Question

Give scientific reasons:

The disaccharide sucrose gives negative Tollens test while the disaccharide maltose gives a positive Tollens test.

Answer 1

1. The structure of sucrose contains glycosidic linkage between C-1 of α-glucose and C-2 of β-fructose.
2. Since the potential aldehyde and ketone groups of both the monosaccharide units are involved in the formation of the glycosidic bond (i.e., α, β-1,2- glycosidic bond), sucrose is a non-reducing sugar and gives negative Tollen’s test.
3. The glycosidic bond in maltose is in between C-1 of one glucose ring and C-4 of the other (i.e., α -1,4-glycosidic linkage).
4. The hemiacetal group at C-1 of the second ring is not involved in the glycosidic linkage. Hence, maltose is a reducing sugar and gives positive Tollen’s test.