Question

Consider the following statements:
Statement I: All monosaccharides are reducing sugars.
Statement II: Sucrose can reduce ammoniacal silver nitrate solution.
Choose the correct answer from the options given below.

• A. Both Statement I and Statement II are correct
• B. Both Statement I and Statement II are incorrect
• C. Statement I is correct but Statement II is incorrect
• D. Statement I is incorrect but Statement II is correct

Hint:

To identify a reducing sugar, check for a hemiacetal group in its cyclic structure. A hemiacetal group (a carbon bonded to both an -OH and an -OR group) indicates that the ring can open to form a free aldehyde. In sucrose, the anomeric carbons are linked in an acetal/ketal linkage, so there are no hemiacetal groups.

Answer 1

The Correct Option is C

Step 1: Analyzing Statement I.
Reducing sugars are carbohydrates that can act as reducing agents because they have a free aldehyde group or a free ketone group. All monosaccharides (like glucose, fructose, galactose) exist in equilibrium with their open-chain forms, which contain either an aldehyde group (aldoses) or a ketone group that can isomerize to an aldehyde (ketoses). This free carbonyl group allows them to be oxidized by mild oxidizing agents like Tollens' reagent or Fehling's solution. Therefore, all monosaccharides are reducing sugars. Statement I is correct.
Step 2: Analyzing Statement II.
Sucrose is a disaccharide made up of two monosaccharide units: \(\alpha\)-glucose and \(\beta\)-fructose. The glycosidic bond in sucrose is formed between the anomeric carbon of glucose (C1) and the anomeric carbon of fructose (C2). These anomeric carbons contain the potentially free aldehyde and ketone groups, respectively. Since both are involved in the glycosidic linkage, neither unit can open up to form a free carbonyl group. Therefore, sucrose does not have reducing properties. The reaction with ammoniacal silver nitrate solution is the Tollens' test, which is a test for reducing sugars. Since sucrose is a non-reducing sugar, it will not give a positive Tollens' test, meaning it cannot reduce the solution. Statement II is incorrect.
Step 3: Final Answer.
Statement I is correct, and Statement II is incorrect.