Post on 20-Feb-2015
CHARBOHYDRATE
General Information:
●Carbohydrates are the most abundant class of organic
compounds found in living organisms.
●The formulas of many carbohydrates can be written as
carbon hydrates, Cn(H2O)n, hence their name.
● The carbohydrates are a major source of metabolic
energy, both for plants and for animals that depend on
plants for food.
A- Simple Sugars:
1- Contain the elements carbon, hydrogen, and oxygen.
2- The name carbohydrate literally means water
compounds of carbon.
3- The general formula for simple sugars is Cn(H2O)n.
A - Methods of Classification:
- Several methods are used to classify carbohydrates.
1-One method of classification is based on whether the
carbohydrate can be broken down into smaller units.
A. Monosaccharides :
cannot be broken down into smaller units by hydrolysis.
Sometimes called simple sugars.( glucose , fructose and
glactose).
B. Disaccharides :
can be broken down (hydrolyzed) into two
monosaccharide units.
e.g. Sucrose (glucose + fructose)
e.g. Lactose (glucose + galactose)
e.g. Maltose (glucose + glucose)
C. Oligosaccharides:
can be broken into three to six monosaccharide units.
D. Polysaccharides :
composed of 7 or more mono-saccharide units.
2-Another method is based on the number of
carbons found in a simple sugar.
- If it has three carbons it is called a triose.
- If it has four carbons it is called a tetrose.
- If it has five carbons it is called a pentose.
- If it has six carbons it is called a hexose.
3-Another method uses the kind of carbonyl group.
A- Aldose - a monosaccharide with an aldehyde group.
B- Ketose - a monosaccharide with a ketone group.
3-Another method according to the reducing activity
of the sugar unit:
1. Reducing sugars:
All monosaccharides & many diasaccharides (e.g.
lactose and maltose) are reducing sugars.
2. Non-reducing sugars :
e.g. sucrose.
QUALITATIVE TESTS FOR CARBOHYDRATES
1.Solubility
All mono-saccharides and some of oligo-saccharides are
easily soluble in cold water and alcoholic solution and
not soluble in non-polar organic solvent such as
benzene, on other hand polysaccharides non-soluble in
cold water or forming colloidal with boiling water such as
starch.
2.General tests for detection of carbohydrates
Molisch’s Test:
- It is the general test for all carbohydrates.
- All carbohydrates. Mono. give a rapid positive test.
Disaccharides and polysaccharides react slower.
- The Molisch reagent dehydrates pentoses to form
furfural (top reaction)
- and dehydrates hexoses to form 5-hydroxymethyl
furfural (bottom reaction).
- The furfurals further react with -naphthol present in the
test reagent to produce a purple product.
Molisch Test: Method:1. 1ml test solution + 2 drops of α-naphthol solution.2. Mix well.3. Add concentration H2SO4 down the side of the tube to form the ring at the interface of the two layers.Observation:A violet colored ring appears as the junction between the two layers.
-ve +ve
2. Reduction tests for detection of reducing
carbohydrates
A.Fehling's Test:
- This test is used to differentiate between reducing and
non reducing sugars.
- A reducing sugar reacts with Fehling's reagent in
alkaline medium to form an orange to red precipitate.
- Positive result is detected by reduction of the deep blue
solution of cupric (II) to a red precipitate of insoluble
cuprous oxide (Cu2O).
Fehling's Test:
-The sucrose does not react with Fehling's reagent.
Sucrose is a disaccharide of glucose and fructose.
Most disaccharides are reducing sugars (e.g. lactose
and maltose)
- Sucrose is non-reducing sugar because the anomeric
carbon of glucose is involved in the glucose- fructose
bond and hence is not free to form the aldehyde in
solution.
Fehling's Reagent:
Two solutions are required:
Fehling's "A" uses 7 g CuSO4.5H2O dissolved in
distilled water containing 2 drops of dilute sulfuric acid.
Fehling's "B" uses 35g of potassium tartrate and 12g of
NaOH in 100 ml of distilled water.
Method:
1. In a test tube add 2ml of the test carbohydrate
solution and put it in a boiling water bath (tube 1). 2. In
another tube add equal volumes of Fehling A & Fehling
B and put it in a boiling water bath for 3 minutes (tube 2).
3. Mix the content of test tubes 1 & 2 and observe any
change in color or precipitate formation.
Observation:
Any change in color from deep blue (cupric ion) to
green, yellow, orange or red (cuprous oxide) indicates a
positive reaction.
B.Benedict's Test:
-This test is used also to differentiate between reducing
and non reducing sugars.
- It works on the same principle but Benedict is more
stable than Fehling's reagent.
Method:
1. To 2 ml of Benedict’s reagent, add 5 drops of the test
carbohydrate solution and mix well.
2- Place the test tube in a boiling water bath for 5
minutes and observe any change in color or precipitate
formation .
Observation:
Any change in color from deep blue (cupric ion) to
green, yellow, orange or red (cuprous oxide) indicates a
positive reaction.
Benedict's and Fehling's test is important which given
positive results: with reducing sugar e.g.; Glucose-
Fructose and Lactose
Negative results: with non-reducing sugar e.g.; Sucrose-
Starch.
C. Barfoid Test:
-Barfoed's test is one the important test which give
positive results with mono-saccharides.
-This test using for difference between mono-reducing
sugars and oligo-reducing sugars.
-The reaction steps in this test like that in Benedicts' and
Fehling's test, but in this test the alkaline medium (OH)
had been substituted by weak acid medium (acetic acid).
- For this reason Barfoed's test give positive result with
mono-reducing sugars faster than oligo-reducing sugars.
Method:
1- 1 ml of the solution to be tested + 2 ml of freshly
prepared Barfoed's reagent.
2- Place test tubes into a boiling water bath and heat for
2 minutes.
3- Remove the tubes from the bath and allow to cool.
Observation:
- Formation of a green, red, or yellow precipitate is a
positive test for reducing monosaccharides.
Do not heat the tubes longer than 3 minutes, as a
positive test can be obtained with disaccharides if they
are heated long enough.
4. Detection of Keto-carbohydrate:
Seliwanoff's Test:
- Seliwanoff's Test is use to distinguish between aldoses
and ketoses
- When heated for only short time with hydrochloric acid
12% Keto-carbohydrates are dehydrate (-3H2O) to
furfural or hydroxymethylfurfural, the aldehyde groups of
which will then condense with resorcinol to form red color.
Method:
1. 5 drops sugar solution
2. 2 drops Seliwanoff's reagent
3. Mix and carfully add 10 drops HCl (conc.)
4. Mix and heat in boiling water bath (3 min)
Observation:
Appearance of deep red color.
Positive result Red color with Ketoses e.g.; Fructose-
Sucrose.
Negative result Colorless with aldoses e.g.; Glucose
5 - Tests for Individual Carbohydrates
Osazone test:
Method:
1- Acidify 3 ml of sugar solution with 5 drops of
glacial acetic acid.
2- Add 1 ml of phenyl hydrazine solution and 1 ml of
sodium acetate solution, mix well.
3- Place the test tube in a boiling water bath.
Observation:
• Osazones of monosaccharides (glucose & fructose)
are formed on hot after about 15 minutes, and have the
same crystal shape needles under the microscope.
• Osazones of reducing disaccharides (maltose &
lactose) are formed after a longer time (up to 30
minutes) and crystal appears slowly after cooling and
can be distinguished under the microscope as follows:
- Lactose gives crystals in the form of a tuft of needles.
- Maltose gives crystals in the form of broad needles.
Detection of starch (polysaccharides)
Iodine Test:
Iodine solution gives a blue color with starch that
disappears on heating and reappears on cooling. Iodine
solution also gives a brown color with glycogen.
Method:
1- Add 2 drops of diluted iodine solution to the test
carbohydrate solution and observe the color obtained.
2- Compare the color obtained with that of water and
iodine (Blank).
DONE BY: DR. NADA