Sugars are categorized as reducing and non-reducing based on their reactivity towards Tollens reagent or Fehling solution, a qualitative test. These solutions contain cupric ions (Cu2+). The cupric ions (Cu2+) oxidize the sugars only when they are in linear form, which exists in equilibrium with the cyclic structures. The carbonyl carbon of sugar is oxidized to a carboxyl group. They form enediols, which are converted into aldonic acids and a mixture of acids under alkaline conditions. This results in the reduction of cupric ions (Cu2+) into a cuprous form (Cu+). The cuprous ions form a red cuprous oxide precipitate. Thus, sugars capable of reducing cupric ions into cuprous ions are called reducing sugars and the others as non-reducing sugars.
The concentration of sugars can also be estimated by measuring the amount of oxidizing agents reduced. Earlier, this test was used to measure blood glucose levels in diabetic patients.
Their reactivity with Fehling’s solution is dependent on whether the sugar molecule has free aldehyde or ketone group available or not. If the sugars have free aldehyde or ketone group, it can be oxidized by cupric ions. Hence, they are categorized as reducing sugars such as all monosaccharides and certain disaccharides like lactose and maltose. The others do not have such free groups; they do not reduce cupric into cuprous ions, are termed as non-reducing sugars such as disaccharides like sucrose and trehalose and polysaccharides.