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Ethers are prepared by the… (Class 12 Chemistry 2026, Q29)

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ChemistryCBSE Class 12 · Board exam 2026, set 56/5/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2026: question 29

Set in the CBSE Class 12 board exam of 2026, paper 56/5/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedAlcohols, Phenols and Ethers

Ethers are prepared by the dehydration of alcohols in presence of protic acids at 413 K. Symmetrical and unsymmetrical ethers can also be prepared by Williamson synthesis. This reaction involves attack of an alkoxide ion on primary alkyl halide. If a tertiary alkyl halide is used, elimination reaction occurs and an alkene is formed, no ether is formed. C – O bond in ethers are cleaved under drastic conditions with excess of HI. When unsymmetrical ethers react with HI, the alkyl halide is formed from smaller alkyl group. If one of the alkyl group is tertiary, the alkyl halide is formed from the tertiary alkyl group because tertiary alkyl carbocation is more stable than the primary carbocation. Cleavage of alkyl aryl ethers takes place at the alkyl-oxygen bond due to more stable aryl-oxygen bond. The order of reactivity of hydrogen halides is HI > HBr > HCl. Aromatic ethers undergo electrophilic substitution reactions. The alkoxy group attached to the aromatic ring activates the ring towards electrophilic substitution and directs the incoming group to ortho- and para-positions.

Answer the following questions :

  1. (a)

    Complete the following equations :
    (i) Anisole
    (ii) Anisole

    2 marks
  2. (b)(i)

    Write the names of alkyl halide and sodium alkoxide used to prepare tert-butyl ethyl ether.

    1 mark

  3. OR

    (b)(ii)

    Anisole on reaction with HI gives phenol and and not methanol and iodobenzene. Justify the statement.

    1 mark
  4. (c)

    Why is C – O – C bond angle in ethers slightly greater than tetrahedral angle ?

    1 mark

Figure in the paper: Both equations in (a) show anisole (benzene ring with OCH3 substituent). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 56/5/1.

Hints

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

Hint 1

–OCH₃ is an o/p-directing activating group, so electrophiles enter at the 2- and 4-positions.

Hint 2

In Williamson synthesis the halide must be primary, so the bulky group must come from the alkoxide. For HI cleavage, compare the strength of the O–aryl and O–alkyl bonds.

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)(i) Friedel–Crafts alkylation at the o- and p-positions:
    2-methoxytoluene (minor) + 4-methoxytoluene (major) + HCl

    1 mark

  2. (a)(ii) Nitration at the o- and p-positions:
    2-methoxynitrobenzene (2-nitroanisole, minor) + 4-methoxynitrobenzene (4-nitroanisole, major) + H₂O

    1 mark

  3. (b)(i) Use ethyl halide (e.g. bromoethane, ) and sodium tert-butoxide, :

    A tertiary halide is not used because it would undergo elimination.

    1 mark

  4. (b)(ii) (Alternative) In anisole, the bond is stronger than the bond. This is because of resonance and the sp² carbon, which give it partial double-bond character. So after protonation, attacks the less hindered group by and breaks the bond, giving phenol and , not iodobenzene.

  5. (c) The C–O–C bond angle is slightly greater than 109°28′ because of repulsive interaction between the two bulky alkyl groups on oxygen.

    1 mark

Answer: (a)(i) 2- and 4-methoxytoluene; (ii) 2- and 4-nitroanisole; (b)(i) ethyl halide + sodium tert-butoxide; (b)(ii) O–C₆H₅ bond stronger (partial double bond), so O–CH₃ cleaves; (c) repulsion between bulky alkyl groups

Where the marks are

  • (a)(i) o- and p-methoxytoluene (½ + ½ marks)
  • (a)(ii) o- and p-nitroanisole (½ + ½ marks)
  • (b)(i) ethyl halide and sodium tert-butoxide (½ + ½ marks) OR (b)(ii) O–C₆H₅ bond stronger due to resonance / partial double-bond character (1 mark)
  • (c) repulsion between alkyl groups (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Alcohols, Phenols and Ethers, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 56/5/1: 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.