Carbohydrates - Introduction, classification, preparation, properties and Uses

CARBOHYDRATES
Prof. C. Nandhakumar
Assistant Professor
Department of Chemistry
Sri Ramakrishna College of Arts & Science
Coimbatore - 641 006
Tamil Nadu, India
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Carbohydrates
• Carbohydrates constitute a major class of naturally
occurring organic compounds. They include such well-
known substances as sugars, starch and cellulose. Table
sugar, wood, cotton, potato starch, honey and milk sugar
are all carbohydrates familiar to most of us. They are
essential to the maintenance of life in both plants and
animals.
• Carbohydrates are poly functional compounds. They
contain two types of functional group and the carbonyl
group. They are poly hydroxy aldehydes (or) poly hydroxy
ketones.
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Sri Ramakrishna College of Arts and Science

Classification of Carbohydrates
The names of most of carbohydrates are characterized by
the ending ‘-ose’. Thus we have glucose, fructose,
sucrose, cellulose…….etc.
Classification:
Carbohydrates may be classified into two broad groups.
1. Sugars
2. Non-sugars (or) Polysaccharides
1. Sugars:
Sugars are sweet crystalline substances and soluble in
water.
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(i) Monosaccharides:
The monosaccharides are polyhydroxy aldehydes (or)
polyhydroxy ketones which cannot be hydrolysed into
simple sugars. Examples are glucose and fructose, both of
which have molecular formula C₆H₁₂O₆.
C₆H₁₂O₆ + H₂O → No reaction
Glucose (or) Fructose
They may again be classified according to the nature of
carbonyl group.
(a) Aldoses, which contain an aldehyde group ( –CHO)
(b) Ketoses, which contain a ketone group (C=O)
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The aldoses and ketoses are further divided into sub-
groups on the basis of the number of carbon atoms in
their molecules as trioses, tetroses, pentoses, hexoses…
etc. Thus monosaccharides are generally referred to as
aldotrisoses, aldotetroses, aldopentoses, aldohexoses,
ketohexoses….etc.
(ii) Oligosaccharides:
Oligosaccharides are sugars that yield two to ten
monosaccharide molecules on hydrolysis and are thus
again classified into various groups depending upon the
number of monosaccharide units formed on hydrolysis.
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(a) Disaccharides:
The disaccharides are sugars which on hydrolysis give two
molecules of the same (or) monosaccharides.
C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆
Sucrose Glucose Fructose
(b)Trisaccharides:
These give three molecules of monosaccharides on
hydrolysis.
C₁₈H₃₂O₁₆ + 2H₂O → C₆H₁₂O₆ + C₆H₁₂O₆ + C₆H₁₂O₆
Raffinose Glucose Fructose Galactose
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2. Non-sugars (or) Polysaccharides:
These are carbohydrates which involve a large number of
monosaccharide units linked to each other by oxide
bridges. These linkages are called glycosidic linkages. The
common and widely distributed polysaccharides
correspond to the general formula (C₆H₁₀O₅)n. They get
hydrolysed to give monosaccharides.
(C₆H₁₀O₅)n + n H₂O → n C₆H₁₂O₆
Starch Glucose
Thus chemically, polysaccharides are long chain polymers
of monosaccharides. Starch, cellulose and insulin are
examples of polysaccharides.
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Glucose
Preparation of Glucose :
1. From Sucrose (Cane Sugar):
In the laboratory glucose is made by the hydrolysis of
sucrose by boiling with dilute HCl (or) H₂SO₄ in alcoholic
solution. Glucose and fructose are obtained in equal
amounts. On cooling the resulting solution, glucose being
much less soluble than fructose separates out.
H⁺
C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆
Sucrose ∆ Glucose Fructose
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Glucose
2. From Starch : Commercially, glucose is obtained by the
hydrolysis of starch by heating with very dilute H₂SO₄ at
120˚ C under a pressure 2 to 3 atmospheres.
H⁺
(C₆H₁₀O₅)n + n H₂O → n C₆H₁₂O₆
Starch 120˚ C Glucose
2-3 atm
3. From Cellulose:
Glucose may also be obtained by the hydrolysis of
cellulose by heating with dilute H₂SO₄.
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Glucose
H⁺
(C₆H₁₀O₅)n + n H₂O → n C₆H₁₂O₆
Cellulose ∆ Glucose
Physical Properties of Glucose :
Naturally occurring glucose is a colourless, odourless,
crystalline solid, melting point 146˚C. It is very soluble in
water, sparingly soluble in alcohols and insoluble in ether.
An aqueous solution of glucose is dextrorotatory and for
this reason it is sometimes called dextrose, grape sugar
(or) Blood sugar.
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Fructose
Preparation of Fructose :
1. From Sucrose (Cane Sugar):
In the laboratory fructose is prepared by hydrolysis of
sucrose by boiling with dilute H₂SO₄.
H⁺
C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆
Sucrose ∆ Glucose Fructose
2. From Inulin :
Fructose is obtained commercially by the hydrolysis of
inulin by heating with dilute H₂SO₄.
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Fructose
H⁺
(C₆H₁₀O₅)n + n H₂O → n C₆H₁₂O₆
Inulin ∆ Fructose
Physical Properties of Fructose:
Naturally occurring fructose is a colourless, odourless,
crystalline solid, melting point 102.4˚C. It is very soluble in
water, sparingly soluble in alcohols and insoluble in ether.
An aqueous solution of fructose is laevorotatory and for
this reason it is sometimes called laevulose, fruit sugar.
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Fructose
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