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MCQEasyJEE 2025Carbohydrates (Glucose, Fructose, Sucrose…)

JEE Chemistry 2025 Question with Solution

Given below are two statements: Statement I: D-glucose pentaacetate reacts with 2,42,4-dinitrophenylhydrazine. Statement II: Starch, on heating with concentrated sulfuric acid at 100°C100\degree \text{C} and 2−32-3 atmosphere pressure, produces glucose. In the light of the above statements, choose the correct answer from the options given below:

  • A

    Both Statement I and Statement II are true

  • B

    Statement I is false but Statement II is true

  • C

    Statement I is true but Statement II is false

  • D

    Both Statement I and Statement II are false

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given:

  • Statement I says D-glucose pentaacetate reacts with 2,42,4-dinitrophenylhydrazine.
  • Statement II says starch, on heating with concentrated sulfuric acid at 100°C100\degree \text{C} and under 2–32\text{–}3 atmosphere pressure, produces glucose.

Find: Which statement is true.

From the solution, 2,42,4-dinitrophenylhydrazine reacts specifically with aldehydes and ketones having a free carbonyl group. D-glucose pentaacetate has its hydroxyl groups esterified and the explanation states that it lacks a free carbonyl group available for this reaction. Therefore, Statement I is false.

The solution further states that starch is a polysaccharide made of glucose units joined by glycosidic bonds. Under acidic conditions, heating with concentrated sulfuric acid under pressure causes acid-catalyzed hydrolysis of these glycosidic linkages, ultimately yielding glucose. Therefore, Statement II is true.

The solution concludes: Statement I is false and Statement II is true. Therefore, the correct option is B.

The answer key lists option 33, but this disagrees with the solution. As per the extracted solution, the answer is B.

Common mistakes

  • Assuming D-glucose pentaacetate behaves like open-chain glucose is incorrect because DNPH requires a free aldehyde or ketone group. Here, the solution states no free carbonyl group is available, so Statement I is false.

  • Confusing esterified hydroxyl groups with the presence of a reactive carbonyl group leads to the wrong conclusion. Check the functional group required by DNPH instead of relying only on the parent compound name glucose.

  • Thinking starch cannot produce glucose on hydrolysis is incorrect. Under acidic conditions and heating under pressure, glycosidic bonds are cleaved, so starch ultimately yields glucose.

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