Introduction to carbohydrates, classification of carbohydrates and identification test for the reducing sugars

1. Experiment :2
QUALITATIVE ANALYSIS OF REDUCING SUGARS

2. Introduction :
Definition :
Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones, primarily
composed of carbon, hydrogen and oxygen. These are the hydrates of carbon.
Carbohydrates are also known as Saccharides (Sakcharon means Sugar,
Greek) having general formula Cn (H2O)n where n≤3.
• Ex: Glucose - C6 (H2O) 6 orC6H12O6

3. • Carbohydrates occur abundantly in nature i.e. in plants and animals.
In plants: (Photosynthesis)
The synthesized glucose is stored in the form of starch and used to
synthesize cellulose of the plant framework.
In animals:
• The metabolic synthesized glucose is stored in the form of glycogen, serves as an important
source of energy for vital activities.

4. • Functions of carbohydrates :
✓ They are the most abundant dietary source of energy (4Cal/g) for all organisms.
✓ Carbohydrates serve as the storage form of energy (glycogen) to meet the immediate energy
demands of the body.

5. ✓Carbohydrates are essential building blocks of important macro molecules such
as RNA, DNA & ATP
.
✓Carbohydrates participate in the structure of cell membrane and cellular
functions such as cell growth, adhesion and fertilization.
✓Carbohydrates help in metabolism of lipids or fats.

6. Classification of carbohydrates:
According to the number of basis sugar or saccharide
incorporated in the molecule carbohydrates can be classified into
units
✓ Monosaccharides
✓Oligosaccharides
✓Polysaccharides

7. • MONOSACCHARIDES:
Monosaccharides are the simplest form of carbohydrates.
➢Consist of one sugar.
➢Usually colourless.
➢Sweat taste.
They cannot be further hydrolyzed into simple carbohydrates.
The monosaccharides that contains an aldehyde group are called aldoses.
Ex: Glucose Those contains a
keto group are called ketoses.
Ex: fructose

8. •Monosaccharides are categorized by the number of carbon.
Trioses :
simplest monosaccharides.
Contains three carbon atoms.
Ex: D- Glyceraldehyde , Dihydroxy acetone
Tetroses :
Contains four carbon atoms.
They have either a functional aldehyde group in position 1 or functional keto group in position 2.
Ex : D- Erythrose.
Pentoses:
Contains five carbon atoms.
Aldopentose have an functional aldehyde group at position 1 and ketopentose have an functional
ketone group at position 2 or 3.
Ex :xylose , ribulose, deoxyribose

9. Hexoses:
Contains six carbon atoms.
Aldohexoses having functional aldehyde group at position 1 and ketohexoses having functional
ketone group at position 2.
Ex : glucose and fructose.
Heptoses:
Contains seven carbon atoms.
Aldophetose have a functional aldehyde group in position 1 or ketoheptose have a functional
ketone group in position 2.
Ex : D-Sedoheptulose.

10. Oligosaccharides:
• Oligosaccharides is a saccharide polymer containing a small number typically three to ten
components of sugar.
• The name is derived from Greek word.
• They generally found either ‘O’ or ‘N’- linked to compatible amino acid side chain in protein /
lipids.
• They joined together by a specific glycosidic bond.
Disaccharide
T
risaccharides

11. • A disaccharide consist of two monosaccharide units (similar or dissimilar) held together
by a glycosidic bond.
Disaccharides
REDUCING
SUGARS
NON REDUCING
SUGARS
Disaccharides with free aldehyde or ketogroup.
Ex: Maltose, Lactose….
Disaccharides with no free aldehyde or ketogroup.
Ex: sucrose, trehalose….

13. Polysaccharides:
• A polysaccharide consists of repeat monosaccharide units (similar or dissimilar) or their
derivatives held together by a glycosidic bond. These are mainly concerned with storage of
energy.
Homopolysaccharides:
These contain only one type of monosaccharide, which on hydrolysis yield only a single type
of monosaccharide.
• Ex: Starch, inulin, Cellulose, glycogen….
• Starch consists of two polysaccharide components, water soluble
• amylose (15-20%) and water insoluble amylopectin (80-85%).
• Heteropolysaccharides :
They contains different type of monosaccharides which on hydrolysis yields a mixture of
monosaccharide units.
Ex: heparin, chondroitin, keratin, dermatan, …

14. Identification test for REDUCING SUGARS :
Molisch’s Test (generaltest):
This is a common test for all carbohydrates larger than tetroses. The test is on the basis that
pentoses and hexoses are dehydrated by conc. sulphuric acid to form furfural or hydroxymethylfurfural, respectively. These
products condense with α-naphthol to form purple condensation product.
Reducing sugar:
The ability of a sugar to reduce alkaline test reagents depends on the availability of an aldehyde or keto
group for reduction reactions. These sugars, therefore, become potential agents capable of reducing Cu+2 to Cu+, Ag+ to Ag
etc.
Fehling’s Test:
This forms the reduction test of carbohydrates. Fehling’s solution contains blue alkaline cupric hydroxide solution,
heated with reducing sugars gets reduced to yellow or red cuprous oxide and is precipitated. Hence, formation of the yellow or
brownish-red colored precipitate helps in the detection of reducing sugars in the test solution.

15. Benedict’s test:
As in Fehling’s test, free aldehyde or keto group in the reducing sugars reduce cupric hydroxide in alkaline medium to
red colored cuprous oxide. Depending on the concentration of sugars, yellow to green color is developed.
Barfoed’s Test:
Barfoed's test is used to detect the presence of monosaccharide (reducing) sugars in solution . Barfoed's reagent, a
mixture of ethanoic (acetic) acid and copper(II) acetate, is combined with the test solution and boiled. A red copper(II) oxide
precipitate is formed will indicates the presence of reducing sugar.
Seliwanoff’s test:
Seliwanoff’s Test distinguishes between aldose and ketose sugars. Ketoses are distinguished from aldoses via their
ketone/aldehyde functionality. This test is based on the fact that, when heated, ketoses are more rapidly dehydrated than
aldoses.

16. • Bial’s test
Used to distinguish between pentoses and hexoses. They react with Bial’s reagent and are converted to furfural.
Orcinol and furfural condense in the presence of ferric ion to form a colored product. Appearance of green colour or
precipitate indicates the presence of pentoses and formation of muddy brown precipitate shows the presence of hexoses.

18. S. No. Test Observation Inference
1. Molisch’s Test
In a test tube, add 2 ml of the test carbohydrate
solution and 2 drops of α- naphthol solution. Carefully
incline the tube and pour dropwise conc. H2SO4, using a
dropper, along the sides of the tube.
A deep violet coloration is
produced at the junction of
two layers.
Presence of carbohydrates
.
2. Fehling's test
About 2 ml of sugar solution is added to about 1 ml of
Fehling’s solution A (aqueous solution of CuSO4) and 1
ml of Fehling solution B (solution of potassium tartrate).
Mix well and then boiled for 10 min.
A red precipitate of cuprous
oxide is formed
Presence of reducing sugar
(glucose, fructose, maltose,
lactose)
3. Benedict’s test
To 2 ml of Benedict's solution, add 5-6 drops of the test
solution and shake each tube. Place the tube in a boiling
water bath and heat for 3 minutes. Remove the tubes
from the heat and allow them to cool.
Color change from blue to
green, yellow, orange or red
according to the amount
present
Presence of reducing sugars
(glucose, fructose, maltose,
lactose)
4. Barfoed’s reagent test
To 1-2 mL of Barfoed’s reagent, add an equal volume of
sugar solution. Boil for 1 min. in a water bath and allow
to stand for few minutes.
a brick-red cuprous oxide
precipitate in the bottom and
along sides of test
tube immediately
Presence of Monosaccharides
(glucose, fructose)

19. S. No. Test Observation Inference
5. Seliwanoff’s test
Heat 1 mL of sugar solution with 3 mL
Seliwanoff’s reagent (0.5 g resorcinol
per liter 10% HCl) in boiling water.
In less than 30 seconds,
appearance of cherry red
color
Upon prolonged heating,
appearance of red colour
Presence of ketoses
(Fructose)
Presence of aldoses
(Glucose)
6. Bial’s test
To 5 mL of Bial’s reagent add 2–3 mL
of test solution and warm gently in a
hot water bath for 2minutes is
indicative of pentoses.
The formation of a
bluish green product
muddy brown products
Presence of pentoses
Presence of hexoses