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Answer the following. (Class 12 Chemistry SQP 2023-24, Q33b)

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ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 5 marks · Long answer

CBSE Class 12 Chemistry sample paper 2023-24: question 33(b)

From the official CBSE sample question paper for 2023-24, section E, question 33(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(b) · 5 marks · Long answerAmines

Answer the following.

  1. (a)

    Account for the following:
    (i) N-ethylbenzenesulphonyl amide is soluble in alkali.
    (ii) Reduction of nitrobenzene using Fe and HCl is preferred over Sn and HCl.

    0 marks
  2. (b)

    Arrange the following in:
    (i) decreasing order of values:
    (ii) increasing order of solubility in water:
    (iii) decreasing boiling point:

    0 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

For (a)(i), think about what the –SO₂– group does to the H attached to N. For (a)(ii), ask what happens to FeCl₂ in water.

Hint 2

Remember that a higher pKb means a weaker base. Aryl amines are weak bases because the lone pair on N is delocalised into the ring.

Hint 3

For solubility and boiling point, compare the strength of hydrogen bonding: O–H is stronger than N–H, a haloalkane or ether has none, and carboxylic acids form dimers.

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) In N-ethylbenzenesulphonamide, , the H on nitrogen is acidic. This is because the strongly electron-withdrawing sulphonyl group () pulls electron density away from N.

    It therefore reacts with alkali to form a water-soluble salt, , so it dissolves.

    1 mark

  2. (a)(ii) During Fe/HCl reduction, the formed is hydrolysed and releases HCl again. So only a small amount of HCl is needed to start the reaction, which makes the process cheaper. Sn/HCl needs a large amount of acid.

    1 mark

  3. **(b)(i) Decreasing (increasing basic strength):**

    Reason: in the aryl amines, the lone pair on N is delocalised into the ring, so they are the weakest bases. In benzylamine, N is not attached to the ring, and alkyl groups (+I effect) raise basicity above that of .

    1 mark

  4. (b)(ii) Increasing solubility in water:

    Reason: cannot form H-bonds with water. O is more electronegative than N, so the alcohol forms stronger H-bonds with water than the amine does.

    1 mark

  5. (b)(iii) Decreasing boiling point:

    Reason: carboxylic acids form dimers through strong H-bonding. O–H···O bonds are stronger than N–H···N bonds. The ether has no H-bonding at all, so it boils lowest.

    1 mark

Answer: (a)(i) The N–H is acidic owing to the –SO₂– group, so the compound forms a soluble salt with alkali; (ii) FeCl₂ hydrolyses to regenerate HCl, so little acid is needed. (b)(i) C₆H₅NH₂ > C₆H₅NHCH₃ > NH₃ > C₆H₅CH₂NH₂ > CH₃NH₂; (ii) C₂H₅Cl < C₂H₅NH₂ < C₂H₅OH; (iii) CH₃COOH > C₂H₅OH > CH₃NH₂ > CH₃OCH₃

Where the marks are

  • States that the N–H is acidic because of the electron-withdrawing sulphonyl group, so the compound dissolves in alkali (1 mark)
  • States that FeCl₂ hydrolyses to release HCl, so only a small amount of acid is needed (1 mark)
  • Gives the correct decreasing pKb order (1 mark)
  • Gives the correct increasing water-solubility order (1 mark)
  • Gives the correct decreasing boiling-point order (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Amines, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2023-24: 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.