This ppt explains the properties of monosaccharides, polysaccharides. the properties like mutarotation, reduction, optical activity, caramerlization, osazone is given in the ppt. Also the determination of ring size of the monosaccharide is explained/
2. Reducing sugar
Aldehydes and keto groups have reducing character and reduce
Tollens reagent and Fehling’s (Benedict’s) solution.
Carbohydrates containing free aldehyde and keto functional group
are thus reducing sugars. Example: Glucose, lactose.
3. Non-Reducing Sugar
Sugar which does not have free aldehyde or ketone functional group is
the non-reducing sugar.
Also, they do not get oxidized. Sucrose is their most common source.
They give a negative reaction for Fehling’s as well as Benedict’s test.
All polysaccharides are non-reducing sugars.
4. Comparison Between Reducing
and Non-Reducing Sugar
Reducing sugars are carbohydrates with free aldehyde or ketone group
while non-reducing one does not have free aldehyde or ketone instead
they are present in bond formation.
Reducing sugar is in hemiacetal or hemiketal form whereas non-
reducing form is in acetal or ketal form.
Reducing sugar exhibits mutarotation, on the other hand, its non-
reducing form does not exhibit it.
Reducing sugar form osazones while non-reducing does not form
osazones.
Reducing sugar is mainly monosaccharides while non-reducing is mainly
polysaccharides.
5. Properties of Monosaccharides
Colour – Monosaccharides are colourless
Shape – They are crystalline compounds
Solubility – They are readily soluble in water
Taste – They have sweet taste
Optical activity – They are optically active. They rotate the plane
polarized light. When a monosaccharide rotates the plane polarized
light in the clockwise direction or to the right (dextrorotatory) the
monosaccharide is called ‘d’ form. When the monosaccharide
rotate in the anticlockwise direction or to the left (levorotatory) the
monosaccharides is called ‘l’ form.
6. Mutarotation – Monosaccharides exhibit mutarotation. The change in
specific rotation of an optically active compound is called mutarotation.
When monosaccharide is dissolved in water, the optical rotatory power
of the solution gradually changes until it reaches a constant value.
The mutarotation is due to the existence of two optical isomers of
glucose, namely α-D-glucose with a specific rotation of +112° and β-D-
glucose with a specific rotation of +19°.
7. Glucoside formation – Glucose reacts with methyl alcohol in the
presence of hydrogen chloride gas to give glucosides. Glucoside
formation is due to the reaction of alcohol with the glucosidic OH
of monosaccharides.
In the same way fructose forms fructosides
Glucose Methyl Alcohol Methyl Glucoside
8. Esterification – Glucose reacts with 5 molecules of acetic anhydride to
form esters.
The ability of sugars to form esters indicates the presence of alcohol
groups.
As glucose yields a penta acetate derivative on acetylation, it
obviously contains five OH groups
9. Reaction with concentrated HCl
When hexoses are treated with concentrated HCl, they form 5-
hydroxymethylfurfural which on further heating yields levulinic
acid and formic acid.
This reaction is the basis of the colour test, Molisch test for sugars.
10. Formation of Oximes
• Aldoses and ketoses react with hydroxylamine to form oximes
Glucose Hydroxylamine Aldoxime Water
11. Reaction with hydrogen Cyanide
• When hydrogen cyanide is added to sugars, cyanohydrin is
produced
Glucose Hydrogen Cyanide Glucose cyanohydrin
12. Kiliani Synthesis
This reaction is the basis for the synthesis of monosaccharides.
This was proposed by Kiliani in 1886 and this method of
monosaccharide synthesis is called Kiliani synthesis
By this method the chain length of a carbohydrate is increased
13. Enolization
Glucose, fructose and mannose are interconvertible in weak alkaline
solutions such as Ca(OH)2 and Ba(OH)2 at low temperature.
This reaction is called Lobry de Bruyn-Alberda Ekenslein
conversion named after its discoverer.
14. Caramelization
When monosaccharide is added with concentrated alkali, it is burnt
and this process is called caramelization.
It produces a series of decomposition products.
Yellow and brown pigments develop, salts may form, many double
bonds between carbon atoms are formed and carbon to carbon
double bonds may rupture.
15. Phosphorylation of hexoses
The formation of a phosphoric acid derivative of hexoses is called
phosphorylation.
On phosphorylation, the hexoses are converted into phosphoric
acid esters
Phosphorylation is an important step in metabolism such as
glycolysis, kreb’s cycle, glycogenesis, glycogenolysis etc…
17. Osazone formation
Osazones are a class of carbohydrate derivatives found in organic
chemistry formed when reducing sugars are reacted with excess of
phenylhydrazine at boiling temperatures.
The osazone formation reaction was developed by famous German
chemist Emil Fischer, who used the reaction as a test to identify
monosaccharides whose stereochemistry differed by only one chiral
carbon.
18. The reaction involves formation of a pair of phenylhydrazone
functionalities, concomitant with the oxidation of the hydroxy
group on the alpha carbon (carbon atom adjacent to the carbonyl
center).
This formation allows two sugars with closely related structures to
give the same osazone.
Glucosazone and fructosazone, for example, are identical.
Since the reaction requires a free carbonyl group, only reducing
sugars can form osazones.
Sucrose, for example, is unable to form an osazone, because it is
non-reducing.
19. The mechanism involves two steps, each of which forms one of the
phenylhydrazone functionalities.
In the first step, a molecule of phenylhydrazine reacts with the
carbonyl group on the sugar to form a phenylhydrazone by
elimination of a water molecule.
The next step involves two molecules of phenylhydrazine.
The first oxidizes the hydroxy group on the reactive alpha carbon
to a carbonyl group, and the second then forms a hydrazone with it.
The end product is stabilized by a hydrogen bond between the two
phenylhydrazone groups.
20.
21. Maltosazone (from maltose) forms petal-shaped crystals
Lactosazone (from lactose) forms powder puff-shaped crystals
Galactosazone (from galactose) forms rhombic-plate shaped
crystals
Glucosazone (from glucose, fructose or mannose) forms
broomstick or needle-shaped crystals
Osazones are highly coloured and crystalline compounds and can be
easily detected. Each different osazone has a characteristic crystal
formation.
22. Properties of Polysaccharides
They do not have sweet taste
They are amorphous substances
Most of them are insoluble in water and they give opalascence
Some polysaccharides are soluble in water eg., Glycogen
They dissolve in boiling water forming colloids
They do not exhibit any of the properties of aldehyde or ketone
They are branched or unbranched
The mucopolysaccharides have a mucous consistency
23. Determination of Ring size
• The ring size of the cyclic hemiacetal structure assumed by many
monosaccharides was determined by oxidative cleavage of a
permethylated derivative.
• Five and six-membered rings are favored over other ring sizes
because of their low angle and eclipsing strain.
• The equations in the following diagram illustrate this approach for
the aldohexose, glucose.
24.
25. • First, a pentamethyl derivative is prepared, as noted earlier.
• One of the methyl ether functions in this derivative is part of an acetal,
and is therefore readily hydrolyzed by aqueous acid.
• The open chain form of this tetramethylglucose derivative is oxidized to
a keto-acid intermediate by nitric acid treatment.
• The location of the ketone carbonyl reflects the size of the initial
heterocyclic ring, C-4 for a pyranose ring and C-3 for a furanose ring.
• Further oxidation cleaves the carbon chain at bonds leading to the
carbonyl group.
• This final oxidation produces a mixture of two dicarboxylic acids,
differing in length by one carbon atom.
• Once these fragments have been identified, the location of the ketone
function is established.
