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(i) Give a chemical test… (Class 12 Chemistry 2023, Q34b)

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ChemistryCBSE Class 12 · Board exam 2023, set 56/5/1 · 5 marks · Long answer

CBSE Class 12 Chemistry 2023: question 34(b)

Set in the CBSE Class 12 board exam of 2023, paper 56/5/1, section E, question 34(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(b) · 5 marks · Long answerAldehydes, Ketones and Carboxylic Acids

(i) Give a chemical test to distinguish between ethanal acid and ethanoic acid.
(ii) Why is the -hydrogens of aldehydes and ketones are acidic in nature ?
(iii) An organic compound ‘A’ with molecular formula undergoes acid hydrolysis to form two compounds ‘B’ and ‘C’. Oxidation of ‘C’ with acidified potassium permanganate also produces ‘B’. Sodium salt of ‘B’ on heating with soda lime gives methane.
(1) Identify ‘A’, ‘B’ and ‘C’.
(2) Out of ‘B’ and ‘C’, which will have higher boiling point ? Give reason.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/5/1.

Hints

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

Hint 1

For (i), think of a reagent that reacts only with an acid and releases a gas.

Hint 2

For (iii), soda-lime decarboxylation removes one carbon. Which acid salt gives CH4?

Hint 3

Once B is known, C must oxidise to B. Then A, with formula C4H8O2, is the ester formed from B and C.

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) Taking the intended pair as ethanal and ethanoic acid: add solution to each.

    • Ethanoic acid gives brisk effervescence of :
    • Ethanal gives no effervescence.

    Alternatively, ethanal gives a silver mirror with Tollens' reagent, while ethanoic acid does not.

    1 mark

  2. (ii) The C=O group is strongly electron-withdrawing (–I effect), which weakens the α C–H bond. The carbanion (enolate ion) formed after losing the α-hydrogen is stabilised by resonance, because the negative charge is delocalised onto the oxygen:

    Hence α-hydrogens are acidic.

    1 mark

  3. (iii)(1) The sodium salt of B gives methane on heating with soda lime (decarboxylation), so B is ethanoic acid:

    C on oxidation also gives B, so C is ethanol. A () is the ester formed from these two.

    **A = (ethyl ethanoate)**

    1 mark

  4. Acid hydrolysis of A gives B and C:

    **B = (ethanoic acid)**

    ½ mark

  5. Oxidation of C gives B:

    **C = (ethanol)**

    ½ mark

  6. (iii)(2) B (ethanoic acid) has the higher boiling point.

    ½ mark

  7. Reason: carboxylic acid molecules are associated more extensively through stronger intermolecular hydrogen bonding, forming dimers. This hydrogen bonding is stronger than that in ethanol.

    ½ mark

Answer: (i) NaHCO3 test: ethanoic acid gives CO2 effervescence, ethanal does not; (ii) –I effect of C=O and resonance-stabilised enolate; (iii) A = CH3COOC2H5, B = CH3COOH, C = C2H5OH; B has the higher b.p. due to stronger H-bonding (dimer formation)

Where the marks are

  • Correct chemical test distinguishing ethanal and ethanoic acid (1 mark)
  • Acidity of α-H explained by electron-withdrawing C=O and resonance-stabilised conjugate base (1 mark)
  • A = ethyl ethanoate (1 mark)
  • B = ethanoic acid (½ mark)
  • C = ethanol (½ mark)
  • B has higher boiling point (½ mark)
  • Reason: more extensive/stronger intermolecular H-bonding in carboxylic acid (½ 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 paper2023 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.