Non-reducing sugars are carbohydrates that do not have a free aldehyde or ketone functional group and, therefore, cannot act as reducing agents. In reducing sugars, the presence of a free aldehyde or ketone group allows them to participate in redox reactions, reducing other substances by donating electrons.
Non-reducing sugars typically include disaccharides and polysaccharides that lack free aldehyde or ketone groups. Common examples of non-reducing sugars include sucrose and trehalose.
- Sucrose:
- Sucrose is a disaccharide composed of one glucose molecule and one fructose molecule linked together by a glycosidic bond. The glycosidic bond is formed between the anomeric carbon of glucose and the hydroxyl group of fructose. Since the glycosidic bond involves the anomeric carbon of glucose and not a free aldehyde or ketone group, sucrose is a non-reducing sugar.
Glucose+Fructose→condensationSucrose+WaterGlucose+Fructosecondensation​Sucrose+Water
- Trehalose:
- Trehalose is another example of a non-reducing sugar. It is a disaccharide consisting of two glucose molecules linked by an α,α-1,1-glycosidic bond. Similar to sucrose, the glycosidic bond in trehalose involves the anomeric carbon of one glucose molecule and the hydroxyl group of the other glucose molecule, resulting in the absence of a free aldehyde or ketone group.
Glucose+Glucose→condensationTrehalose+WaterGlucose+Glucosecondensation​Trehalose+Water
In contrast, reducing sugars, such as glucose and fructose, have a free aldehyde or ketone group and can donate electrons, reducing other substances. The ability to reduce certain chemical reagents, like Benedict’s solution, is a common test to distinguish between reducing and non-reducing sugars. Non-reducing sugars do not react with Benedict’s solution because they lack a free aldehyde or ketone group to participate in the redox reaction.