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Why are haloarenes less… (Class 12 Chemistry 2025, Q20)

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ChemistryCBSE Class 12 · Board exam 2025, set 56/7/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2025: question 20

Set in the CBSE Class 12 board exam of 2025, paper 56/7/1, section B, question 20, for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20 · 2 marks · Very short answerHaloalkanes and Haloarenes

Why are haloarenes less reactive towards nucleophilic substitution reaction ? How does the presence of nitro () group at ortho- and para-positions in haloarenes increase the reactivity towards nucleophilic substitution reaction ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 56/7/1.

Hints

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

Hint 1

Think about the halogen lone pair interacting with the ring.

Hint 2

Consider the hybridisation of the carbon that carries the halogen.

Hint 3

Where can the negative charge of the intermediate go when is ortho or para?

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. Low reactivity: the lone pair on halogen is in resonance with the benzene ring, so the C–X bond gets partial double bond character. In addition, the carbon of C–X is hybridised, so the bond is shorter and stronger. Both effects make the bond hard to break, and haloarenes are less reactive towards nucleophilic substitution.

    1 mark

  2. **Effect of at o/p positions:** the nitro group withdraws electron density from the ring by its and effects, which makes the ring carbon easier for a nucleophile to attack. It also stabilises the intermediate carbanion, because the negative charge is delocalised onto the oxygen atoms of . This is possible only when is at the ortho or para position, so reactivity increases.

    1 mark

Answer: C–X has partial double bond character (resonance, C), so it is hard to break; o/p withdraws electrons and stabilises the carbanion intermediate.

Where the marks are

  • Resonance / partial double bond character or carbon giving a shorter C–X bond (1 mark)
  • withdraws electrons and stabilises the intermediate carbanion at o/p positions (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 paper2025 board paper, set 56/7/1: 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.