Name of the sugar : Sucrose
Sucrose is the common table sugar, obtained mainly from sugarcane and sugar beet. It is a disaccharide of formula $C_{12}H_{22}O_{11}$.
Hydrolysis products
$$\underset{\text{sucrose}}{C_{12}H_{22}O_{11}} + H_2O \xrightarrow{H^+ \text{ or invertase}} \underset{\text{glucose}}{C_6H_{12}O_6} + \underset{\text{fructose}}{C_6H_{12}O_6}$$
Products : D-(+)-glucose and D-(−)-fructose, formed in equal amounts.
The two units in sucrose are joined by an $\alpha$-1,2-glycosidic linkage between C-1 of glucose and C-2 of fructose.
Is it a reducing sugar ?
No — sucrose is a non-reducing sugar.
Reason : the glycosidic linkage in sucrose involves the anomeric carbon of both units :
- C-1 of glucose, its anomeric carbon
- C-2 of fructose, its anomeric carbon
With both anomeric carbons locked into the linkage, neither ring can open to expose a free aldehyde or ketone group. There is therefore nothing available to reduce Tollens' reagent or Fehling's solution.
Contrast with maltose and lactose : in each of those, only one anomeric carbon is used in the linkage, so the other remains free and both are reducing sugars.
A related point — invert sugar
The equimolar mixture produced on hydrolysis is called invert sugar, because the sign of optical rotation reverses :
| Species | Specific rotation |
|---|
| Sucrose | $+66 \cdot 5^\circ$ (dextrorotatory) |
| Product mixture | $-19 \cdot 9^\circ$ (laevorotatory) |
Fructose $(-92 \cdot 4^\circ)$ rotates light to the left far more strongly than glucose $(+52 \cdot 5^\circ)$ rotates it to the right.
Interestingly, the products of hydrolysis are reducing sugars, even though sucrose itself is not.
