Fructose and the disaccharides: sucrose, maltose and lactose
Fructose is a ketohexose with a five-membered ring. Two monosaccharides join through a glycosidic linkage to make sucrose, maltose or lactose.
Fructose
Fructose is found in fruits and honey, and, joined to glucose, in sucrose. Its formula is C₆H₁₂O₆, the same as glucose, but it has a keto group at C2, so it is a ketohexose. Its six carbon atoms are in a straight chain. It belongs to the D-series and is laevorotatory, so it is written D-(−)-fructose. In its cyclic form, the OH at C5 adds to the C=O at C2, giving a five-membered ring of four C and one O. This ring is like furan, so it is called a furanose structure.
How to draw D-fructose (Fischer projection)
- Draw a vertical chain of six carbons with C1 at the top.
- C1: CH₂OH. C2: C=O.
- C3: HO on the left, H on the right.
- C4 and C5: H on the left, OH on the right.
- C6: CH₂OH.
- C3 to C6 are the same as in D-glucose; only C1 and C2 differ.
The three disaccharides
| Disaccharide | Units on hydrolysis | Glycosidic linkage | Reducing sugar? |
|---|---|---|---|
| Sucrose (cane sugar) | α-D-glucose + β-D-fructose | C1 of glucose to C2 of fructose | No: both reducing groups are used in the link |
| Maltose (malt sugar) | Two α-D-glucose | C1 of one glucose to C4 of the other | Yes: a free CHO can form at C1 of the second glucose |
| Lactose (milk sugar) | β-D-galactose + β-D-glucose | C1 of galactose to C4 of glucose | Yes: a free CHO can form at C1 of the glucose unit |
Hydrolysis of sucrose and invert sugar
- 1C₁₂H₂₂O₁₁ (sucrose) + H₂O → C₆H₁₂O₆ (D-(+)-glucose) + C₆H₁₂O₆ (D-(−)-fructose)Dilute acid or the enzyme invertase (also called sucrase). Equal numbers of moles of the two sugars form.
- 2Sucrose is dextrorotatory, but the mixture after hydrolysis is laevorotatory.Fructose turns light to the left (−92.4°) more strongly than glucose turns it to the right (+52.5°).
- 3★This change in the sign of rotation is called inversion, and the mixture is called invert sugar.
★ key step
Mass of products from sucrose
34.2 g of sucrose is completely hydrolysed. What mass of glucose is formed? (C = 12, H = 1, O = 16)
- Molar mass of sucrose, C₁₂H₂₂O₁₁ = 12(12) + 22(1) + 11(16) = 342 g mol⁻¹.
- Moles of sucrose = 34.2 g ÷ 342 g mol⁻¹ = 0.100 mol.
- 1 mol of sucrose gives 1 mol of glucose and 1 mol of fructose, so 0.100 mol of each.
- Molar mass of C₆H₁₂O₆ = 6(12) + 12(1) + 6(16) = 180 g mol⁻¹. Mass of glucose = 0.100 mol × 180 g mol⁻¹ = 18.0 g.
- Check: 34.2 g of sucrose + 1.8 g of water = 36.0 g = 18.0 g of glucose + 18.0 g of fructose.
18.0 g of glucose (and 18.0 g of fructose).
Lactose is the sugar in milk. Many adults feel bloated after drinking milk because they make less lactase, the enzyme that splits lactose. Curd and paneer contain much less lactose and are often easier to digest.
2 marks: “What is invert sugar? Why is it so called?” The equimolar mixture of glucose and fructose formed by hydrolysis of sucrose. The sign of rotation changes from dextro (sucrose) to laevo (mixture), because fructose (−92.4°) outweighs glucose (+52.5°).
2 marks: “Why is sucrose a non-reducing sugar while maltose is a reducing sugar?” In sucrose, C1 of glucose and C2 of fructose are both in the glycosidic bond, so no free reducing group remains. In maltose, C1 of the second glucose is free and can open to a CHO group.
3 marks: “Name the products of hydrolysis of sucrose, maltose and lactose, and state the glycosidic linkage in each.” Sucrose: α-D-glucose + β-D-fructose, C1 to C2. Maltose: two α-D-glucose, C1 to C4. Lactose: β-D-galactose + β-D-glucose, C1 (galactose) to C4 (glucose).
Sucrose: glucose C1 to fructose C2, not reducing. Maltose: glucose-glucose, C1 to C4. Lactose: galactose-glucose, C1 to C4. Fructose ring is a pentagon (furanose); glucose ring is a hexagon (pyranose).