Chemical Properties of Monosaccharides
The most important chemical property of monosaccharides is the reducing power
The reducing power of monosaccharides comes from the hydroxyl group or aldehyde group. If it is an aldose, the aldehyde group gives the significant reducing power. Otherwise, it is the hydroxyl group.
When an aldose is oxidized, it would be turned into the corresponding acid. e.g.
Glucose -------- > gluconic acid
At the same time, the aldehyde group of the glucose is oxidized into the acid group.
If it is a ketose, the molecule would be cut into two parts from the location of the ketone group.
The standardized oxidizing power required in this reaction is provided by Cu2+ ion. When the Cu2+ ion oxidizes glucose into gluconic acid, at the same time, glucose would reduce Cu2+ into Cu22+.
The Cu2+ ion provided in the reagent is in the form of ions. It can be prepared in copper (II) sulphate solution.
Finally, the Cu22+ would exist in the form of Cu2O (copper (I) oxide). It is a brick-red colour precipitate.
This result acts as the core of the tests of monosaccharides. Two packages are designed at this basis for the purpose of testing monosaccharides. One is called the Benedict's Test, another is the Fehling's Test.