A sugar is reducing if it possesses a free hemiacetal or hemiketal group (which is in equilibrium with a free aldehyde or ketone in aqueous solution).

• Sucrose is composed of $\alpha$-D-Glucose and $\beta$-D-Fructose.
• The glycosidic linkage occurs between $C_1$ of glucose (the aldehyde carbon) and $C_2$ of fructose (the ketone carbon).
• Since both anomeric carbons are involved in the bond, there is no free carbonyl group available to reduce Tollen's or Fehling's reagents.