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Answer the following. (Class 12 Chemistry 2024, Q25)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 3 marks · Short answer

CBSE Class 12 Chemistry 2024: question 25

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerHaloalkanes and Haloarenes

Answer the following.

  1. (a)

    Which of the following is an allylic halide ?
    (i)
    (ii)

    1 mark
  2. (b)

    Out of chlorobenzene and 2,4,6-trinitrochlorobenzene, which is more reactive towards nucleophilic substitution and why ?

    1 mark
  3. (c)

    Which isomer of has the lowest boiling point ?

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 56/1/1.

Hints

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

Hint 1

Allylic carbon is the carbon next to a C=C, not on it.

Hint 2

Electron-withdrawing groups at o/p positions help a nucleophile attack the ring; branching reduces surface area.

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) Compound (ii) is allylic, as Br is on an sp³ carbon next to the C=C. (Compound (i) is a vinylic halide — Br is on the doubly bonded carbon.)

    1 mark

  2. (b) 2,4,6-Trinitrochlorobenzene is more reactive, because the –NO₂ groups at ortho and para positions withdraw electrons and stabilise the carbanion intermediate formed on attack of the nucleophile.

    1 mark

  3. (c) (2-chloro-2-methylpropane, tert-butyl chloride) has the lowest boiling point; it is the most branched and nearly spherical, so it has the least surface area and weakest van der Waals forces.

    1 mark

Answer: (a) (ii); (b) 2,4,6-trinitrochlorobenzene (–NO₂ electron withdrawal); (c) tert-butyl chloride

Where the marks are

  • Identifies (ii) as allylic halide (1 mark)
  • 2,4,6-trinitrochlorobenzene with reason of electron-withdrawing –NO₂ (½ + ½ mark)
  • tert-Butyl chloride (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 paper2024 board paper, set 56/1/1: 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.