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Answer the following. (Class 12 Chemistry SQP 2023-24, Q19)

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ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 2 marks · Very short answer

CBSE Class 12 Chemistry sample paper 2023-24: question 19

From the official CBSE sample question paper for 2023-24, section B, question 19, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q19 · 2 marks · Very short answerHaloalkanes and Haloarenes

Answer the following.

  1. (a)

    Arrange the isomeric dichlorobenzene in the increasing order of their boiling point and melting points.

    0 marks
  2. (b)

    Explain why the electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions as compared to those in benzene.

    0 marks

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

Melting point depends mainly on how well molecules pack in the solid.

Hint 2

Boiling point depends on intermolecular forces such as dipole–dipole attraction.

Hint 3

For (b), think whether the halogen adds electron density to the ring or removes it.

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) Melting point: m-dichlorobenzene < o-dichlorobenzene < p-dichlorobenzene. The para isomer is symmetrical and packs best in the crystal lattice, so it has the highest melting point.

    ½ mark

  2. Boiling point: m-dichlorobenzene < p-dichlorobenzene < o-dichlorobenzene (446 K, 448 K, 453 K). The ortho isomer has the largest dipole moment, so stronger dipole–dipole attractions and the highest boiling point.

    ½ mark

  3. (b) The halogen withdraws electrons from the ring by its strong –I effect (stronger than its +R effect), lowering the electron density of the ring. The ring is deactivated compared with benzene, so electrophilic substitution is slower and needs harsher conditions.

    1 mark

Answer: (a) m.p.: meta < ortho < para; b.p.: meta < para < ortho; (b) –I effect of halogen deactivates the ring

Where the marks are

  • Correct order with the para-isomer packing reason for melting point (½ mark)
  • Ortho isomer has stronger dipole–dipole interaction than meta (½ mark)
  • –I effect of halogen deactivates ring, so substitution slower/drastic conditions (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Haloalkanes and Haloarenes, 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.