Sucrose is not a reducing sugar because it lacks the ability to form either aldehyde or a ketone in a basic solution. It cannot perform as a reducing agent in solutions at a pH above 7, according to Reference.com.
Sucrose, the primary ingredient in table sugar, is a disaccharide consisting of a fructose ring and a glucose ring. It has the chemical formula C6H12O6. Due to the molecular shape of sucrose, there is no place for the aldehyde or ketone formation.
To be classified as a reducing sugar, there must be an open-chain form, including an aldehyde or free hemiacetal group. A monosaccharide that contains the aldehyde group is an aldose, and one containing a ketone group is a ketos. A redox reaction oxidizes the aldehyde and reduces the other compound in solution. The ketone group in the open chain allows the sugar to isomerize to produce an aldehyde when in solution, so fructose and other ketone-bearing sugars are also reducing sugars, even though the ketone cannot be oxidized unless the sugar decomposes.
Chemists use reagents, such as Benedict's solution or other test reagents, to check for reducing sugars. As these reagents react with the sugar, they bring a color change not observed with non-reducing sugars, including sucrose.