Sucrose is not a reducing sugar because it does not have a free aldehyde or ketone group in its structure. Reducing sugars are carbohydrates that have a free aldehyde or ketone group that can undergo oxidation reactions. These groups are responsible for the reducing properties of the sugar.
Sucrose is a disaccharide composed of glucose and fructose units linked together by a glycosidic bond. The glucose unit has an aldehyde group at the anomeric carbon (C1), which is capable of reducing other compounds. However, in sucrose, this aldehyde group is involved in the glycosidic bond formation with the fructose unit, and it is no longer free to undergo oxidation.
The glycosidic bond between glucose and fructose in sucrose prevents the open-chain form of glucose from existing and participating in the typical reactions of reducing sugars. As a result, sucrose does not exhibit the reducing properties that are characteristic of monosaccharides or certain other disaccharides like maltose or lactose.
In summary, sucrose is not a reducing sugar because its structure lacks a free aldehyde or ketone group necessary for the reducing behavior.