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Give reasons for the… (Class 12 Chemistry 2024, Q20b)

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ChemistryCBSE Class 12 · Board exam 2024, set 56/1/1 · 2 marks · Very short answer

CBSE Class 12 Chemistry 2024: question 20(b)

Set in the CBSE Class 12 board exam of 2024, paper 56/1/1, section B, question 20(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q20(b) · 2 marks · Very short answerHaloalkanes and Haloarenes

Give reasons for the following :
(i) Grignard reagent should be prepared under anhydrous conditions.
(ii) Alkyl halides give alcohol with aqueous KOH whereas in the presence of alcoholic KOH, alkenes are formed.

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

The carbon in RMgX carries a partial negative charge — what does it do with an acidic H?

Hint 2

Compare the behaviour of a nucleophile and a base; which species is formed when KOH dissolves in ethanol?

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) Grignard reagents are highly reactive and react with even traces of moisture to form alkanes, so the reagent is destroyed: . Hence they are prepared under anhydrous conditions.

    1 mark

  2. (ii) In aqueous KOH, the OH⁻ ions are highly hydrated and act mainly as nucleophiles, giving substitution (alcohol). In alcoholic KOH, alkoxide ions () are formed, which are stronger bases than OH⁻; they abstract a β-hydrogen, so dehydrohalogenation (elimination) gives alkenes.

    1 mark

Answer: (i) RMgX reacts with water to give alkane; (ii) alkoxide ion in alcoholic KOH is a strong base causing elimination, while aqueous OH⁻ acts as nucleophile.

Where the marks are

  • Reaction of Grignard reagent with water forming alkane (1 mark)
  • Alkoxide in alcoholic KOH is a stronger base → elimination; aqueous OH⁻ → substitution (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.