Transcript Lab 6 Identification of unknown carbohydrates

Carbohydrates
Identification of Unknown Carbohydrates
By
Dr. Mohammed Golam Rasul
2
3
Objective
• Study the various qualitative tests of carbohydrates.
• Identify an unknown carbohydrate.
Definition
• Organic compounds composed of C, H and O with H
and O present in the same ratio as in water.
• Example: Glucose C6H12O6. [Cm(H2O) n ] m, n= where m and n are
integer.
• Exceptions:
– Deoxy sugars such as Rhamnose C6H12O5, Acetic acid
C2H4O2. Formaldehyde HCHO, Lactic acid C3H6O3
• New definition:
Optically active Polyhydroxy aldehydes or ketones, or
substances that hydrolyze to yield polyhydroxy
aldehydes or ketones.
Physical Characteristics
•
•
•
State:
Sugars are white, crystalline in shape and with
sharp melting points, while polysaccharides
are white amorphous solids.
Taste:
Sugars have a sweet taste. Polysaccharides
are tasteless.
Solubility:
Monosaccharides are soluble in cold water and
hot alcohol. Polysaccharides are partially
soluble in hot water.
Qualitative Analysis Test
• Is
concerned
with
determining the identity of a
substance.
• Enables us to detect the
presence of things which
may be beyond the reach of
our senses.
Types of Carbohydrates
• Monosaccharides are the simplest carbohydrates
- Also called “simple sugars”
- Examples: glucose, fructose, galactose, ribose
• Disaccharides are two monosaccharides bonded
together.
- Examples: sucrose (table sugar), lactose (milk
sugar)
• Polysaccharides are polymers of monosaccharides
- Can be split into many monosaccharides with acid or
enzymes
- Examples: starch, cellulose, glycogen
Carbohydrates Characterization
• Molisch’s Test : carbohydrate
• Benedict’ s Test : color change indicator for simple
sugars
• Fehling Test: Reducing sugars
• Barfoed’s
Test:
Monosaccharides
and
Oligosaccharides
• Seliwanoff Test : Differentiation of Ketose from
Aldose
• Iodine Test : Cellulose, Starch, Dextrin
Qualitative Tests for Carbohydrates
1. Molisch’s Test
- test for carbohydrates
Pentose/
Hexose
conc.H2SO4
- H2O
a-naphthol
Furfural
5-hydroxymethyl furfural
(Purple complex)
2. Barfoed’s Test
- test for reducing sugars
- test for distinguishing monosaccharides and disaccharides
Reducing sugar + 2Cu2+
Slightly acidic
2Cu+ + oxidized sugar
Cu2O
(brick red ppt)
3. Seliwanoff Test
- test for distinguishing ketose from aldose
Ketohexose
conc. HCl
dehydration
4. Iodine Test
- test for starch
Starch + I2
5-hydromethyl
furfural
resorcinol
starch - I2 complex (deep blue)
red complex
Qualitative Tests for Carbohydrates
5. Fehling’s Test
- test for reducing sugars
- test for distinguishing monosaccharides and
disaccharides
Reducing sugar + Fehling reagent
Boiling in water bath
Cu2O
( red ppt)
6. Benedict’s Test
- test for reducing sugar
Reducing sugar + Benedict’s reagent
Boil
red ppt
Molisch’s Test
• Test for carbohydrate materials
Procedure: Using 2 ml of test solution in a test
tube, add 2 drops of 1% a 1-naphthol, mix and then
allow 2 ml of conc. Sulphuric acid to flow down the
side of the inclined tube to layer under the mixture.
Note the colour at the junction.
Barfoed’s Test
• Test for Monosaccharides and Oligosaccharides
Procedure: To 2 ml of a sample solution is placed in
a test tube. 2 ml of Barfoed's reagent is added. The
solution is then heated in a boiling water bath for
three minutes.
Seliwanoff Test
• Test for Differentiation of Ketose from Aldose
Procedure: Add 0.5 ml of the test solution to 2.5 ml
of the Seliwanoff’s reagent. Place the tubes in boiling
water and note the results and continue heating,
observe the colour change at after 1 minute.
Iodine Test
• Test for Cellulose, Starch, Dextrin
Procedure: Add a drop of dil. HCl to 1 ml test
solution to acidify the solution. Add few drops of
iodine solution. Record the colours observed. Gently
warm the solution and then cool it. Note he change.
Fehling’s Test
• Test for Monosaccharides and Oligosaccharides
Procedure: Using 0.5 ml of test solution, mix
with 2.0 ml of Fehling’s solution in a test tube and
heat the mixture. Red precipitate indicates the
presence of reducing sugar.
Benedict’s Test
• Test for Monosaccharides and Oligosaccharides
Procedure: Using 0.5 ml of test solution, mix with
5 ml of Benedict’s reagent in a test tube and heat
the mixture. Appearance of red precipitate indicate
the presence of reducing sugar.