Carbohydrates Chemistry Introductory

8/14/2019 Carbohydrates Chemistry Introductory

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Dr. Maha Ismail


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Carbohydrates

Carbohydrates are the most abundant

compounds found in nature.

The name carbohydrates indicatesthat these compounds are

composed of carbon,

hydrogen and oxygen.


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Carbohydrates

Definition:

Arepolyhydroxyl

alcohols havingpotentially active

aldehyde or ketone

group attached to them.


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Function of Carbohydrates

1. Themain source of energy in the diet.2. Storage form of energy.

3. Cell membrane component that mediate

some form of intracellularcommunications.

4. Serve as structural component of manyorganisms including:

The cell wall of bacteria. The fibrous cellulose of plants. The exoskeleton of some insects


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Classification of Carbohydrates


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Classification of Carbohydrates

Monosaccharide

Are sugars which can not be degraded

further into simpler sugars.

Monosaccharides are classified according

to the number of carbon atoms they contain.


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Monosaccharide

Monosaccharides are further classified

according to the nature of the functional

group.

Thus there areAldoses for aldehyde

containing monosaccharides, and Ketoses

for those having a ketone group.

These two classifications may be used

together for exampleAldohexose and

ketohexose.


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Monosaccharide Aldehyde Ketone

Trioses (3 C Glyceraldehydes Dihydroxyacetone

Tetroses (4C Erythrose Erythrulose

Pentose (5 C Ribose Ribulose

Hexoses (6 C Glucose Fructose


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Physical properties of Monosaccharide

1. Soluble in water.

2. Aldose and ketose isomer

3. Contain asymmetric carbon atom. i.e.attached to 4 different groups e.g.

Carbon no.2 in glucose.

4. Optically active compounds.

.


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2.Aldose and Ketose isomers

Are compounds having the same

molecular formula but differ in the

functional group.

Fructose has a keto group in position 2

whereas glucose has an aldehyde

group in position 1.


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Optically active compounds.

Monosaccharides have the power of

rotating plane of polarized light through

a certain angle, but must contain at least

one asymmetric carbon atom.

Optically active substances are

classified into types according to the

direction in which they rotate thepolarized light.


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When equal amounts of dextro and

levo isomers are present in a solution

the resulting mixture has no optical

activity and the compounds are knownasRacemic Compounds


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Isomers posses the same general

physical and chemical properties.


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L and D configuration or Enantiomers

Enantiomer is a special type of isomer

found in pairs that they are mirror image

of each other.

Any compound containing asymmetric

carbon atom can be present in the space

in 2 different forms which are mirror

image and are called stereo isomers.


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L and D configuration

When the OH group of the carbon atom

adjacent to the last carbon is on the right

side it is called D- sugar, if on the left side

it is called L- sugar.

Nearly all naturally occurring sugars are D-

sugars.


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Carbohydrates


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Epimers

Isomers differ in configuration of the OH

and H around one specific carbon atom, 2,

3, and 4 of glucose.

Glucose and Galactose are epimers in

carbon 4.

Glucose and Mannose are epimers in

carbon 2.


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Cyclization of monosaccharides

The anomeric carbon of aldose or ketose

will react with the alcohol group on the

same sugar to form hemiactal or hemiketal.

If the resulting ring has six members

(5 carbons and 1 oxygen it is a pyranose

ring.

If it is five membered ( 4 carbons and

oxygen it is called furanose ring ( ex.

Fructose


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- Anomers

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- Anomers

Two molecules of D- glucose can differ from

each other, one is the other is .

The two sugars are glucose but they are

anomers of each others.


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Disaccharides

These are glycosides formed by the

condensation of 2 sugars, they

include:

1. Lactose.

2. Maltose.

3. Sucrose.


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Starch

Starch is the most important food source of

carbohydrates.

It is found in cereals, potatoes, grains, tubers &vegetable.

Starch is formed of an glycosidic chain.

It yield only glucose on hydrolysis, as it is ahomoploymer of glucose (Glucosan, or Glucan.


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>play 15-20%

80-85%

(24-30,

glucose

residues.


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It is called animal starch.

10-12,glucose


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Inulin


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Inulin

Is a starch found in tubers.

Is a homoplymer of fructose ( Fructosan.

This starch is unlike potato starch, is readily

soluble in water & has been used to determine the

glomerular filteration rate.

Chitin:

Is an important structural polysaccharide of

invertebrates.

It is found in the skeleton of insects.


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Isomers

The number of possible isomers of a

compound depends on the number of

asymmetric carbon atoms (n and is

equal to 2 n.Glucose with 4 asymmetric carbon

atoms, so it has 16 isomer

Carbohydrates Chemistry Introductory

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