Enzymes

Sucrose molecule is made up of a glucose pyranose and a fructo furanose

Carbohydrates which can reduce Tollen's reagent or Fehling solution are called reducing sugars. All the monosaccharides and the disaccharides (except sucrose) are reducing agent.

Sucrose does not have free aldehyde or keto group adjacent to group, therefore it fails towards Tollen's reagent and Fehling solution.