Which of the following are reducing sugar?

A. Sucrose

B. Maltose

C. Lactose

D. Mannose

E. Glucose

Choose the correct answer from the options given below:

B, C, D, E only

The correct answer is option 2. B, C, D, E only.

A reducing sugar is a carbohydrate that can act as a reducing agent because it has a free aldehyde or ketone group in its open-chain form. This allows it to participate in oxidation-reduction reactions. In solution, sugars can exist in both a cyclic (ring) form and an open-chain form. In the open-chain form, sugars with a free aldehyde group (as in aldoses) or a free ketone group (as in ketoses) can act as reducing agents. These sugars are capable of reducing oxidizing agents, such as:

Tollens' reagent (which produces a silver mirror if a reducing sugar is present),

Benedict's solution (which forms a red precipitate of copper(I) oxide when reduced),

Fehling's solution (which also detects the presence of reducing sugars by forming a red precipitate).

In the cyclic form, the aldehyde or ketone group is part of a hemiacetal or hemiketal linkage, which can open up to form the reactive open-chain form. The reducing properties of a sugar depend on whether it can return to its open-chain form.

A. Sucrose 

Sucrose is a disaccharide made up of glucose and fructose. In sucrose, the glucose is linked to the fructose via an α-1,2-glycosidic bond. The linkage occurs between the anomeric carbon of glucose (C1) and the anomeric carbon of fructose (C2).

Why Non-reducing: Both anomeric carbons in sucrose are involved in the glycosidic bond, meaning neither the glucose nor the fructose units can convert into their open-chain forms. Since neither has a free anomeric carbon, sucrose cannot act as a reducing agent.

Conclusion: Non-reducing sugar.

B. Maltose

Maltose is a disaccharide consisting of two glucose units linked by an α-1,4-glycosidic bond. Only one of the glucose units has its anomeric carbon involved in the glycosidic bond.

Why Reducing: The other glucose unit has a free anomeric carbon (C1), which can open up to form the aldehyde group in the open-chain form. This makes maltose capable of acting as a reducing sugar.

Conclusion: Reducing sugar.

C. Lactose 

Lactose is a disaccharide composed of glucose and galactose, connected via a β-1,4-glycosidic bon. The glycosidic bond involves the C1 of galactose and the C4 of glucose.

Why Reducing: In lactose, the glucose unit has a free anomeric carbon (C1) that is not involved in the glycosidic bond. This allows the glucose to revert to its open-chain form, revealing a free aldehyde group that can act as a reducing agent.

Conclusion: Reducing sugar.

D. Mannose

Mannose is a monosaccharide and an aldose. It is a stereoisomer of glucose with the same chemical formula but differs in the configuration of one of its chiral carbons.

Why Reducing: Being a monosaccharide, mannose can readily interconvert between its cyclic form and open-chain form. In its open-chain form, it has a free aldehyde group at the C1 position, which can reduce oxidizing agents.

Conclusion: Reducing sugar.

E. Glucose

Glucose is a monosaccharide and the most common aldose sugar. In its cyclic form, glucose exists as either α-glucose or β-glucose, depending on the position of the hydroxyl group at the anomeric carbon.

Why Reducing: Glucose can easily switch between its cyclic and open-chain forms. In the open-chain form, glucose exposes its aldehyde group (at C1), making it capable of reducing oxidizing agents.

Conclusion: Reducing sugar.

Reducing sugars: Maltose, Lactose, Mannose, and Glucose.

Non-reducing sugar: Sucrose.

The correct answer is option 2: B, C, D, E only.