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Explain the following… (Class 12 Chemistry SQP 2025-26, Q24)

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ChemistryCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2025-26: question 24

From the official CBSE sample question paper for 2025-26, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerHaloalkanes and Haloarenes

Explain the following:
I. Toluene on treatment with in sunlight gives benzyl chloride whereas when treated with in dark gives o-chlorobenzene and p-chlorobenzene.
II. Finkelstein reaction is carried out in the presence of dry acetone.
III. neo pentylchloride has lower boiling point than isopentylchloride.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Ask what kind of reactive species chlorine forms in light and in the dark.

Hint 2

For II, compare the solubility of NaI and NaCl in acetone.

Hint 3

For III, link branching to surface area and intermolecular forces.

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. In sunlight Cl undergoes homolysis to Cl free radicals. These attack the side-chain CH (free-radical substitution), giving benzyl chloride, CHCHCl.

    In the dark with a Lewis acid (FeCl), Cl (an electrophile) forms and attacks the ring (electrophilic substitution). Since –CH is ortho/para directing, o- and p-chlorotoluene are formed.

    1 mark

  2. II. In the Finkelstein reaction, . NaI dissolves in dry acetone but NaCl/NaBr do not, so they precipitate out. This shifts the equilibrium forward (Le Chatelier's principle). Water would dissolve NaCl and allow the reverse reaction.

    1 mark

  3. III. Neopentyl chloride is more branched than isopentyl chloride, so it is more compact (nearly spherical) with a smaller surface area. Its van der Waals forces are weaker, so its boiling point is lower.

    1 mark

Answer: I. Sunlight: free-radical side-chain substitution gives benzyl chloride; dark with FeCl₃: electrophilic ring substitution gives o/p-chlorotoluene. II. NaCl/NaBr precipitate in dry acetone, driving the reaction forward. III. Greater branching means smaller surface area, weaker van der Waals forces and a lower boiling point.

Where the marks are

  • Free radical (sunlight) vs electrophile Cl⁺ (dark), giving side-chain vs ring substitution (1 mark)
  • NaCl insoluble in dry acetone, so equilibrium shifts forward (1 mark)
  • Branching lowers surface area and van der Waals forces, so lower b.p. (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 2025-26: 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.