Illustrated Glossary of Organic Chemistry

Reducing sugar: Any carbohydrate whose structure contains an aldehyde, or a hemiacetal in equilibrium with an aldehyde. This aldehyde group can be oxidized, with resultant reduction of the oxidizing agent. A nonreducing sugar is a sugar lacking this aldehyde, and therefore it is a sugar that does not give this reduction reaction.

Glucopyranose		Acyclic glucose

Glucose is a reducing sugar. In aqueous solution glucose exists as an equilibrium greatly favoring the glucopyranose form with traces of the acyclic form also present. The glucopyranose hemiacetal and acyclic glucose aldehyde are both shown in red.

	+				+
Acyclic glucose		Benedict's solution		Oxidized glucose		CuO precipitate
		(aqueous Cu2+)

The glucose aldehyde reduces blue Cu²⁺ ions in Benedicts' solution to Cu⁺, which subsequently forms a deep red CuO precipitate. The aldehyde is oxidized to a carboxylic acid.

	+			No reaction
Fructose		Benedict's solution

Fructose is a ketose. Acyclic fructose lacks an aldehyde, and so does not give a CuO precipitate with Benedict's solution.