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57.3 MONOSACCHARIDES

As pointed out earlier, these are the simplest carbohydrates and cannot be hydrolysed to give still simpler carbohydrates. Monosaccharides containing aldehyde group are called aldoses while those containing ketonic group are called ketoses. They can be further classified into different
categories depending upon the number of carbon atoms. For example,

Monosaccharides with three carbon atoms are called trioses.

Monosaccharides with four carbon atoms are called tetroses.

Monosaccharides with five, six and seven carbon atoms are called pentoses, hexoses and heptoses respectively. 

GLUCOSE

(DEXTROSE OR GRAPE SUGAR), C6H12O6

Glucose occurs in nature in free state as well as in combined state. In free state, it occurs in sweet fruits (such as grapes, mangoes, oranges) and honey. Ripe grapes contain about 20% glucose and hence the name grape sugar. In combined state, glucose is present in di-and polysaccharides. Maltose and starch on hydrolysis yield only glucose.

Preparation of Glucose 

1.      By Hydrolysis of Cane-sugar. In laboratory, glucose can be prepared by hydrolysis of cane-sugar in the presence of alcohol using dilute hydrochloric acid. Glucose and fructose are formed in equal amounts. Glucose, being less soluble in ethyl alcohol than fructose, crystallizes out.

H+

C12H22O11 + H2O     →     C6H12O6 + C6H12O6

                                                                                 Glucose      Fructose

2.      By Hydrolysis of Starch. Glucose is obtained, on commercial scale, by hydrolysis of starch by boiling it with dilute H2SO4 acid at 393 K under a pressure of 2-3 atm.

H+

(C6H10O5)n + nH2O                     →                nC6H12O6

Starch                                 393K.2-3 atm.           Glucose

Physical Properties of Glucose

1.      Glucose is a white crystalline solid. Its melting point is 419 K.

2.      It is sweet in taste.

3.      It is very soluble in water due to its ability to form hydrogen bonds with water. Glucose is insoluble in organic solvents such as ether.

Structure of Glucose

Glucose is an aldohexose. It is monomer of many of the larger carbohydrates such as starch, cellulose. The reactions of glucose indicate that its molecule contains one  primary (-CH2OH) and four secondary (-CHOH) by hydroxyl groups. Glucose was assigned the following structure:

Glucose also exists in two cyclic forms, α-D-glucose and β-D-glucose. The cyclic structures of glucose are shown below:

α-D-glucose

β-D-glucose

FRUCTOSE (LAEVULOSE), C6H12O6

Fructose is a ketohexose. It is obtained along with glucose by the hydrolysis of sucrose. It is found in nature in ripe fruits and honey.

Physical Properties of Fructose

1.      Fructose is a white crystalline solid. Its melting point in 377 K

2.      It is highly soluble in water. It is insoluble in organic solvents such as ether.

3.      It is very sweet in taste and is used in making sweets,

Structure of Fructose

Fructose has the molecular formula C6H12O6 and on the basis of its reactions it was found to contain a ketonic functional group at C-2 and six carbon atoms in straight chain as in case of glucose. Its open chain structure may be written as:

Fructose also exists in two cyclic forms, (α- D- fructose and β-D-fructose.

The two cyclic structures ‘of fructose are represented by Haworth structures as given: