3.2 Pests of Sorghum_Identification, Symptoms and nature of damage, Binomics,...
Qualitative tests to identify functional groups of carbohydrates
1. Qualitative tests to identify
functional groups of carbohydrates
in given solutions
(Glucose, Fructose, Sucrose,
Lactose)
DR. UDITA MUKHERJEE
UNIVERSITY OF DELHI
2. MATERIALS REQUIRED
• Test tubes, test tube stands, test tube holders,
water bath, droppers, cotton, slides, coverslips
etc.
• Conc. Sulphuric acid, a- Naphthol, iodine
solution, Barfoed’s reagent, Benedict’s
reagent, Seliwanoff’s reagent, glacial acetic
acid, phenylhydrazine, sodium acetate.
3. INTRODUCTION
• Carbohydrate, class of naturally occurring compounds and
derivatives formed from them. In the early part of the 19th
century, substances such as wood, starch, and linen were
found to be composed mainly of molecules
containing atoms of carbon (C), hydrogen (H), and oxygen (O)
and to have the general formula C6H12O6; other organic
molecules with similar formulas were found to have a similar
ratio of hydrogen to oxygen. The general formula Cx(H2O)y is
commonly used to represent many carbohydrates, which
means “watered carbon.”
4. CLASSIFICATION OF CARBOHYDRATES
• Although a number of classification schemes have been
devised for carbohydrates, the division into four major
groups—monosaccharides, disaccharides, oligosaccharides,
and polysaccharides—used here is among the most common.
• Most monosaccharides, or simple sugars, are found in grapes,
other fruits, and honey. Although they can contain from three
to nine carbon atoms, the most common representatives
consist of five or six joined together to form a
chainlike molecule. Three of the most important simple
sugars—glucose (also known as dextrose, grape sugar, and
corn sugar), fructose (fruit sugar), and galactose—have the
same molecular formula, (C6H12O6), but, because their atoms
have different structural arrangements, the sugars have
different characteristics; i.e., they are isomers.
5. • Two molecules of a simple sugar that are linked to each other
form a disaccharide, or double sugar. The
disaccharide sucrose, or table sugar, consists of one molecule
of glucose and one molecule of fructose; the most familiar
sources of sucrose are sugar beets and cane sugar. Milk sugar,
or lactose, and maltose are also disaccharides. Before the
energy in disaccharides can be utilized by living things, the
molecules must be broken down into their respective
monosaccharides.
• Oligosaccharides, which consist of three to six
monosaccharide units, are rather infrequently found in
natural sources, although a few plant derivatives have been
identified.
6. • Polysaccharides (the term means many sugars) represent
most of the structural and energy-reserve carbohydrates
found in nature. Large molecules that may consist of as many
as 10,000 monosaccharide units linked together,
polysaccharides vary considerably in size, in structural
complexity, and in sugar content; several hundred distinct
types have thus far been identified.
• Cellulose, the principal structural component of plants, is a
complex polysaccharide comprising many glucose units linked
together; it is the most common polysaccharide. The starch
found in plants and the glycogen found in animals also are
complex glucose polysaccharides. Starch (from the Old English
word stercan, meaning “to stiffen”) is found mostly in seeds,
roots, and stems, where it is stored as an available energy
source for plants.
7. QUALITATIVE TESTS FOR
CARBOHYDRATES
• Molisch’s test: General for carbohydrates.
• Iodine test: For glycans (starch, glycogen).
• Barfoed’s test: Distinguishing between
reducing sugars and disaccharides.
• Seliwanoff’s test: Distinguishing between
aldoses and ketoses.
• Benedict’s test: For reducing sugars.
• Osazone Test: For crystal formation.
8. MOLISCH’S TEST
• Molisch's test (named after Austrian
botanist Hans Molisch) is a sensitive chemical
test for the presence of carbohydrates, based
on the dehydration of the carbohydrate by
sulfuric acid or hydrochloric acid to produce
an aldehyde, which condenses with two
molecules of phenol (usually α-naphthol,
though other phenols (e.g. resorcinol, thymol)
also give colored products), resulting in a red-
or purple-colored compound.
9. PROCEDURE FOR MOLISCH’S TEST
• Chemicals required: Concentrated sulphuric acid,
freshly prepared a- Naphthol (5% w/v) in
ethanol.
• Add 2 to 3 drops of a- Naphthol to 2ml of the
testube containing the given solution. Gently
pippette out 2ml concentrated sulphuric acid and
add along the sides of the test tube, so that two
distinct layers are formed.
• Appearance of a purple ring at the junction,
indicates the presence of carbohydrates.
11. IODINE TEST
• Iodine forms colored adsorption complexes
with polysaccharides like starch. The reaction
of starch with iodine to give a blue- black color
while glycogen reacts with iodine to form a
reddish brown complex.
• Used as a convenient and a ready test for
amylose, amylopectin and glycogen.
12. PROCEDURE FOR IODINE TEST
• Chemicals required: Iodine solution (Prepare
0.005N Iodine solution in 3% w/v of potassium
iodide solution)
• Take 1 ml of the sample extract or the test
solution in a test tube and 4-5 drops of iodine
solution. Mix the contents gently and note the
formation of a colored product.
13. EXPECTED RESULTS
• Glucose, fructose, lactose and maltose do not
react with iodine whereas starch reacts with
iodine to give a blue black compound,
indicating that starch is a polysaccharide.
14. BARFOED’S TEST
• This test is used for distinguishing between
reducing monosaccharides and reducing
disaccharides. Monosaccharides usually react
in 1-2 minutes, while disaccharides take
between 7- 12 minutes to get hydrolyzed and
then react with the reagent. Brick red color is
obtained due to formation of Cu2O (cuprous
oxide).
15. PROCEDURE FOR BARFOED’S TEST
• Chemicals required: Barfoed’s reagent:
Dissolve 13.3 gm of copper acetate in 200ml
water and add 1.8ml of glacial acetic acid.
• Take 2ml of Barfoed’s reagent in a test tube
and add 1ml of sample solution to it. Keep the
test tube in a boiling water bath. Appearance
of brick red color should be observed. Note
the time taken for the color to appear.
16. EXPECTED RESULTS
• Reducing monosaccharides give brick red
color while starch and sucrose remain
unaffected.
17. SELIWANOFF’S TEST
• This test is used to distinguish aldoses from
ketoses. Ketoses undergo dehydration to give
furfural derivatives which then condense with
resorcinol to give a red colored complex.
• In case of disaccharides, prolonged heating
should be done, so that they are converted
into monosaccharides and eventually produce
the red colored complex.
18. PROCEDURE FOR SELIWANOFF’S TEST
• Chemicals required: Seliwanoff’s reagent
0.05% w/v, resorcinol in 3N HCl.
• Add 1 ml of test solution to 2ml of
Seliwanoff’s reagent and warm in boiling
water bath for 1 minute. Note the appearance
of a deep red color, which indicates that the
sample contains a ketose sugar.
19. EXPECTED RESULTS
• Only sucrose and fructose give a red complex,
indicating the presence of ketones, whereas
the others remain unaffected.
20. BENEDICT’S TEST
• Benedict's reagent is a chemical reagent named after American chemist Stanley Rossiter
Benedict.
• It is a complex mixture of sodium carbonate, sodium citrate and copper(II)
sulfate pentahydrate. It is often used in place of Fehling's solution to detect the presence
of reducing sugars. The presence of other reducing substances also gives a positive reaction.
Such tests that use this reagent are called the Benedict's tests. A positive test with Benedict's
reagent is shown by a color change from clear blue to a brick-red precipitate.
• Generally, Benedict's test detects the presence of aldehydes and alpha-hydroxy-ketones, also
by hemiacetal, including those that occur in certain ketoses. Thus, although the
ketose fructose is not strictly a reducing sugar, it is an alpha-hydroxy-ketone, and gives a
positive test because it is converted to the aldoses glucose and mannose by the base in the
reagent.
• The principle of Benedict's test is that when reducing sugars are heated in the presence of an
alkali they are converted to powerful reducing species known as enediols. Enediols reduce
the cupric compounds (Cu2+) present in the Benedict's reagent to cuprous compounds (Cu+)
which are precipitated as insoluble red copper(I) oxide(Cu2O).
21. PROCEDURE FOR BENEDICT’S TEST
• Chemicals required: (1) Dissolve 173g of sodium
citrate and 100g of anhydrous sodium carbonate
in 600ml of hot water, which is to be diluted to
800ml with hot water.
• (2) Dissolve 17.3g of copper sulphate
pentahydrate in 100ml of fresh water. Cool and
dilute to 100ml.
Add reagent number 2 to reagent number 3, slowly
with constant stirring making the final volume to 1l.
22. • Add 0.5ml to 1ml of test solution to 2ml of
Benedict’s reagent. Keep test tubes in boiling
water bath. Observe the formation of red
precipitate whose appearance would suggest
the presence of reducing sugars in the given
solution.
23. EXPECTED RESULTS
• Sucrose and starch do not give the test, while the rest do. The
color of the obtained precipitate gives an idea about the
quantity of sugar present in the solution, hence the test is
semi-quantitative. A greenish precipitate indicates about 0.5
g% concentration; yellow precipitate indicates 1 g%
concentration; orange indicates 1.5 g% and red indicates 2 g%
or higher concentration.
24. OSAZONE TEST
• This test is also known as the phenylhydrazine
test. It involves the reaction of a
monosaccharide with phenylhydrazine. All
reducing sugars form osazones with excess of
phenylhydrazine when kept at boiling
temperatures.
25. PROCEDURE FOR OSAZONE TEST
• Chemicals required: Glacial acetic acid,
phenylhydrazine, sodium acetate.
• Take 5ml of test solution and add 10 drops of glacial
acetic acid. Add a pinch of phenylhydrazine to it, and
twice the amount of sodium acetate. Heat the solution
to dissolve the solute. Keep the solution in boiling
water bath with a cotton plug for 30- 40 minutes.
Observe the change of color. Once the color changes,
take out the test tube and cool it under tap water. Note
the shape of the crystals and the time needed for its
formation.
26. EXPECTED RESULTS
• Glucose and fructose containing tubes shall
turn cloudy in approximately 10 minutes,
while the other sugars would take longer.
Fructose Glucose Sucrosee
Lactose