Qualitative analysis of carbohydrates

BY PR OF. LIWAYWAY MEMIJ E - C R U Z
QUALITATIVE ANALYSIS OF
CARBOHYDRATES

WHAT ARE CARBOHYDRATES?
 most abundant organic compounds found in living organisms
and are composed of carbon, hydrogen and oxygen.
 act as the primary source to provide energy for functioning of
living organisms.
 are called carbohydrates because they can be considered as
hydrates of carbon
 have the general formula Cx(H2O)y
 Carbohydrates are called saccharides.
 Some of them have
sweet taste and are
called sugars.

HOW DO YOU CLASSIFY CARBOHYDRATES
BASED ON REACTIVITY?
Based on the reactivity with several laboratory tests,
carbohydrates are classified as;
1. Reducing sugars
Carbohydrates that can reduce Tollen’s, Benedict’s or
Fehling’s reagents are called reducing sugars (sugar
with free aldehyde or ketone group). All
monosaccharides and most of the disaccharides are
reducing sugars. Some examples are Maltose and Lactose.
2. Non-reducing sugars
Carbohydrates that cannot reduce Tollen’s, Benedict’s
or Fehling’s reagents are called non-reducing sugars.
Sucrose is a non-reducing sugar.

THEORY
1. Carbohydrates are polyhydroxy aldehydes and ketones or substances that hydrolyze to
yield polyhydroxy aldehydes and ketones. Aldehydes (–CHO) and ketones ( = CO)
constitute the major groups in carbohydrates.
2. Carbohydrates are mainly divided into monosaccharides, disaccharides and
polysaccharides. The commonly occurring monosaccharides includes glucose, fructose,
galactose, ribose, etc. The two monosaccharides combine together to form disaccharides
which include sucrose, lactose and maltose. Starch and cellulose fall into the category of
polysaccharides, which consist of many monosaccharide residues.

MOLISCH’S TEST:
• Molisch’s reagent is 10%
alcoholic solution of α-
naphthol. This is a common
chemical test to detect the
presence of carbohydrates.
Carbohydrates undergo
dehydration by sulphuric acid
to form furfural
(furfuraldehyde) that reacts
with α-naphthol to form a
violet colored product

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.

BENEDICT’S TEST
• Benedict’s test distinguishes
reducing sugar from non-
reducing sugar.
• Benedict’s reagent contains
blue copper (II) ions (Cu2+,
cupric ions) that are reduced
to copper (I) ions (Cu+,
cuprous ions) by
carbohydrates. These ions
form precipitate as red
colored cuprous (copper (I)
oxide.

BARFOED’S TEST
 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.
 The reaction will be negative in the
presence of disaccharide sugars
because they are weaker reducing
agents. This test is specific for
monosaccharides . Due to the weakly
acidic nature of Barfoed's reagent, it is
reduced only by monosaccharides.

SELIWANOFF’S TEST:
It is a color reaction specific for
ketoses. When conce: HCl is added.
ketoses undergo dehydration to yield
furfural derivatives more rapidly than
aldoses. These derivatives form
complexes with resorcinol to yield
deep red color. The test reagent
causes the dehydration of
ketohexoses to form 5-
hydroxymethylfurfural. 5-
hydroxymethylfurfural reacts with
resorcinol present in the test reagent
to produce a red product within two
minutes (reaction not shown).
Aldohexoses reacts so more slowly to
form the same product.

BIAL’S TEST
Bial’s test is 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.

NYLANDER'S TEST
 It is a chemical test used
for detecting the
presence of reducing
sugars.
 When Nylander's
reagent, which consists of
bismuth nitrate,
potassium sodium tartrate
and potassium hydroxide,
is added to a solution with
reducing sugars, a black
precipitate of metallic
bismuth is formed.

IODINE TEST
 Iodine test is used to detect the
presence of starch. Iodine is not
much soluble in water so iodine
solution is prepared by
dissolving iodine in water in
presence of potassium iodide.
 Iodine dissolved in an aqueous
solution of potassium iodide
reacts with starch to form a
starch/iodine complex which
gives characteristics blue black
color to the reaction mixture.

OSAZONE TEST
 The ketoses and aldoses react
with phenylhydrazine to produce a
phenylhydrazone which further
reacts with another two molecules
of phenylhydrazine to yield
osazone.
 Needle-shaped yellow osazone
crystals are produced by glucose,
fructose and mannose, whereas
lactosazone produces mushroom
shaped crystals. Crystals of
different shapes will be shown by
different osazones. Flower-shaped
crystals are produced by maltose.

REFERENCES
• http://vlab.amrita.edu/?sub=3&brch=63&sim=631&cnt=1
• https://www.slideshare.net/namarta28/qualitative-tests-
for-carbohydrates-35884145
• https://biochemistryisagoodthing.wordpress.com/2013/02
/17/lab-review-1/
• http://amrita.olabs.edu.in/?sub=73&brch=8&sim=209&cn
t=1

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