Disaccharides: Sucrose as a Non-Reducing Sugar

A disaccharide is formed when two monosaccharide units are joined together by a glycosidic bond, an ether-type linkage formed, in effect, by the loss of one molecule of water between an $-\text{OH}$ group of one monosaccharide and an $-\text{OH}$ group (very often the anomeric $-\text{OH}$) of the other.

Sucrose, common table sugar, is the disaccharide this chapter focuses on. It is built from one unit of $\alpha$-D-glucose and one unit of $\beta$-D-fructose. What makes sucrose structurally distinctive -- and gives rise to its most important chemical property -- is exactly which carbons form the glycosidic bond: the linkage in sucrose runs from C1 (the anomeric carbon) of the glucose unit to C2 (the anomeric carbon) of the fructose unit. In other words, the glycosidic bond in sucrose uses up the anomeric carbon of both monosaccharide units simultaneously, unlike most other disaccharides (such as maltose), where the glycosidic bond ties up the anomeric carbon of only one unit, leaving the other unit's anomeric carbon free.

This has a direct chemical consequence. A sugar's ability to act as a reducing sugar -- to reduce Fehling's solution or Tollens' reagent -- depends on having at least one anomeric carbon free to reopen into an aldehyde (or, via isomerisation, an oxidisable) form. Because both of sucrose's monosaccharide units have their anomeric carbon locked into the glycosidic bond, neither ring can open back up to expose a free carbonyl group. As a result, sucrose is a non-reducing sugar: it gives a negative test with Fehling's solution and Tollens' reagent, unlike glucose, fructose, or most other disaccharides. …