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The compound which undergoes… (Class 12 Chemistry 2024, Q6)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 1 mark · MCQ

CBSE Class 12 Chemistry 2024: question 6

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section A, question 6, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q6 · 1 mark · MCQHaloalkanes and Haloarenes

The compound which undergoes reaction most rapidly is :

  1. (A)Br attached at the allylic position of a cyclohexene ring (3-bromocyclohexene)
  2. (B)Cyclohexyl-
  3. (C)Bromocyclohexane
  4. (D)Bromobenzene-type ring (Br on a cyclohexadiene/vinylic ring carbon)

Figure in the paper: Four ring structures bearing Br: (A) cyclohexene ring with Br on the carbon next to the double bond; (B) cyclohexane ring with CH2-Br substituent; (C) cyclohexane ring with Br; (D) ring with double bonds drawn, Br attached to a ring carbon. See it in the official paper.

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

S1 reactions go through a carbocation.

Hint 2

Which carbocation can be stabilised by resonance with a C=C?

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. The S1 rate depends on how stable the carbocation intermediate is.

  2. 3-Bromocyclohexene forms an allylic carbocation, which is stabilised by resonance, so it reacts fastest.

    1 mark

Answer: Option (A)

Where the marks are

  • Identifies the allylic carbocation as most stable (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.