Disaccharides

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 elimination 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.

Now the question comes How do monosacharides combine to form disaccharides? The answer is through glycosidec bond or glycosydic linkage.

What is glycosidic linkage ?

Glycosidic linkage refers to the linkage formed between two monosaccharide units through an oxygen atom by the loss of a water molecule. For example maltose is formed by the combination two Glucose aunits. The C-1 and C-4 make glycosidic bond with elimination of water.
 

 Maltose (C₁₂H₂₂O₁₁)

Maltose or malt sugar is formed by condensation of two molecule of glucose joined with an α bond .Maltose is a reducing sugar. It reduces Tollens’ and Fehling’s reagent. It reacts with phenylhydrazine to yield hydrazone. It is oxidized by bromine water to maltobinoic acid. On hydrolysis it give two glucose units.

Maltose can be hydrolysed by enzyme maltase. 

You can also refer to the following video for disaccharides

Sucrose (C₁₂H₂₂O₁₁)

 
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 of sucrose ((Invert Sugar or Invertose) 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.

Lactose (C₁₂H₂₂O₁₁)

Lactose or milk sugar is formed by condensation of one  molecule of galactose and  one molecule of glucos .Maltose is a reducing sugar. It reduces Tollens’ and Fehling’s reagent. It reacts with phenylhydrazine to yield hydrazone. Lactose exist in alpha and beta form which undergo mutarotation. It makes up about 5% of human milk and of cow’s milk. Milk sours when lactose in converted to lactic acids by bacterial (e.g. Lactobacillus bulgaricus). Acidic hydrolysis of lactose converts it into equal about of D- glucose and D- galactose.
 

Cellobiose (C₁₂H₂₂O₁₁) 

Cellobiose consists of two glucose molecules linked by a β bond. Octaacetate of cellobiose is obtained by treatement of cellulose with sulfuric acid. Alkaline hydrolysis of  octaacetate yeilds cellobiose. Cellobiose is also a reducing sugar just like maltose. It forms osazone and exist in alpha and beta forms that undergi mutarotation. Cellobiose can be hydrolysed to two molecules of D-Glucose.Cellobiose differ from amtose in respect of enzyme which causes hydrolysis. Cellobiose is hydrolysed by emulsin and not be maltase.

Some other Related Links 

Monosaccharides

JEE Organic Chemistry Syllabus

Reference books of Organic Chemistry

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