Reducing sugars are carbohydrates that can reduce reagents like Fehling's or Tollens' solution. To be a reducing sugar, the carbohydrate must have a free aldehyde or ketone group (or a hemiacetal/hemiketal group that can open to yield a free carbonyl in solution).

• Glucose is an aldohexose and has a free aldehyde group, making it a reducing sugar.
• Maltose is a disaccharide composed of two glucose units linked by an $\alpha(1 \rightarrow 4)$ glycosidic bond. The second glucose unit retains a free hemiacetal group at its anomeric carbon (C-1), making it a reducing sugar.
• Fructose is a ketohexose. Under the alkaline conditions of Fehling's test, it undergoes isomerization (Lobry de Bruyn-van Ekenstein transformation) to form glucose and mannose, which then reduce the reagent.
• Sucrose is a disaccharide formed by an $\alpha, \beta(1 \rightarrow 2)$ glycosidic linkage between the anomeric carbon of $\alpha$-D-glucose (C-1) and the anomeric carbon of $\beta$-D-fructose (C-2). Because both anomeric carbons are involved in the bond, there is no free reducing group available. Hence, sucrose is a non-reducing sugar and does not reduce Fehling's solution.