Which of the following sugar is non-reducing?

Sucrose

The correct answer is option 4. Sucrose.

Reducing Sugars: A reducing sugar is one that can donate electrons to another molecule, specifically to a mild oxidizing agent like Benedict's reagent, which results in the reduction of the reagent and the oxidation of the sugar. This ability to act as a reducing agent comes from the presence of a free anomeric carbon. An anomeric carbon is the carbon atom that was the carbonyl carbon \((C=O)\) in the open-chain form of the sugar. In the ring form of the sugar, this carbon becomes a part of the glycosidic bond. For a sugar to be reducing, it must be able to open up to its linear form, exposing the aldehyde (in aldoses) or ketone (in ketoses) group that can act as a reducing agent.

Non-Reducing Sugars: Non-reducing sugars, on the other hand, do not have a free anomeric carbon because their anomeric carbons are involved in a glycosidic bond, which prevents them from opening up into the linear form. As a result, they cannot act as reducing agents and do not react with Benedict's reagent or similar oxidizing agents.

Analysis of the Given Sugars:

1. Maltose: Maltose is a disaccharide composed of two glucose units linked by an α(1→4) glycosidic bond. The glycosidic bond is formed between the anomeric carbon of one glucose and the hydroxyl group on the fourth carbon of the second glucose. The second glucose unit in maltose has a free anomeric carbon that can open to its aldehyde form, making maltose a reducing sugar.

2. Lactose: Lactose is a disaccharide composed of galactose and glucose linked by a β(1→4) glycosidic bond. The glycosidic bond is between the anomeric carbon of galactose and the fourth carbon of glucose. The glucose unit has a free anomeric carbon that can open to its aldehyde form, making lactose a reducing sugar.

3. Glucose: Glucose is a monosaccharide, specifically an aldohexose, with an aldehyde group at the anomeric carbon in its linear form. In its cyclic form, glucose can open up to expose the aldehyde group, making it a reducing sugar.

4. Sucrose: Sucrose is a disaccharide composed of glucose and fructose linked by an α(1→2) glycosidic bond. The glycosidic bond involves the anomeric carbon of glucose and the anomeric carbon of fructose. Because both anomeric carbons are involved in the glycosidic bond, neither glucose nor fructose can open up to their aldehyde or ketone forms. As a result, sucrose cannot act as a reducing agent, making it a non-reducing sugar.

Conclusion:

Among the options provided, sucrose is the only non-reducing sugar because its structure prevents it from opening up into a form that can participate in reduction reactions. Maltose, lactose, and glucose all have free anomeric carbons that allow them to act as reducing sugars.