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You are given four… (Class 12 Chemistry SQP 2023-24, Q25)

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ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2023-24: question 25

From the official CBSE sample question paper for 2023-24, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerAldehydes, Ketones and Carboxylic Acids

You are given four organic compounds "A", "B", "C" and "D". The compounds "A", "B" and "C" form an orange-red precipitate with 2,4 DNP reagent. Compounds "A" and "B" reduce Tollen's reagent while compounds "C" and "D" do not. Both "B" and "C" give a yellow precipitate when heated with iodine in the presence of NaOH. Compound "D" gives brisk effervescence with sodium bicarbonate solution. Identify "A", "B", "C" and "D" given the number of carbon atoms in three of these carbon compounds is three while one has two carbon atoms. Give an explanation for your answer.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Work out what each test tells you: 2,4-DNP for C=O, Tollens' for –CHO, iodoform for CH₃CO–, NaHCO₃ for –COOH.

Hint 2

Only one aldehyde gives the iodoform test — which one?

Hint 3

Once the two-carbon compound is fixed, the other three must each have three carbons.

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. A, B and C give an orange-red precipitate with 2,4-DNP, so they contain a carbonyl group. A and B reduce Tollens' reagent, so they are aldehydes. C does not, so it is a ketone.

    ½ mark

  2. B and C give the iodoform test, so both contain a group: C is a methyl ketone and B is an aldehyde with a CH₃CO– unit. D gives brisk effervescence (CO₂) with NaHCO₃, so it is a carboxylic acid.

    ½ mark

  3. The only aldehyde that gives a positive iodoform test is ethanal, so B (the two-carbon compound). Hence A, C and D have three carbon atoms each.

    ½ mark

  4. A = (propanal): an aldehyde that does not give iodoform. C = (propanone): a 3-C methyl ketone. D = (propanoic acid): a 3-C acid.

    1½ marks

Answer: A = CH₃CH₂CHO (propanal), B = CH₃CHO (ethanal), C = CH₃COCH₃ (propanone), D = CH₃CH₂COOH (propanoic acid)

Where the marks are

  • 2,4-DNP and Tollens' tests show A, B are aldehydes and C a ketone (½ mark)
  • Iodoform test shows CH₃CO– in B and C; NaHCO₃ test shows D is an acid (½ mark)
  • B identified as CH₃CHO, the only iodoform-positive aldehyde (½ mark)
  • A = propanal, C = propanone, D = propanoic acid (½ mark each)

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 paperSample paper 2023-24: 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.