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MCQEasyJEE 2024Carbohydrates (Glucose, Fructose, Sucrose…)

JEE Chemistry 2024 Question with Solution

Sugar which does not give reddish brown precipitate with Fehling’s reagent is:

  • A

    Sucrose

  • B

    Lactose

  • C

    Glucose

  • D

    Maltose

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: The sugar must be identified which does not give a reddish-brown precipitate with Fehling’s reagent.

Find: The correct option.

Fehling’s solution tests for reducing sugars. A reducing sugar reduces copper(II) ions to copper(I) oxide, giving a reddish-brown precipitate.

2Cu2++R−CHO+4OH−→Cu2O (s)+2H2O+R−COOH2Cu^{2+} + R-CHO + 4OH^- \rightarrow Cu_2O\,(s) + 2H_2O + R-COOH

Now evaluate the given sugars:

  1. Lactose is a reducing sugar because it has a free anomeric carbon.
  2. Maltose is also a reducing sugar because it has a free anomeric carbon.
  3. Glucose is a reducing sugar and gives a positive Fehling’s test.
  4. Sucrose is a non-reducing sugar because its glycosidic bond involves the anomeric carbon of both glucose and fructose, so no free anomeric carbon is available.

Therefore, sucrose does not give the reddish-brown precipitate with Fehling’s reagent.

The correct option is A.

Reducing Sugar Check

Given: Fehling’s reagent reacts with reducing sugars.

Find: Which sugar is non-reducing.

Use the quick rule: a sugar gives Fehling’s test only if it has a free hemiacetal / free anomeric carbon. Lactose, glucose, and maltose satisfy this condition, but sucrose does not because both anomeric carbons are involved in glycosidic linkage.

Therefore, the sugar that does not give the test is sucrose, so the correct option is A.

Common mistakes

  • Confusing non-reducing sugar with sweet taste or common usage. Fehling’s test depends on the presence of a free anomeric carbon, not on sweetness.

  • Assuming all disaccharides are non-reducing. Lactose and maltose are reducing disaccharides because they still have a free hemiacetal group.

  • Treating glucose as non-reactive in Fehling’s test. Glucose is a classic reducing sugar and gives the reddish-brown precipitate.

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