Oxidation of Reducing Sugars

Reducing sugars can be oxidized by a variety of oxidizing agents. The oxidizing agent can oxidize the aldehyde functional group and in some cases the terminal -CH₂-OH group is oxidized by selective oxidizing agents. Scientists often perform oxidation of sugars to determine the stereochemistry, identify functional groups, or to synthesize their derivatives. Following are some oxidation reactions of reducing sugars.

Bromine Water solution:

Bromine (Br₂) is used as a mild oxidizing agent for the oxidation of aldehydes to corresponding carboxylic acids. It cannot oxidize ketones and alcohols. For this reason, the bromine water test is used to distinguish between aldoses and ketoses. The reddish brown color of bromine disappears when the reacting sugar is an aldose. This is because the Br₂ is reduced to Br^- which is colorless.

Following are some specific examples of oxidation of aldoses by bromine water.

Nitric Acid:

Nitric acid (HNO₃) is a strong oxidizing agent. When nitric acid is reacted with monosaccharides, it oxidizes both the aldehyde functional group and terminal -CH₂-OH (primary alcohol) group into carboxylic acid groups. It cannot oxidize secondary alcohols. The resulting product is called aldaric acid.

Following are some specific examples of oxidation of aldoses by HNO₃.

Tollens Test:

Tollens’ reagent (Ag⁺, NH₃, HO^-) react with aldehyde to produce carboxylate ions and a reduced silver metal. The metallic silver forms mirror on the inside of the test tube.

Since this reaction is performed in basic medium, the sugars containing α-hydroxy ketone functional groups tautomerize into aldehyde which are then oxidized by Tollens’ reagent. For this reason, this test cannot be used to distinguish between aldoses and ketoses.