20.5: Reactions of Monosaccharides

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Learning Objectives

Identify the structures of sucrose, lactose, and maltose.
Identify the monosaccharides that are needed to form sucrose, lactose, and maltose

Reducing Sugars

Sugar Alcohols

Disaccharide Formation

Previously, you learned that monosaccharides can form cyclic structures by the reaction of the carbonyl group with an OH group. These cyclic molecules can in turn react with another alcohol. Disaccharides (C₁₂H₂₂O₁₁) are sugars composed of two monosaccharide units that are joined by a carbon–oxygen-carbon linkage known as a glycosidic linkage. This linkage is formed from the reaction of the anomeric carbon of one cyclic monosaccharide with the OH group of a second monosaccharide.

The disaccharides differ from one another in their monosaccharide constituents and in the specific type of glycosidic linkage connecting them. There are three common disaccharides: maltose, lactose, and sucrose. All three are white crystalline solids at room temperature and are soluble in water. We’ll consider each sugar in more detail.