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Acidity of substitute… (Class 12 Chemistry SQP 2026-27, Q29)

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ChemistryCBSE Class 12 · Sample paper, session 2026-27 · 4 marks · Case-based

CBSE Class 12 Chemistry sample paper 2026-27: question 29

From the official CBSE sample question paper for 2026-27, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedAlcohols, Phenols and Ethers

Acidity of substituted phenols
Phenol and substituted phenols are relatively more acidic than aliphatic alcohols. The pKa value is a quantitative measure of the acidic strength. The effect of substituents on acidity of phenols (pKa values) is given in Figure 4.

Study the graph and answer the following questions

  1. (I)

    The pKa value of 2,4,6- trinitrophenol will be:
    A. more than 8.36
    B. between 7.2 and 8.3
    C. between 4.09 and 2.3
    D. less than 4.09

    1 mark
  2. (II)

    The order of acidity of p-halophenols is :
    A. p-bromophenol > p-chlorophenol > p-flourophenol > phenol
    B. phenol > p-flourophenol > p-chlorophenol > p-bromophenol
    C. p-chlorophenol > p-bromophenol > p-flourophenol > phenol
    D. phenol > p-flourophenol > p-bromophenol > p-chlorophenol

    1 mark
  3. (IIIA)

    Predict which one of the following two compounds will have lower value of pKa: cyclohexanol OR phenol. Give reason in support of your reason.

    2 marks

  4. OR

    (IIIB)

    Predict whether 4 chloro-2, 6- dintirophenol is more or less acidic than 2,4,6-trinitrophenol? Use the values in the graph to support your answer.

    2 marks

Figure in the paper: Figure 4: bar graph of pKa values (y-axis 0-12): o-cresol 10.29, m-cresol 10.09, p-cresol 10.26, o-nitrophenol 7.23, m-nitrophenol 8.36, p-nitrophenol 7.2, 2,4-dinitrophenol 4.09, p-bromophenol 9.34, p-fluorophenol 9.9, p-chlorophenol 9.38, phenol 10. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Lower pKa means a stronger acid.

Hint 2

Read how each additional nitro group changes the pKa in the graph.

Hint 3

For (III), compare how well each conjugate base spreads its negative charge, or compare the para –NO₂ and para –Cl values.

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) Each group lowers the pKa (p-nitrophenol 7.2, 2,4-dinitrophenol 4.09). A third nitro group lowers it further, so the pKa of 2,4,6-trinitrophenol is less than 4.09. Option D.

    1 mark

  2. (II) From the graph: p-bromophenol 9.34 < p-chlorophenol 9.38 < p-fluorophenol 9.9 < phenol 10. Lower pKa means a stronger acid, so p-bromophenol > p-chlorophenol > p-fluorophenol > phenol. Option A.

    1 mark

  3. (IIIA) Phenol has the lower pKa (≈10), because it is a stronger acid than cyclohexanol.

    1 mark

  4. (IIIA) The phenoxide ion is stabilised by resonance, as its negative charge is delocalised over the ring. In cyclohexoxide the negative charge stays localised on oxygen, so this conjugate base is less stable and cyclohexanol is a weaker acid with higher pKa.

    1 mark

  5. (IIIB) 4-Chloro-2,6-dinitrophenol is less acidic than 2,4,6-trinitrophenol.

    1 mark

  6. (IIIB) The two compounds differ only in the para group: Cl versus NO₂. The graph shows p-nitrophenol (pKa 7.2) is much more acidic than p-chlorophenol (pKa 9.38). So replacing the para –NO₂ by –Cl raises the pKa, and the trinitro compound with its third strong electron-withdrawing group has the lower pKa (stronger acid).

    1 mark

Answer: (I) D; (II) A; (IIIA) phenol (resonance-stabilised phenoxide); (IIIB) less acidic than picric acid.

Where the marks are

  • (I) Option D (1 mark)
  • (II) Option A (1 mark)
  • (IIIA) Phenol has the lower pKa (1 mark)
  • (IIIA) Resonance stabilisation of phenoxide vs localised charge in cyclohexoxide (1 mark)
  • (IIIB) Less acidic (1 mark)
  • (IIIB) Justification using p-nitrophenol 7.2 vs p-chlorophenol 9.38 (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Alcohols, Phenols and Ethers, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: 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.