Question

A lost property. Glucose and fructose are reducing sugars. Sucrose, or table sugar, is a disaccharide consisting of both fructose and glucose. Is sucrose a reducing sugar? Explain.

Short answer

Sucrose is not a reducing sugar because it does not have a free aldehyde or ketone group.

Step-by-step solution

Understanding Reducing Sugars

Reducing sugars are carbohydrates that can act as reducing agents because they have free aldehyde or ketone groups. Glucose and fructose, being monosaccharides with such groups, are reducing sugars.

Analyzing Disaccharides

Disaccharides are formed when two monosaccharides are linked together via a glycosidic bond. Whether a disaccharide is a reducing sugar depends on the presence of a free aldehyde or ketone group after forming the glycosidic bond.

Examining the Structure of Sucrose

Sucrose is composed of glucose and fructose linked by a glycosidic bond. In sucrose, this bond involves the anomeric carbon of both glucose and fructose, thus blocking the free aldehyde or ketone group that would allow it to act as a reducing sugar.

Conclusion

Since the glycosidic bond in sucrose prevents the presence of a free aldehyde or ketone group, sucrose is not a reducing sugar despite being composed of two reducing monosaccharides.

Key concepts

Disaccharides

A disaccharide is a carbohydrate molecule composed of two monosaccharides. These simpler sugar units are connected through a specific type of chemical bond called a glycosidic bond. Examples of disaccharides include sucrose, lactose, and maltose. Each has unique properties based on the types of monosaccharides involved and the positions at which they bond.
When considering if a disaccharide can act as a reducing sugar, it's important to look at its structure. The potential for reduction hinges on whether the resulting compound maintains a free aldehyde or ketone group. If both reactive ends (anomeric carbons) of the monosaccharides are tied up in the bond, the disaccharide typically loses its reducing capability.

Glycosidic Bond

Glycosidic bonds are the links that form between two sugar molecules, enabling the creation of disaccharides. They occur when the hydroxyl group (–OH) of one sugar reacts with the hemiacetal (aldehyde) or hemiketal (ketone) group of another, resulting in the release of a water molecule. This condensation reaction forms a bridge that holds the two sugar molecules together.
The position and type of glycosidic bond can significantly influence the sugar's properties, including whether it remains a reducing sugar. For instance, in sucrose, the bond forms between the anomeric carbon of glucose and the anomeric carbon of fructose. This specific connection inactivates both reducing ends, thereby classifying sucrose as a non-reducing sugar.

Aldehyde Groups

An aldehyde group is a functional group characterized by the presence of a carbonyl center (C=O) bonded to at least one hydrogen atom. In sugars, an aldehyde group becomes crucial in determining whether it can function as a reducing sugar. In open-chain formations, such as glucose, the aldehyde group can donate electrons to other molecules, making it a reducing sugar.
The presence of an aldehyde group is a key factor in the reactivity of monosaccharides like glucose. It allows the sugar to engage in redox reactions, moving electrons from the sugar to other atoms or molecules. However, once a glycosidic bond forms utilizing the aldehyde end, as seen in sucrose, this reactivity may be lost, leading to the sugar not being reducing.

Ketone Groups

A ketone group is composed of a carbon atom double bonded to an oxygen atom (C=O) with two other carbon atoms attached. This group is found in sugars known as ketoses, such as fructose. Ketone groups play a similar role to aldehyde groups in determining a sugar's reducing capabilities. If the ketone's carbonyl carbon is involved in a glycosidic bond, the sugar typically loses its ability to be a reducing sugar.
Like with aldehydes, the capacity for reduction in monosaccharides with ketone groups is significant, making them reactive in forming additional bonds or participating in redox reactions. However, sucrose, despite being derived from fructose, which contains a ketone, cannot act as a reducing sugar because its glycosidic bond involves the ketonic carbon, preventing it from opening up to facilitate reduction.