Carbohydrates which upon hydrolysis give two molecules of the same or different monosaccharides are called disaccharides. Their general formula is C12H22O11. The three most important disaccharides are sucrose, maltose, and lactose. Each one of these on hydrolysis with either an acid or an enzyme gives two molecules of the same or different monosaccharides as shown below:
Disaccharides may also be considered to be formed by a condensation reaction between two molecules of the same or different monosaccharides with the eliminatioin of a molecule of water. This reaction involves the formation of an acetal from a hemiacetal and an alcohol – in which one of the monosaccharides acts as the hemiacetal while the other acts as the alcohol.
It is formed by condensation of one molecule of glucose and one molecule of fructose. Unlike maltose and lactose, it is non-reducing sugar since both glucose (C1 - α) and fructose (C2 -β) are connected to each other through their reducing centres. Its structure is shown below:
Hydrolysis: (Invert Sugar or Invertose). Hydrolysis of sucrose with hot dilute acid yields
D-glucose and D-fructose.
Sucrose is dextrorotatory, its specific rotation being +66.5%, D-glucose is also dextrorotatory, [α]D = +53°, but D-fructose has a large negative rotation, [α]D = -92°. Since D-fructose has a greater specific rotation than D-glucose, the resulting mixture is laevorotatory. Because of this the hydrolysis of sucrose is known as the inversion of sucrose, and the equimolecular mixture of glucose and fructose is known is invert sugar or invertose.
Solved example. The correct haworth projection of sucrose is
Solution: (B) Sucrose is composed of α -D- glucose & β - D – fructose.
Solved example. When sucrose is heated with conc. HNO3 the product is
(A) sucrosenitrate (B) oxalic acid
(C) formic acid (D) citric acid