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Answer the following. (Class 12 Chemistry 2024, Q33b)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/5/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2024: question 33(b)

Set in the CBSE Class 12 board exam of 2024, paper 56/5/1, section E, question 33(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(b) · 5 marks · Long answerAldehydes, Ketones and Carboxylic Acids

Answer the following.

  1. (i)

    Account for the following :
    (1) The alpha ()-hydrogens of aldehydes and ketones are acidic in nature.
    (2) Oxidation of aldehydes is easier than ketones.

    2 marks
  2. (ii)

    Arrange the following in :
    (1) Decreasing reactivity towards nucleophilic addition reaction propanal, acetone, benzaldehyde.
    (2) Increasing order of boiling point : Propane, Ethanol, Dimethylether, Propanal

    2 marks
  3. (iii)

    Give simple chemical test to distinguish between Benzoic acid and Benzaldehyde.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

For α-H acidity, think about what happens to the negative charge once the α-H leaves.

Hint 2

For reactivity, compare the steric hindrance and the electron donation around the carbonyl carbon; for boiling points, compare the intermolecular forces (none, dipole, H-bond).

Hint 3

A carboxylic acid is acidic enough to react with a weak base such as NaHCO.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i)(1) The carbonyl group is strongly electron-withdrawing, so it weakens the α C–H bond. When the α-H leaves as H, the carbanion formed (enolate ion) is resonance-stabilised, with the negative charge delocalised onto the oxygen. This makes the α-hydrogens acidic.

    1 mark

  2. (i)(2) Aldehydes have a hydrogen on the carbonyl carbon, so oxidation only needs a C–H bond to break, which happens easily even with mild oxidants. Ketones can be oxidised only by breaking a C–C bond, which needs strong oxidising agents. So aldehydes are oxidised more easily.

    1 mark

  3. (ii)(1) Decreasing reactivity towards nucleophilic addition: Propanal > Benzaldehyde > Acetone.

    Propanal has only one alkyl group, so it has the least steric hindrance and an electrophilic carbonyl carbon. In benzaldehyde, resonance with the ring lowers the electrophilicity of the carbonyl carbon. Acetone has two electron-releasing methyl groups, which give it the most steric hindrance and the least positive carbonyl carbon.

    1 mark

  4. (ii)(2) Increasing order of boiling point: Propane < Dimethyl ether < Propanal < Ethanol.

    Propane is non-polar and has only weak van der Waals forces. The ether and the aldehyde have dipole–dipole forces, and these are stronger in the more polar propanal. Ethanol has intermolecular hydrogen bonding, so it boils highest.

    1 mark

  5. (iii) Add sodium bicarbonate solution to each compound. Benzoic acid gives brisk effervescence of CO: . Benzaldehyde gives no effervescence.

    Alternatively, benzaldehyde gives a silver mirror with Tollens' reagent, while benzoic acid does not.

    1 mark

Answer: (i) Electron-withdrawing C=O and resonance-stabilised enolate; aldehyde oxidation breaks only a C–H bond, not a C–C bond. (ii)(1) Propanal > Benzaldehyde > Acetone; (2) Propane < Dimethyl ether < Propanal < Ethanol. (iii) NaHCO test: benzoic acid effervesces, benzaldehyde does not.

Where the marks are

  • Explains α-H acidity by the electron-withdrawing carbonyl group or resonance-stabilised conjugate base (1 mark)
  • Explains easier oxidation of aldehydes by C–H cleavage versus C–C cleavage in ketones (1 mark)
  • Gives the order propanal > benzaldehyde > acetone (1 mark)
  • Gives the order propane < dimethyl ether < propanal < ethanol (1 mark)
  • Gives a correct distinguishing test with its observations (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Aldehydes, Ketones and Carboxylic Acids, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 56/5/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.