Skip to content

When sucrose is… (Class 12 Chemistry SQP 2023-24, Q26)

questionbuddy.in/q/cbse-12-chemistry-2023-24-sqp-q26-sucrose-hydrolysed-optical-rotation-values

ChemistryCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Chemistry sample paper 2023-24: question 26

From the official CBSE sample question paper for 2023-24, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerBiomolecules

When sucrose is hydrolysed the optical rotation values are measured using a polarimeter and are given in the following table:

S.No.Time (hours)Specific Rotation
10+ 66.5°
2∞-39.9°
  1. (a)

    Account for the two specific rotation values.

    0 marks
  2. (b)

    What is the specific name given to sucrose based on the above observation?

    0 marks
  3. (c)

    One of the products formed during the hydrolysis of sucrose is a glucose, that reacts with hydroxylamine to give compound A. Identify compound A.

    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

What does sucrose break down into on hydrolysis?

Hint 2

Compare the sizes of the rotations of glucose and fructose.

Hint 3

Hydroxylamine reacts with the aldehyde group of glucose.

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) At t = 0 only sucrose is present, which is dextrorotatory (+66.5°). At t = ∞ sucrose is fully hydrolysed to an equimolar mixture of D-(+)-glucose (+52.5°) and D-(–)-fructose (–92.4°). The laevorotation of fructose exceeds the dextrorotation of glucose, so the mixture is laevorotatory (–39.9°).

    1 mark

  2. (b) The sign of rotation changes from dextro (+) to laevo (–) on hydrolysis, so sucrose is called invert sugar; this change is called inversion.

    1 mark

  3. (c) Glucose reacts with hydroxylamine to form glucose oxime: . A = glucose oxime.

    1 mark

Answer: (a) +66.5° is sucrose; –39.9° is the glucose (+52.5°) + fructose (–92.4°) mixture; (b) Invert sugar; (c) A = glucose oxime, HON=CH(CHOH)₄CH₂OH

Where the marks are

  • Sucrose dextrorotatory; products glucose (+) and fructose (–), fructose's larger laevorotation makes the mixture laevo (1 mark)
  • Invert sugar, owing to the change in sign of rotation (1 mark)
  • Compound A is glucose oxime (1 mark)

Revise the chapter

  • Chapter: Biomolecules (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Biomolecules, 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.