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:

2446_disaccharide.JPG

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 refre 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.

 2418_disaccharide.JPG

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 cows 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  rlinks are

Monosaccharides

JEE Organic Chemistry Syllabus

Reference books of Organic Chemistry

Related Resources
Nucleosides and Nucleotides

NUCLEOSIDES AND NUCLEOTIDES Nucleosides:...

Esterification

Esterification Note: HCl first converts the...

Aldotetroses Aldopentoses Aldopentoses and Epimers

Aldotetroses If we examine the general formula of...

Isoelectric Point

ISOELECTRIC POINT (i) When ionized form of amino...

Objective Questions on Carbohydrates Part I

Objective: Prob 1. Peptide linkage is Sol. (D)....

Definition and Classification of Carbohydrates

INTRODUCTION Old Definition The group of compounds...

Cyclic Structure of Monosaccharides

Cyclic structure of Monosaccharides We know that...

Nucleic Acids

Nucleic Acids Nucleic acids are biologically...

Zwitter Ion

ZWITTER ION (i) Amino acids contain both acidic...

Functions of Nucleic Acids

Biological functions of nucleic acids Important...

Structure of Proteins

Structure of Proteins (i) Proteins have three...

Classification of Carbohydrates

Classification of Carbohydrates The carbohydrates...

Polysaccharides

Polysaccharides Polysaccharides are formed when a...

Cyclic Structure of Fructose

Cyclic structure of Fructose Like glucose,...

Cyclic Structure of Anomers

Cyclic Structure of Glucose – Anomers We...

Amino Acids

Amino acids Amino acids are molecules, which...

Essential And Non-Essential Amino Acid

Essential amino acids (a) These must be supplied...

DNA And RNA

DNA And RNA DNA RNA 1. Sugar present in DNA is...

Aldopentoses Aldopentoses and Epimers

Aldopentoses If we examine the general formula of...

Tests of Proteins

Tests of protein Biuret test (i) On adding a...

Triose

Trioses D and L Terminology: The simplest of all...

Problems on Carbohydrates

MISCELLANEOUS EXERCISES Exercise 1: Explain the...

Monosaccharide

Monosaccharides Monosaccharides are the simplest...

Classification of Amino Acid

Classification Of Amino Acid Amino acid with non...

Synthesis of Alpha Amino Acids

Synthesis of α - amino acids Protein can be...

Classification of Proteins

Classification of Proteins There are two methods...

Peptides And Proteins

Peptide & Proteins Proteins are formed by...

Uses of Proteins

Uses of Proteins (i) Protein constitute as...

Mutarotation

MUTAROTATION The two stereoisomeric forms of...

Solved Problems on Organic Chemistry

Solved Problems on Biomolecules Subjective...