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