Unit-1

Carbohydrates

Carbohydrates are a group of naturally occurring carbonyl compounds (aldehydes or

ketones) that also contain several hydroxyl groups. It may also include their derivatives
which produce such compounds on hydrolysis. They are the most abundant organic
molecules in nature and are also referred to as “saccharides”. The carbohydrates which are
soluble in water and sweet in taste are called “sugars”.

Structure of Carbohydrates
 Carbohydrates consist of carbon, hydrogen, and oxygen.
 The general empirical structure for carbohydrates is (CH2O)n.
 They are organic compounds organized in the form of aldehydes or ketones with multiple
hydroxyl groups coming off the carbon chain.
 The building blocks of all carbohydrates are simple sugars called monosaccharides.
 A monosaccharide can be a polyhydroxy aldehyde (aldose) or a polyhydroxy ketone (ketose).
The carbohydrates can be structurally represented in any of the three forms:
 Open chain structure.
 Hemi-acetal structure.
 Haworth structure.
Open chain structure – It is the long straight-chain form of carbohydrates.
Hemi-acetal structure – Here the 1st carbon of the glucose condenses with the -OH group of
the 5th carbon to form a ring structure.
Haworth structure – It is the presence of the pyranose ring structure.

Physical Properties of Carbohydrates

 Stereoisomerism – Compound shaving the same structural formula but they differ in spatial
configuration. Example: Glucose has two isomers with respect to the penultimate carbon
atom. They are D-glucose and L-glucose.
 Optical Activity – It is the rotation of plane-polarized light forming (+) glucose and (-)
glucose.
 Diastereo isomers – It the configurational changes with regard to C2, C3, or C4 in glucose.
Example: Mannose, galactose.
 Annomerism – It is the spatial configuration with respect to the first carbon atom in aldoses
and the second carbon atom in ketoses.

Chemical Properties of Carbohydrates

 Osazone formation: Osazone are carbohydrate derivatives when sugars are reacted with an
excess of phenylhydrazine. eg. Glucosazone
 Benedict’s test: Reducing sugars when heated in the presence of an alkali gets converted to
powerful reducing species known as enediols. When Benedict’s reagent solution and reducing
sugars are heated together, the solution changes its color to orange-red/ brick red.
 Oxidation: Monosaccharides are reducing sugars if their carbonyl groups oxidize to give
carboxylic acids. In Benedict’s test, D-glucose is oxidized to D-gluconic acid thus, glucose is
considered a reducing sugar.
 Reduction to alcohols: The C=O groups in open-chain forms of carbohydrates can be
reduced to alcohols by sodium borohydride, NaBH4, or catalytic hydrogenation (H2, Ni,
EtOH/H2O). The products are known as “alditols”.

Classification of Carbohydrates (Types of Carbohydrates)

The simple carbohydrates include single sugars (monosaccharides) and polymers,

oligosaccharides, and polysaccharides.
Monosaccharides
 The simplest group of carbohydrates and often called simple sugars since they cannot be
further hydrolyzed.
 Colorless, crystalline solids that are soluble in water and insoluble in a non-polar solvent.
 These are compound that possesses a free aldehyde or ketone group.
 The general formula is Cn(H2O)nor CnH2nOn.
 They are classified according to the number of carbon atoms they contain and also on the
basis of the functional group present.
 The monosaccharides thus with 3, 4, 5, 6, 7 … carbons are called trioses, tetroses, pentoses,
hexoses, heptoses, etc., and also as aldoses or ketoses depending upon whether they contain
aldehyde or ketone group.
 Examples: Glucose, Fructose, Erythrulose, Ribulose.
Properties of Monosaccharides
 Most monosaccharides have a sweet taste (fructose is sweetest; 73% sweeter than sucrose).
 They are solids at room temperature.
 They are extremely soluble in water: – Despite their high molecular weights, the presence of
large numbers of OH groups makes the monosaccharides much more water-soluble than most
molecules of similar MW.
 Glucose can dissolve in minute amounts of water to make a syrup (1 g / 1 ml H2O).

More Properties of Monosaccharide continued……….

Epimerization:-
Epimer in stereochemistry specifies one of a pair of stereoisomers. At the stereogenic centre,
two isomers present in the molecule differ, while the rest remains identical. A molecule may
contain numerous stereocenters leading to several stereoisomers.

Stereoisomers are isomeric molecules that possess the same constitution and molecular formula,
but they vary in three-dimensional orientations of their atoms in space.

Epimers – Example

Below, stereoisomers illustrate the D and L configurations of glucose. Here, glucose is referred
the D and L on the basis of last chiral carbon atom.

Glucose involves the formation of glycogen, starch, glucose, oligosaccharides and

polysaccharides. Due to the presence of carbon in glucose molecule it may exhibit
stereoisomerism, that is enantiomers and diastereomers.

Enantiomers

Optical isomers or enantiomers are two isomers that are relevant to each other by reflection.
They are non – superimposable mirror images of each other. They are comprised of the same
physical properties except in a way they interact with several optical isomers of other
compounds. Hence, different optical isomers may have variant biological effects.

Diastereomers
Diastereomers are stereoisomers that are not mirror images of each other in that they are not
linked with reflection operation unlike of enantiomers. They possess the same physical
properties. One of the example include meso compounds. The below structure is mesotartaric
acid.

 Epirubicin and Doxorubicin are epimers that are used in drugs.

Isomerization of Carbohydrates

Aldoses and ketoses undergo epimerization and other isomerization in basic

solutions through keto-enol tautomerization. For example, a solution of D-glucose was found to
establish an equilibrium containing both D-glucose and D-mannose:

This type of reaction, characteristic of monosaccharides, was first discovered in 1895 by two
Dutch chemists, Cornelius Adriaan van Troostenbery Lobry de Bruyn and Willem Alberda van
Ekenstein. They are therefore known as Lobry de Bruyn–Alberda van Ekenstein reactions.

For example, let’s see the mechanism behind the transformation of glucose to mannose. When
exposed to a base, the ɑ carbon of glucose is deprotonated forming an enolate ion:
After this, it is simply about the protonation of this enolate.
Mannose is formed by protonation at one face of the double bond while protonation at the other
face regenerates glucose:

This mixture is also obtained when D-mannose, rather than D-Glucose, is exposed to a base.

If we summarize what happens during this process, we see that one isomer is transformed
into two epimers. Remember that D-Glucose and D-mannose are epimers at C-2 because that is
the only chiral center with a different configuration. This process is therefore
called epimerization and to avoid it when not desired, carbohydrates should not be stored in
basic conditions.
Oligosaccharides
 Oligosaccharides are compound sugars that yield 2 to 10 molecules of the same or different
monosaccharides on hydrolysis.
 The monosaccharide units are joined by glycosidic linkage.
 Based on the number of monosaccharide units, it is further classified as a disaccharide,
trisaccharide, tetrasaccharide, etc.
 Oligosaccharides yielding 2 molecules of monosaccharides on hydrolysis is known as a
disaccharide, and the ones yielding 3 or 4 monosaccharides are known as trisaccharides and
tetrasaccharides respectively, and so on.
 The general formula of disaccharides is C n(H2O)n-1and that of trisaccharides is Cn(H2O)n-2 and
so on.
 Examples: Disaccharides include sucrose, lactose, maltose, etc.

Trisaccharides are Raffinose, Rabinose.

Polysaccharides
 They are also called “glycans”.
 Polysaccharides contain more than 10 monosaccharide units
and can be hundreds of sugar units in length.
 They yield more than 10 molecules of monosaccharides on
hydrolysis.
 Polysaccharides differ from each other in the identity of their
recurring monosaccharide units, in the length of their chains,
in the types of bond linking units and in the degree of
branching.
 They are primarily concerned with two important functions ie.
Structural functions and the storage of energy.
 They are further classified depending on the type of molecules
produced as a result of hydrolysis.
 They may be homopolysaccharidese, containing
monosaccharides of the same type
or heteropolysaccharides i.e., monosaccharides of different
types.
 Examples of Homopolysaccharides are starch, glycogen,
cellulose, pectin.
 Heteropolysaccharides are Hyaluronic acid, Chondroitin.
Functions of Carbohydrates
Carbohydrates are widely distributed molecules in plant and animal tissues. In plants and
arthropods, carbohydrates from the skeletal structures, they also serve as food reserves in plants
and animals. They are important energy sources required for various metabolic activities, the
energy is derived by oxidation.
Some of their major functions include
 Living organisms use carbohydrates as accessible energy to fuel cellular reactions. They are
the most abundant dietary source of energy (4kcal/gram) for all living beings.
 Carbohydrates along with being the chief energy source, in many animals, are instant sources
of energy. Glucose is broken down by glycolysis/ Kreb’s cycle to yield ATP.
 Serve as energy stores, fuels, and metabolic intermediates. It is stored as glycogen in animals
and starch in plants.
 Stored carbohydrates act as an energy source instead of proteins.
 They form structural and protective components, like in the cell wall of plants and
microorganisms. Structural elements in the cell walls of bacteria (peptidoglycan or murein),
plants (cellulose), and animals (chitin).
 Carbohydrates are intermediates in the biosynthesis of fats and proteins.
 Carbohydrates aid in the regulation of nerve tissue and is the energy source for the brain.
 Carbohydrates get associated with lipids and proteins to form surface antigens, receptor
molecules, vitamins, and antibiotics.
 Formation of the structural framework of RNA and DNA (ribonucleic acid and
deoxyribonucleic acid).
 They are linked to many proteins and lipids. Such linked carbohydrates are important in cell-
cell communication and in interactions between cells and other elements in the cellular
environment.
 In animals, they are an important constituent of connective tissues.
 Carbohydrates that are rich in fiber content help to prevent constipation.
 Also, they help in the modulation of the immune system.
 Qualitative Tests for Carbohydrates

Potato consists of different carbohydrates like starch, reducing sugars etc. Difficulties are
encountered in the qualitative and quantitative analysis of samples containing mixtures of
carbohydrates, particularly the sugars, because of their structural and chemical similarity and
also with respect to their stereoisomers. During biochemical investigations it may because
necessary to establish whether a given sample, particularly of a purified preparation, consist
carbohydrates or not. Several rapid tests are available the presence or absence of a sugar or a
carbohydrate in a sample. These tests are based on specific colour reactions typical for their
group and are described below

The types of carbohydrates detected by these tests are:

Name of the test Application

1. Molisch’s Test General test for carbohydrates

2. Anthone Test General test for carbohydrates
3. Iodine Test For glycans (starch, glycogen)
4. Barfoed’s Test To distinguish between mono-saccharides from reducing diasaccharides
5. Seliwanoff’s Test For Ketones
6. Fehling’s Test For reducing sugars
7. Bendict’s Test For reducing sugars
8. Picric acid Test For reducing sugars
9. Bial’s Test For pentoses

1) MOLISCH’S TEST

Principle
This is a general test for all carbohydrates. Conc. H 2SO4 hydrates glycosidic bonds to yield
monosaccharides which in the presence of an acid get dehydrated to form furfural and its
derivatives. These products react with sulphonated α-naphthol to give a purple complex.
Polysaccharides and glycoproteins also give a positive reaction.
 Reaction

2) ANTHRONE TEST

Principle

Anthrone reaction is another general test for carbohydrates. In this the furfural produced reacts
with anthrone to give bluish green colored complex.

Reaction

3) IODINE TEST

Principle
Iodine forms colored adsorption complexes with polysacchaides. Starch gives blue color with
iodine, while glycogen reacts to form reddish brown complex. Hence it is useful, convenient and
rapid test for detection of amylase, amylopectin and glycogen.

4) Barfoed’s Test

Principle

This test is used for distinguishing monosaccharides from reducing disaccharides.

Monosaccharides usually react in about 1 - 2 min while the reducing disaccharides take much
longer time between 7 - 12 min to get hydrolysed and then react with the reagent. Brick red color
is obtained in this test which is due to the formation of cuprous oxide.

Reaction

(CH3COO)2Cu2 + H2O ® 2CH3COOH + Cu(OH)2

Cupric acetate Cupric hydroxide

Cu(OH)2 ® CuO+H2O

5) SELIWANOFF’S TEST

Principle
This test is used to distinguish aldoses from ketoses. Ketoses undergo dehydration to give
furfural derivatives, which then condense with resorcinol to form a red complex. Prolonged
heating will hydrolyze disaccharides and other monosaccharides will also eventually give color.

Reaction

6) Fehling’s Test

Principle

Fehling’s test is a specific and highly sensitive for detection of reducing sugars. Formation of
yellow or red ppt of cuprous oxide denotes the presence of reducing sugars. Rochelle salt acts as
the chelating agent in this reaction.

Reaction

7) Benedict’s test
Principle

Benedict’s test is more convenient and this reagent in more stable. In this method sodium citrate
functions as a chelating agent. Presence of reducing sugars results in the formation of red ppt of
cuprous oxide.

8) Picric acid test

Principle

It is another test for detection of reducing sugars. The reducing sugars react with picric acid to
form a red colored picramic acid.

Reaction

9) Bial’s test
Principle

This test is useful in the determination of pentose sugars. Reaction is due to formation of
furfural in the acid medium which condenses with orcinol in presence of ferric ions to give
a blue-green colored complex which is soluble in butyl alcohol.

Reaction

Note:
1-Frame the Notes in your language.
2-Reagent should be remembered more than the reaction.
3- All the basic of carbohydrate discussed here…from the Uni-1
4- Next notes will continue with Carbohydrate metabolism.