answer: (D) Fehling's solution

Aldehydes reduce Fehling's solution (Cu²⁺ complex) to red-brown Cu₂O. Ketones (and aromatic aldehydes) do not respond.

NCERT Class 12

easy

11. Which of the following pairs of compounds cannot be distinguished from each other by using Tollens' reagent alone?

  1. Ethanal and propanone
  2. Benzaldehyde and acetophenone
  3. Acetaldehyde and butan-2-one
  4. Propanal and benzaldehyde
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Answer: (D) Propanal and benzaldehyde

Both propanal (an aliphatic aldehyde) and benzaldehyde (an aromatic aldehyde) reduce Tollens' reagent to give a positive silver mirror test. Hence, Tollens' reagent cannot be used to distinguish between them. All other given pairs contain one aldehyde and one ketone, which can be distinguished.

hard

12. Consider the oxidation of aldehydes by Fehling's solution: R-CHO + 2Cu²⁺ + 5OH⁻ → R-COO⁻ + Cu₂O↓ + 3H₂O How many moles of Cu₂O precipitate are obtained per mole of aldehyde oxidized, and what is the change in oxidation state of copper?

  1. 2 moles of Cu₂O; Cu changes from +2 to +1
  2. 1 mole of Cu₂O; Cu changes from +2 to +1
  3. 1 mole of Cu₂O; Cu changes from +2 to 0
  4. 0.5 mole of Cu₂O; Cu changes from +1 to 0
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Answer: (B) 1 mole of Cu₂O; Cu changes from +2 to +1

From the balanced stoichiometric equation, 1 mole of aldehyde reduces 2 moles of Cu²⁺ to yield exactly 1 mole of Cu₂O precipitate. The oxidation state of copper decreases from +2 in Cu²⁺ (Fehling's complex) to +1 in copper(I) oxide (Cu₂O).

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