Name the disaccharide (shown at right) formed by glycosidic linkage of D-Glucose and D-Fructose. ? Name this disaccharide: Do you expect this disaccharide to be a reducing sugar? Briefly explain. H OH HOCH2 H CH2OH H OH H O H HO OH H H O OH H CH2OH
The Correct Answer and Explanation is:
The disaccharide formed by the glycosidic linkage of D-Glucose and D-Fructose is sucrose.
Is Sucrose a Reducing Sugar?
No, sucrose is not a reducing sugar.
Disaccharides are carbohydrates composed of two monosaccharide units linked by a glycosidic bond. In the case of sucrose, the two monosaccharides involved are D-glucose and D-fructose. Specifically, sucrose is formed when the anomeric carbon of α-D-glucose (C1) forms a glycosidic bond with the anomeric carbon of β-D-fructose (C2). This linkage is referred to as an α(1→2)β glycosidic bond.
To understand whether a disaccharide is a reducing sugar, it is important to consider the availability of a free anomeric carbon capable of acting as a reducing agent. A reducing sugar must possess a hemiacetal (for aldoses like glucose) or a hemiketal (for ketoses like fructose) group that can open to form a free aldehyde or ketone group in solution. This open-chain form is essential for the sugar to participate in redox reactions, such as reducing Benedict’s or Fehling’s solution.
In sucrose, both the anomeric carbons (C1 of glucose and C2 of fructose) are involved in the glycosidic bond. As a result, neither glucose nor fructose in sucrose has a free anomeric carbon that can revert to the open-chain form. Therefore, sucrose lacks the necessary functional group to act as a reducing agent.
This is why sucrose does not react with Tollens’, Benedict’s, or Fehling’s reagents, which are commonly used to test for reducing sugars. It remains non-reactive in these tests, distinguishing it from reducing disaccharides like maltose or lactose, which do contain a free hemiacetal group.
In summary, sucrose is the disaccharide formed by D-glucose and D-fructose, and due to the involvement of both anomeric carbons in the glycosidic bond, it is non-reducing.
