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Chemistry Question 51 – NEET-UG 2024

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The reagents with which glucose does not react to give the corresponding tests/products are A. Tollen's reagent B. Schiff's reagent C. HCN D. NH2OH E. NaHSO3 Choose the correct options from the given below:

Recall the functional groups present in glucose and their characteristic reactions. Glucose is an aldohexose, meaning it contains an aldehyde group and multiple hydroxyl groups.

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Ninja Strategy•Identify Non-Reactive Reagents

Quickly identify the reagents that are known to react with the aldehyde group of glucose (Tollen's, HCN, NH2OH) and eliminate options containing them, leaving the reagents that do not react readily (Schiff's, NaHSO3).

Step 1: Analyze the structure and common reactions of glucose✦ Active

Glucose is an aldohexose, meaning it contains an aldehyde group (-CHO) and multiple hydroxyl groups (-OH). It exists in equilibrium between its open-chain aldehyde form and cyclic hemiacetal forms. The aldehyde group is responsible for many characteristic reactions of glucose.

💡 Teacher's Secret Hint

Remember that the aldehyde group is responsible for many characteristic reactions of glucose.

Step 2: Evaluate the reactivity of glucose with each given reagent○ Expand

Let's assess each reagent: * **A. Tollen's reagent:** Glucose is a reducing sugar and reacts with Tollen's reagent (ammoniacal silver nitrate) to form a silver mirror, indicating the presence of an aldehyde group. (Reacts) * **B. Schiff's reagent:** While Schiff's reagent tests for aldehydes, glucose exists predominantly in its cyclic hemiacetal forms, with only a very small concentration of the open-chain aldehyde. This low concentration makes the reaction with Schiff's reagent very slow or practically negative under typical conditions. (Does NOT react readily) * **C. HCN:** The aldehyde group of glucose reacts with hydrogen cyanide (HCN) to form a cyanohydrin. (Reacts) * **D. NH2OH:** The aldehyde group of glucose reacts with hydroxylamine (NH2OH) to form an oxime. (Reacts) * **E. NaHSO3:** Sodium bisulfite (NaHSO3) forms addition products with simple aldehydes and methyl ketones. However, due to steric hindrance from the multiple hydroxyl groups and the preference for cyclic forms, glucose generally does not form a stable bisulfite addition product. (Does NOT react)

💡 Teacher's Secret Hint

Pay close attention to the "not react" condition in the question.

Step 3: Identify the reagents with which glucose does not react○ Expand

Based on the analysis, glucose does not react readily with Schiff's reagent (B) and does not react with sodium bisulfite (NaHSO3) (E). Therefore, the reagents with which glucose does not react are B and E.

💡 Teacher's Secret Hint

Differentiate between reactions that are slow/weak due to equilibrium and those that do not occur due to steric hindrance or lack of suitable functional groups.

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