Carbohydrates PPT is useful for Students and teachers

1. By
Dr. Prabhakar W. Chavan
Assistant Professor
Dept. of Chemistry (PG), Sahyadri Science College, Shivamogga-577203
Karnataka, India. Email:prabhakarchavan7@gmail.com

2. Each year, 100 metric tons of CO2 is converted to
Carbohydrates by plants

3. Introduction
• Molecular formula of is C6H12O6,fits into this general formula, C6(H2O)6
• Carbohydrates are the most abundant organic molecules in nature.
• Composed of carbon, hydrogen and oxygen
• Some common examples of carbohydrates are cane sugar, glucose, starch, etc.
• Most of them have a general formula, Cx(H2O)y, and were considered as
hydrates of carbon from where the name carbohydrate was derived.
• For example,

4.  But all the compounds which fit into this formula may not be classified as
carbohydrates.
 For example Acetic acid (CH3COOH) which canwritten as
C2(H2O)2 not a carbohydrate.
 Similarly, Rhamnose, C6H12O5 is a carbohydrate but does not fit in the mentioned
formula.
Therefore chemically, the carbohydrates may be defined as optically active
polyhydroxy aldehydes or ketones or the compounds which produce such units
on hydrolysis.
Some of the carbohydrates, which are sweet in taste, are also called sugars.

5. Functions of
Carbohydrates
Main sources of ENERGY in body (4kcal/g)
RBCs and Brain cells have an absolute requirement of carbohydrates.
Storage form of energy (starch and glycogen)
Excess carbohydrate is converted to fat.
Glycoproteins and glycolipids are components of cell membranes and
receptors.
Structural basis of many organisms.e.g. Cellulose in plants, exoskeleton of
insects, cell wall of microbes, mucopolysaccharides and ground substance in higher
organisms.
Certain carbohydrate derivatives are used as
drugs, like cardiac glycosides/
antibiotics.

6. CLASSIFICATION OF CARBOHYDRATES
CARBOHYDRATES

7. FURTHER CLASSIFICATION OF CARBOHYDRATES
Functional
group
Ketoses
Eg: Fructose
Aldoses
Eg: Glucose
Number of
carbon
atoms
Trioses
Tetroses
Pentoses
Hexoses
Heptoses
Di-
Saccharide
Tri- Tetra-
Saccharide Saccharide
Maltose
Lactose
Sucrose
Raffinose Stachyose Starch
Dextrin
Glycogen
Cellulose
Inulin
Hyaluronic
acid
Heparin
Chondrotan
Sulfate
Homopoly-
Saccharide
Heteropoly-
Saccharide
Monosaccharide Oligosaccharide Polysaccharide

8. 1. Monosaccharaides
•They contain single polyhydroxy aldehyde or ketone unit.
•These carbohydrates cannot be hydrolyzed into simpler compounds
•Example: Glyceraldehyde
2. Disaccharides
•Consists of 2 monosaccharaides units linked together by covalent bond.
•They give two monomeric units on hydrolyses.
•Example: Maltose, Sucrose, Lactose
(Maltose) ɑ-1 ----4 glysosidic linkage
Glyceraldehyde

9. 3. Oligosaccharides
•Contain 2-10 monosaccharide units.
•Give 2-10 monomeric units on hydrolysis.
•Example: Raffinose
4. Polysaccharides
• Contain very long chain of hundreds and thousands of monosaccharide units.
• They can be straight or branched.
• They are made of 1 or different types of sugar.
• All monomeric units are linked together by glycosidic linkage.
• They are carbohydrates of higher molecular weight.
• Mostly insoluble in water.
• Example: Starch,Cellulose
They are further classified into 2 types
a) Homopolysaccharides:
b) Heteropolysaccharides:

10. a) Homopolysaccharides:
These are polysaccharides made up of only one type of monosaccharide.
On hydrolysis they give only one type of monosaccharide.
Example : Starch, Cellulose
Starch
b) Heteropolysaccharides:
These are polysaccharides made up of more than one type of monosaccharide.
On hydrolysis they give two or many types of monomers.
Example: Heparin, Hyaluronic Acid
Heparin

11. Lactose Maltose Galactose Glucose Sucrose Fructose
16 32 32 74 100 173
Sugar and non – sugar :
Sugar :
The carbohydrates which are sweet in taste and dissolve in water are called sugars.
They are generally crystalline in nature.
All monosaccharides and oligosaccharides are sugars.
Eg. glucose, fructose sugar, lactose, etc.
Non – sugar :
The carbohydrates which are not sweet in taste are called non –sugars.
These are insoluble in water
They are generally amorphous in nature.
Eg. starch, cellulose, etc.
The sweetness of sucrose is taken as standard and is given a value of 100
The relative sweetness of some sugars

12. EXPLANATION OF CLASSIFICATION
1. Monosaccharides.
I. They are further classified based on functional group
a) Aldoses
When the functional group in a monosaccharide is an aldehyde (-HC=O) they are
known as aldoses
Example: Glucose, Glyceraldehyde.
b) Ketoses
When the functional group in monosaccharide is keto group (-C=O) they are
known as ketoses
Example: Fructose, Dihydroxyacetone

13. II. They are further classified based on number of
carbon atoms.
a) Trioses: contain 3 carbon atoms (example: glyceraldehyde)
b) Tetroses: contain 4 carbon atoms (example- erythrose)
c) Pentoses: contain 5 carbon atoms (example- ribose, xylose)
d) Hexoses: contain 6 carbon atoms ( example- glucose, fructose)
e) Heptoses: contain 7 carbon atoms (example- glucoheptose)
Glyceraldehyde Erythrose Ribose Fructose Glucoheptose

14. D and L Notations Of
Carbohydrates
(Monosaccharides)
1. Stereoisomerism
It is an important character of monosaccharides.
Stereoisomers are the compounds that have the same structural formulae but differ
in their spatial configuration. (three dimension structure)
Asymmetric carbon
A carbon is said to be asymmetric (chiral) when it is attached to four different
atoms or groups.
A carbon atom is said to be asymmetric when its mirror images are non-
superimposable on each other

15. D and L notations used for monosaccharides to describe their configuration.
D and L-isomers are mirror images of each other .
They differ in the spatial arrangement of –H and –OH groups on carbon
atom.
The carbon adjacent to terminal primary alcohol determines weather the sugar is
D or L- isomer.
If the –OH group on the bottom-most asymmetric carbon is on the right side,
the notation is D
If the –OH group on the bottom-most asymmetric carbon is on the left side,
the notation is L

16. In case of glucose we consider C4 carbon and in case of glyceraldehyde C2.
Enantiomers
•They are chiral molecules.
•Enantiomers are stereoisomers that are mirror images of each other.
•Non-superimposible mirror images.
•Enantiomeric pair have identical physical property.
•Enantiomers are optically active. (they differ in arrangement at the carbon atom
just above terminal carbon/alcohol.)
Naturally occurring monosaccharides in mammalian tissues are mostly of D- configuration as the
enzyme mechanism in the body is specific to metabolize D- series of monosaccharides.

18. D and L sugars are not the same as d and l sugars.
•These two abbreviations denote entirely different configurations.
•D and L denotes the position of hydroxyl group at the asymmetric carbon of a
monosaccharide,
•Whereas d and l denotes the rotation of plane polarized light i.e. d: Dextrorotatory l:
Levorotatory

20. ❖Optical activity of sugars
• The compounds that have tendency to rotate the plane polarized light are called
optically active.
• Optical activity is characteristic feature of compounds
with asymmetric carbon atom.
• When a beam if polarized light is passed
through the solution of an optical isomer, it will rotate the
light to left or right.
• The optical rotation is measured by an instrument called polarimeter.
a) Dextrorotatory (d)
•The compounds that rotate the plane polarized light to right side.
•They are denoted by (+) sign.
b) Levorotatory (l)
•The compounds that rotate the plane polarized light to left side or left.
•They are denoted by (-) sign.
c) Racemic mixture
•If D and L isomers are present in equal concentration, it is known as racemic
mixture or DL mixture.
•They do not exhibit any optical activity since dextro and levo cancel out each
other.

21. Polarimeter
Optically active molecules cause the rotation of plane-polarized light by an
amount specific to each molecule.
The measurement generated by a polarimeter is known as the observed rotation
or alphaα (observed) because it is dependent on
•The concentration of the compound,
•The length of the tube holding the solution, and
•The temperature

22. a) Anomeric carbon
• The carbon atom next to oxygen after cyclization ( not the one attached to CH2OH)
group is called anomeric carbon.
• Which means they differ in configuration at C1 (cyclic) .
•If the CH2OH group is on same side of –OH it is beta.
•If CH2OH is on opposite side of –OH it is alfa.
•An important feature is the direction of the OH group attached to the anomeric
carbon.
• Depending on the direction of the OH group, the anomeric carbon is either α or β.
•
•
α: equatorial DOWN or axial DOWN
β: equatorial UP or axial UP
b) Alfa and beta

23. • It is the change in specific optical rotation representing interconversion
of alfa and beta forms of glucose to an equilibrium mixture.
• Interconversion of alfa and beta anomers.
• The alfa and beta anomers as solids are stable but in solution they are
interconvertable.
• The alfa and beta forms of glucose are interconvertable
Mutarotation

25. • Fischer Projection: A way of representing an acyclic (open chain) carbohydrate
structure. Vertical lines point away from the viewer and horizontal lines point
toward the viewer.
• Haworth Projection: A way of representing a cyclic (closed chain)
carbohydrate. Substituents can either point up or down on this ring.
25

26. 20

27. 21

28. FISCHER TO HAWORTH STRUCTURE
To know weather the groups will lie on left or right we interchange the position of
groups on C-5 carbon.
1.The group facing right will project down. (right on ficher projection is down on haworth)
2.The group facing left will project up. (left on ficher projection is
up on haworth)
Therefore, the –CH2OH group will be pointing up in the Haworth projection.
Moreover, the–OH group on the C-2 will be pointing down and its –H substituent will
be pointing up. And, the –OH group on the C-3 will be pointing up while its – H
substituent points down, and so on.

29. • As useful as the Fischer projection is (it is an excellent way to keep track of relative
stereochemistry).
• It gives a poor sense of the real structure of carbohydrates.
• The Haworth projection is a way around this limitation that does not require you to try to
convey the complete 3D image of the molecule.
• Sugars in Haworth projection can be classified accordingto
the "ring size" (five- furanoses or six-pyranoses ) which they
assume in solution.
• A sugar with fewer than five carbons can not form a stable ring.
Haworth Projection Formulas
Furanoses
Pyranose

30. Furanoses
• The furanoses are 5-member ring hemiacetals
• These are drawn with the oxygen at the top of a pentagon.
• In solution, fructose, ribose, and deoxyribose will exist as five member furanose rings.
• The furanose ring resembles the cyclic ether called furan.
• Afuranose form of the sugar ribose is a good example.
We divide Haworth projections into two classes:
Furanoses And Pyranoses.
rings ("pyranoses") have a slightly different but quite similar Haworth
Pyranoses
•6-member
projection.
• Ahexagon is placed so that one horizontal bond runs along the bottom.
• The oxygen in the ring is placed at the upper right.
• Usually, the hemiacetal carbon (the anomeric position) is placed at the extreme right.
• In solution, glucose, galactose, and mannose will exist as six member pyranose rings.
• The sugar ring resembles the cyclic ether called pyran.

31. Pyranose and Furanose forms of Glucose

32. Pyranose and Furanose forms of Ribose

33. Reactions of Monosaccharides
Oxidation Reactions
Reduction reactions
Osazone formation
Chain Elongation (Kiliani-Fischer Synthesis)
Chain Shortening (Ruff Degradation)
Hemiacetal Formation
Acylation andAlkylation
Glycoside Formation
Anomeric Effect

34. •Glucose with different oxidizing agents gives different products.
•Glucose gives gluconic acid, glucaric acid and glucoronic acid
•Sugar on oxidation gives acid.
•The oxidation product depends
upon the oxidizing agent used in
reaction.

38. The product of this reaction is a
polyalcohol and commonly called
as an alditol.
The carbonyl group in
aldoses and ketoses can be
reduced by NaBH4 to a 1o and
2o alcohol respectively.

40. Significance of reduction
⚫ Sorbitol, mannitol and dulcitol are used to identify
bacterial colonies.
⚫ Mannitol is also used to reduce intracranial
tension by forced diuresis.
⚫ The osmotic effect of sorbitol produces changes in tissues when
they accumulate in abnormal amounts, e.g. cataract of
lens.

41. is heated with
When sugar
phenylhydrazine,
compounds are
yellow crystalline
formed. These are
called as OSAZONES.

42. •Osazones of different sugars are identified from their crystalline form.
•In this reaction monosaccharaides give needle shaped crystals.
•Maltose give sunflower shaped crystals.
•Lactose give powder-puff shaped crystals.

43. Because these are identical in their lower four carbon atoms.
Why glucose, fructose, and mannose give same type of osazones?
Non-reducing sugars like sucrose cannot form osazone due to the absence of a free carbonyl
group.

44. Formation of osazones

46. Osazone Mechanism (Fischer’s mechanism)
• Glucose and fructose reacts with phenyl hydrazine to give corresponding phenylhydrazones.
Osazone formation requires 3 molecules of phenyl hydrazine.
• In the presence of excess of reagent phenylhydrazones are converted into osazones.
• Mechanism for osazone
formation was
proposed by
Fischer and Weygand.
• Emil Fischer, who first prepare
osazone, explained the
mechanism of its formation as
follows:
First molecule
phenylhydrazine
of
reacts
with aldehydic group
the expected manner
in
to
give phenylhydrazone.

47. molecule of
hydrazine
Second
phenyl
oxidizes the
alcoholic
ketonic
secondary
group to
group.
Third
phenyl
molecule of
hydrazine
reacts with the keto
group, obtained after
oxidation, to give
osazone.

49. Carbohydrate acetals are generally called glycosides.
An acetal of glucose is called a glucoside, acetals of mannose are mannosides and acetals
of fructose are called as fructosides.
A glycosidic bond is a type of covalent bond that joins a carbohydrate (sugar) molecule to
another group, which may or may not be another carbohydrate.
A glycosidic bond is formed between the hemiacetal or hemiketal group of a saccharide
and the hydroxyl group of some compound such as an alcohol.
A compound containing a glycosidic bond is a glycoside.

50. glycoside
alcohol
bottom
is formed; when the
approaches from the of
the plane, the a-
glycoside is formed.
The reaction of a single anomer with an alcohol leads to the formation of both the - and
-glycosides. The mechanism of the reaction shows why both glycosides are formed.
•The acid protonates the OH group bonded to the anomeric carbon.
•A lone pair on the ring oxygen helps eliminate a molecule of water.
•The anomeric carbon in the resulting oxocarbenium ion is sp2 hybridized, so that part of the
molecule is planar. (An oxocarbenium ion has a positive charge that is shared by a carbon and
an oxygen.)
•When the alcohol approaches
from the top of the plane, the b-
Mechanism of glycoside formation

51. Glycosides are stable in basic solutions because they are acetals
In acidic solutions, however glycosides undergo hydrolysis to produce a sugar and an
alcohol.
The alcohol obtained by hydrolysis of glycoside is known as aglycone.
In acidic solution glycosides show mutarotation.
Example
• Salicin is a compound found in the bark of
willow trees and are used to relieve pain in
earlir days.
• Eventually scientists isolated salicin from
plants and used as analgesic.
• This can be converted into salicylic acid
which further can be converted to most
widely used modern analgesic i.e. ,Aspirin

53. In order to increase the chain length, Heinrich Kiliani suggested a method which involves
transformation of an aldose to the epimeric aldonic acids having one additional carbon
through the addition of hydrogen cyanide and subsequent hydrolysis of the epimeric
cyanohydrins.
Fischer later extended this method by showing that aldonolactones obtained from the
aldonic acids can be reduced to aldoses.
Therefore this method of transforming aldose to higher aldoses is known as
Kiliani-Fisher synthesis.
The Kiliani-Fischer Synthesis is
a method for extending a
carbohydrate chain by a single
carbon.
The Kiliani-Fischer Synthesis involves addition of cyanide ion to an open-chain aldehyde
(in the case of aldoses) which is then partially reduced and then hydrolyzed to give a new
aldehyde. In the absence of chiral reagents, a mixture of diastereomers will be produced.

54. For example, take the simplest sugar, the three-carbon aldose, glyceraldehyde.
Step 1: Formation of Cyanohydrins From Aldoses
•First nucleophilic attack of cyanide ion on the
aldehyde results in a cyanohydrin, extending the length of the
longest carbon chain from three to four.
•It also creates a new stereocenter, giving rise to
a mixture of products with
(R) and (S) configurations.
•Since the stereocenter at the C-3 carbon (R) remains unchanged by this process, in the
absence of any chiral reagents this process results in a mixture of diastereomers: (2R,
3R) and (2S, 3R).
Step 2: Reduction and Hydrolysis Converts The Nitrile Into An Aldehyde
•While these cyanohydrins can be hydrolyzed to carboxylic acids (with aqueous acid), it’s
often more useful to adopt the process for the creating of a new aldose.
•Using a poisoned catalyst (Pd/BaSO4) in the presence of hydrogen gas (H2) will reduce
the
nitrile to an imine.
•In the presence of water, the imine will then be rapidly hydrolyzed to an aldehyde.

55. Which on hydrogenation produces
two distereomeric imines followed
by hydrolysis to form long chain
aldoses.
Addition
produces
because
of HCN to glyceraldehyde two
epimeric cyanohydrins the
reaction creates a new
chirality center.
For example, application of this procedure
to D-glyceraldehyde results in the two
diastereomers D-erythrose (2R, 3R) and D-
threose (2R, 3S).
(Since these two diastereomers only differ in
the configuration at a single carbon, they are
often called “epimers”. )
The result is an extension of a sugar by one carbon (as a mixture).

56. • In 1898, Otto Ruff published his work on the transformation of D-Glucose to D-
Arabinose. Which is later called as the Ruff degradation.
• The Ruff Degradation is a method for peforming the reverse reaction.
• Ruff degradation involves oxidation of the aldose followed by oxidative decarboxylation
to form short chain aldose.
Degradation
The Ruff
method for
is a
shortening
a
carbohydrate chain by a single
carbon.
D-(-)Ribose D-Ribonic acid D-(-)Erythrose

57. Step 1: Oxidation
The aldehyde is selectively oxidized to a respective aldonic acid using bromine water.
Step 2: Oxidative decarboxylation
It involves adding an iron (III) salt [Fe2(SO4)3]i.e., ferric sulfate with hydrogen peroxide,
which involves the loss of carbon dioxide and oxidation of the adjacent C2-OH to an aldehyde.

59. Hemiacetal formation
 The product formed when one equivalent of an alcohol adds to an aldehyde or a ketone
is called a hemiacetal.
 The product formed when a second equivalent of alcohol is added is called an acetal.
 Like water, an alcohol is a poor nucleophile, so an acid catalyst is required for the
reaction to take place at a reasonable rate.
Likewise when an alcohol adds to a ketone the resulting product is known as hemiketal.
Hemi is the Greek word for “half.” When one equivalent of alcohol has added to an aldehyde
or a ketone, the compound is halfway to the final acetal, which contains groups from two
equivalents of alcohol.

60. The conversion of an alcohol and aldehyde
(or ketone) to a hemiacetal (or hemiketal)
is a reversible process. The generalized
mechanism for the process at
physiological pH is shown here:
Hemiacetal formation in monosaccharides
D-Glucose exists in three different forms: The open-chain form of
D-glucose, and two cyclic forms -D-glucose and -D-glucose.
We know that the two cyclic forms are different because they have
different melting points and different specific rotations.
How can D-glucose exist in a cyclic form?
We saw that an aldehyde reacts with an alcohol to form a hemiacetal.
The reaction of the alcohol group bonded to C-5 of D-glucose with
the aldehyde group forms two cyclic (six- membered ring)
hemiacetals.

61. • To see that the OH group on C-5 is in the proper position to attack the aldehyde group, we need to
convert the Fischer projection of D-glucose to a flat ring structure.
• To do this, draw the primary alcohol group up from the back left-hand corner.
• Groups on the right in a Fischer projection (blue) are down in the cyclic structure, and groups on the
left in a Fischer projection (red) are up in the cyclic structure.
• The cyclic hemiacetals shown here are drawn as Haworth projections.
• In a Haworth projection, the six-membered ring is represented as flat and is viewed edge-on.
• The ring oxygen is always placed in the back right-hand corner of the ring, with C-1 on the right-
hand side, and the primary alcohol group attached to C-5 is drawn up from the back left-hand
corner.

62. • There are two different cyclic hemiacetals because the carbonyl carbon of the open-chain aldehyde
becomes a new asymmetric center in the cyclic hemiacetal.
• If the OH group bonded to the new asymmetric center points down (is trans to the primary alcohol
group at C-5), the hemiacetal is -D-glucose ;
• if the OH group points up (is cis to the primary alcohol group at C-5), the hemiacetal is -D-glucose.
 The individual sugar units in a carbohydrate are held together by acetal groups.
 For example, the reaction of the aldehyde group and an alcohol group of D-glucose forms a cyclic
compound that is a hemiacetal.
 Molecules of the cyclic compound are then hooked together by the reaction of the hemiacetal
group of one molecule with an OH group of another, resulting in the formation of an
acetal.
 Hundreds of cyclic glucose molecules hooked together by acetal groups is the major component
of both starch and cellulose

63. Anomeric Effect
The preference of certain substituents bonded to the anomeric carbon for the axial
position is called the anomeric effect.
-D-glucose and -D-glucose are anomers.
Anomers are two sugars that differ in configuration only at the carbon that was the
carbonyl carbon in the open- chain form.
This carbon is called the anomeric carbon. The prefixes - and - denote the
configuration about the anomeric carbon.
Ano is greek for “upper”; thus, anomers differ in configuration at the upper-most
asymmetric carbon.

64. What is responsible for the anomeric effect?
If the substituent is axial, one of the ring oxygen’s lone pairs is in an orbital that is parallel
to the s* antibonding orbital of the C–Z bond.
The molecule then can be stabilized by hyperconjugation—some of the electron density
moves from the sp3 orbital of oxygen into the s* antibonding orbital.
If the substituent is equatorial, neither of the orbitals that contain a lone pair is aligned
correctly for overlap.

65. • The preference of certain substituents bonded to the anomeric carbon for the axial
position is called the anomeric effect.
• The anomeric carbon is the only carbon in the molecule that is bonded to two oxygen
atoms.
• The anomer with the anomeric –oh group down (axial) is called the α-anomer, and
• The one with the anomeric –oh group up (equatorial) is called the β-anomer.

66. Acylation and Alkylation
Monosaccharides undergo acetylation when heated with acetic anhydride and a little
anhydrous zinc chloride. Thus glucose land fructose form penta·acetyl derivatives.
Monosaccharides undergo alkylation with excess methyl iodide in the presence of base,
we get the penta-ether derivatives in high yield.

67. –OH
Penta acetyl
derivative
5
times
 
STEP 1
(A)

68. (B)
STEP 3
STEP 2
(C)

69. 4
(D)
STEP 5
STEP 4
4
(E)

70.  Reducingsugar:Sugars that reduce Tollens’ or Benedict’sreagents.
 Benedict’s reagent (an alkaline solution containing a cupric citrate complex ion) and
Tollens’ solution [Ag+(NH3)2OH-] oxidize and thus give positive tests with aldoses and
ketoses. The tests are positive even though aldoses and ketoses exist primarily as cyclic
hemiacetals.
 Benedict’s solution and the related Fehling’s solution (which contains a cupric tartrate
complex ion) give brick red precipitates of Cu2O when they oxidize an aldose (Scheme 6)
.
 In alkaline solution ketoses are converted to aldoses, which are then oxidized by the cupric
complexes.
 Since the solutions of cupric tartrates and citrates are blue, the appearance of a brick-red
precipitate is a vivid and unmistakable indication of a positive test.
 All sugars that contain hemiacetal or hemiketal groups (and therefore are in equilibrium
with aldehydes or α-hydroxy-ketones) are reducing sugars.
 In aqueous solution the hemiacetal form of sugars exists in equilibrium with relatively small,
but not insignificant, concentrations of noncyclic aldehydes or a-hydroxy ketones. It is
the latter two that undergo the oxidation, perturbing the equilibrium to produce more
aldehyde or α-hydroxy ketone, which then undergoes oxidation until one reactant is
ReducingSugar&Non-reducingSugar

71. tests with
Non-reducing Sugar: Carbohydrates that contain only acetal groups do not give positive
Benedict’sorTollens’ solutions, andtheyarecalled nonreducing sugars.
Oxidationof aldose using Benedict’s solution.
Fundamental difference between reducing and non-reducing sugar
Acetals do not exist in equilibrium with aldehydesor a-hydroxy ketones in the basic
aqueous media of the test reagents.

72. Disaccharides
Structural Elucidation of
Sucrose
Lactose
Cellobiose
Maltose

73. • The main disaccharides: Sucrose
Maltose
Cellobiose
Lactose
•All are isomers with molecular formula
C12H22O11
•On hydrolysis they yield 2
monosaccharide units, which soluble in
water.
•Even though they are soluble
in water, they are too large to pass 76

74. Disaccharides

75. SUCROSE
• Sucrose is a molecule composed of two monosaccharides, namely glucose and
fructose. This non-reducing disaccharide has a chemical formula of C12H22O11.
• Sucrose is commonly referred to as table sugar or cane sugar.
• In sucrose molecule, the fructose and glucose molecules are connected via a
glycosidic bond. This type of linking of two monosaccharides called glycosidic
linkage.
• William Miller, an English chemist, coined the word sucrose in the year 1857.
• Sucrose has a monoclinic crystal structure and is quite soluble in water. It is
characterized by its sweet taste. It is widely used as a sweetener in food.

76. • Sucrose is the commonest sugar known. The most common sources
are sugar cane and sugar beets and other sources are maple saps,
honey and fruit juices.
• On hydrolysis with acids or enzymes, sucrose gives equal parts of
glucose and fructose; which thus constitute the two
monosaccharides units of sucrose.
• Sucrose neither reacts with phenylhydrazine nor reduce Fehling’s
solution indicating that the carbonyl group of the both
monosaccharides involved in linkage.
of
• The glucose linked via its to the
of fructose.

77. emulsin , thus indicating an
• Sucrose is hydrolysed by maltase but not by
-D–glucose unit.
• Sucrose is also hydrolysed by an enzyme called takainvertase and thus indicates the
- D- fructofuranose unit in sucrose.
• The identities of these products demonstrate that the glucose portion is a
pyranoside(1:5 linkage) and that the fructose portion is a furanoside(2:5 linkage)

78. LACTOSE
• Lactose is a disaccharide found in milk. It is a sugar composed of galactose and glucose subunits and has
the molecular formula C12H22O11.
• Lactose is commonly referred to as milk sugar.
• It constitutes 4.5% of cow’s milk and 6.5% of human milk by weight.
• The D-galactose subunit is an acetal, and the d-glucose subunit is a hemiacetal. The subunits are joined
by a by 1,4-glycosidic linkage. Because one of the subunits is a hemiacetal.
• Lactose is a reducing sugar and undergoes mutarotation. The compound is a white, water-soluble, non-
hygroscopic solid with a mildly sweet taste. It is used in the food industry.
• Lactose was identified as a sugar in 1780 by Carl Wilhelm Scheele.

79. • Lactose reduces on fehling’s solution, Tollen’s solution and Benedict’s solution, it forms an
osazone and exhibits mutarotation. Therefore, lactose must possess at least one carbonyl
group which is not involved in he disaccharide linkage.
• On acidic or enzymatic (lactase) hydrolysis lactose gives equimolar amounts of glucose and
galactose.
• Since lactose is hydrolysed only by lactase (identical with emulisin) the two monosaccharide
units are linked through beta –glycosidic linkage. This is also indicated by its low specific
rotation.

80. • When lactose is methylated, it yields methyl heptamethyl lactose which, on vigorous
hydrolysis, yields 2,3,6-tri-o-mehyl-D-glucose and 2,3,4,6-tetra-O-methyl-D-galactose.
• The formation of these products reveals that glucose is the reducing half.
• When lactose is oxidised by bromine water, it yields lactobionic acid which, on methylation
and hydrolysis yields 2,3,5,6-tetra-O-methyl-D-gluconic acid and 2,3,6- tetra-O- methyl-D-
galactose.
• The formation of these products reveals that in lactose C-1 of glucose is linked to C-4 of
galactose.
• This further reveals that glucose is a reducing part and D- galactose in non-reducing part in
lactose.
• The point of linkage (C-4) of
galactose unit is further
confirmed by osazone
formation.

81. • When lactosazone is subjected to acid hydrolysis, it yields D-galactose and D-glucosazone.
• This reaction show that in lactose it is the glucose unit which possesses a reducing group

82. MALTOSE
•Maltose also known as maltobiose or malt sugar, is a disaccharide.
•Maltose, a disaccharide obtained from the hydrolysis of starch, contains two D-glucose subunits.
•These subunits are connected by a glycosidic linkage and this particular linkage is called an
,1,4′-glycosidic linkage.
•Maltose was 'discovered' by Augustin-Pierre Dubrunfaut.
•Maltose is a reducing sugar .

83. • When 1 mol of maltose is subjected to acid-catalyzed hydrolysis, it yields 2 mol of D-(+)-
glucose.
• Unlike sucrose, maltose is a reducing sugar; it gives positive tests with Fehling’s, Benedict’s,
and Tollens’ solutions.
• Maltose also reacts with phenylhydrazine to form a monophenylosazone (i.e., it incorporates
two molecules of phenylhydrazine).
• Maltose exists in two anomeric forms: α-(+)-maltose, [α] D 25 = + 168, and β-(+)-maltose, [α]
D 25 = + 112. The maltose anomers undergo mutarotation to yield an equilibrium mixture,
[α]D
25 = +136.
• Maltose is a reducing sugar because the right-hand
subunit is a hemiacetal
and, therefore, is in equilibrium with the open-chain aldehyde that is easily oxidized.

84. CELLOBIOSE
• Cellobiose is a disaccharide with the formula C12H22O11.
• Cellobiose, obtained from the hydrolysis of cellulose, also
contains two d-glucose subunits. Cellobiose is different from maltose.
• It is classified as a reducing sugar.
• In terms of its chemical structure, it is derived from
the condensation of a pair β- glucose molecules
forging a β(1→4) bond.
• It can be hydrolyzed to glucose enzymatically or with acid.

85. • Cellobiose has eight free alcohol (OH) groups, one acetal linkage and one hemiacetal linkage,
which give rise to strong inter- and intramolecular hydrogen bonds. It is a white solid.
• Cellobiose is different from maltose, however, because the two glucose subunits are hooked
together by a , ,4′-glycosidic linkage. Thus, the only difference in the structures of maltose
and cellobiose is the configuration of the glycosidic linkage.
• Like maltose, cellobiose exists in both  and  forms because the OH group bonded to the
anomeric carbon not involved in acetal formation can be in either the axial position (in -
cellobiose) or the equatorial position (in -cellobiose).
• Cellobiose is a reducing sugar and undergoes mutarotation because the subunit on the right is
a hemiacetal.

86. Polysaccharides
Structural Elucidation of
Starch (Amylose &Amylopectin)
Cellulose
Glycogen

87. more
•Polysaccharides are large molecules containing 10 or
monosaccharide units.
•The main polysaccharides are: Starch
Cellulose Glycogen
•Polysaccharides are complex carbohydrates made uplinked
or their derivatives held together by
monosaccharide units
•Monosaccharides
glycosidic bonds.
•Sources of polysaccharides
Microbial fermentation
Higher plants

88. Poly
Saccharides

90. Poly
saccharides

91. STARCH
•Starch is a storage compound in plants, and made of glucose units
•It is a homopolysaccharide made up of two components: amylose and amylopectin. Starch is a
mixture of amylose and amylopectin and is found in plant foods.
•Most starch is 10-30% amylose and 70-90% amylopectin.
•Amylose makes up 20% of plant starch and is made up of 250–4000 D-glucose units
bonded α(1→4) in a continuous chain.
•Long chains of amylose tend to coil.
•Amylopectin makes up 80% of plant starch and is made up of D-glucose units
connected by α(1→4) glycosidic bonds.

92. Amylose
H
O
H
O
H
OH
CH2OH
H O H
H
H
H
H O
H
O
O
H
H O
H
OH
CH2OH
H O H
H O
H
H
H
HO
O
H
OH
CH2OH
H
O
H
H
O
H
1
6CH2OH
5
4 H
O
H
3
H
1
O
H
2
OH
CH2OH
H O H
H
OH
A straight chain structure formed by 1,4 glycosidic bonds between α-D-glucose
molecules.
amylose
Structure ofAmylose Fraction of Starch
• The amylose chain forms a helix.
• This causes the blue colour change on reaction with iodine.
• Amylose is poorly soluble in water, but forms micellar
suspensions

93. • Amylopectin causes a red-violet colour
change on reaction with iodine.
• This change is
usually mased by the
much darker reaction of amylose to iodine.
Amylopectin
• Amylopectin-a glucose polymer with mainly α -(1-4)
Amylopectin
linkages, but it also has
branches formed by α -(1-6) linkages. Branches are generally longer than shown
above.
H
OH
O
H
OH
CH2OH
H OH
H
O H
H
OH
CH2OH
H OH
H
H H
O
O
H
OH
O
H
OH
CH2OH
H OH
H OH
H
H H
O
O
H
OH
CH2OH
H
OH
H
O
H H
1 4
O
H
OH
CH2OH
H OH
H
H H O
H
OH
CH2OH
H OH
H
O
H
1
O
H
OH
H
O 4
6 CH2
5
H OH
3
H 2
amylopectin
Structure of Amylopectin Fraction of Starch

94. Starch therefore consists of amylose helices entangled on
branches of amylopectin.

95. • Storage polysaccharide in animals.
• Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass.
• Glycogen is stored energy for the organism
• Similar in structureto amylopectin, only difference
from starch: number of
branches; Alpha(1,6) branches every 8-12 residues.
• Like amylopectin, glycogen gives a red-violet color with iodine.
• Glycogen is a storage polysaccharide found in animals. Glycogen is stored in
the liver and muscles.
• Its structure is identical to amylopectin, except that α(1→6) branching occurs
about every 12 glucoseunits.
• When glucose is needed, glycogen is hydrolyzed in the liver to glucose.
GLYCOGEN

96. glycogen

97. CELLULOSE
• Cellulose contains glucose units bonded (1→4).
• This glycosidic bond configuration changes the three-
dimensional shape of cellulose compared with that of amylose.
• The chain of glucose units is straight. This allows chains to align next to each other
to form a strong rigid structure.
• Cellulose is an insoluble fiber in our diet because we lack the enzyme cellulase to
hydrolyze the (1→4) glycosidic bond.
• Cellulose is important in our diet because it assists with digestive movement
in the
small and large intestine. Whole grains are a good source of cellulose.
• Some animals and insects can digest cellulose because they contain bacteria that
produce cellulase.

98. • The β-glucose molecules are joined by condensation, i.e. the removal of water, forming β-
(1,4) glycosidic linkages.
• Note however that every second β -glucose molecule has to flip over to allow the bond
to form. This produces a “heads-tails-heads” sequence.
• The glucose units are linked into straight chains each 100-1000units long.
• Weak hydrogen bonds
form between
parallel chains
binding them
into cellulose
microfibrils.
• Cellulose micro fibrils arrange themselves into thicker bundles called microfibrils. (These
are usually referred to as fibres.)
• The cellulose fibres are often “glued” together by other compounds such as
hemicelluloses