Qualitative Identification of Carbohydrates PDF

Qualitative identification of carbohydrates A) Physical Properties B) Chemical properties Color (Specific tests) Shape Solubility (1) Molisch’s Test: Molisch’s test is a general test for ca...

Qualitative identification of carbohydrates A) Physical Properties B) Chemical properties Color (Specific tests) Shape Solubility (1) Molisch’s Test: Molisch’s test is a general test for carbohydrates. This test is given by almost all of the carbohydrates. In this test, concentrated sulfuric acid converts the given carbohydrate into furfural or its derivatives, which react with α-naphthol to form a purple-coloured product. The chemical reaction is given below. 1. Take 2ml of the given sample solution in a clean test tube. 2. Add 2-3 drops of Molisch reagent slowly. 3. Now add concentrated sulfuric acid along the sides of the test tube. 4. The acid layer forms a layer at the bottom. 1  5. Note the junction of the two layers. 6. If there is a formation of the violet ring then the presence of carbohydrate is confirmed. (2) Action of heat: When sugars are heated, they can undergo caramelization, resulting in the formation of a brown, flavorful substance. This process involves the breakdown of sugars and the formation of new compounds, contributing to color and flavor in foods. Direct flame Solid on a slide odour of burnt sugar (3) Barfoed’s Test: Barfoed's test is 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. Objectives of Barfoed’s Test To distinguish reducing monosaccharides from disaccharides. 2 Principle of Barfoed’s Test The Barfoed reagent is made up of copper acetate in a dilute solution of acetic acid. Since acidic pH is unfavorable for reduction, monosaccharides, which are strong reducing agents, react in about 1-2 min. However, the reducing disaccharides take a longer time of about 7-8 minutes, having first to get hydrolyzed in the acidic solution and then react with the reagent. Once the reaction takes place, thin red precipitate forms at the bottom of the sides of the tube. The difference in the time of appearance of precipitate thus helps distinguish reducing monosaccharides from reducing disaccharides. Reaction D-glucose + 2CuO → D-gluconic acid + Cu2O 1. Take 1 ml of a given sample in a clean, dry test tube. 2. Take control of 1 ml of distilled water in another tube. 3. Add about 2-3 drops of Barfoed’s reagent to both the tubes and mix them in a vortex. 4. Keep the test tubes in the water bath for 1-2 minutes. The boiling should not be done for more than 2 minutes as the disaccharides might hydrolyze into monosaccharides and give a positive result. 5. Observe the appearance of color in the test tubes. 6. Noted own the time taken for the appearance of color in the tubes. 3 (4) Seliwanoff’s test definition Seliwanoff’s test is used to differentiate between sugars that have a ketone group (ketose) and sugars that have an aldehyde group (aldoses). This test is a timed color reaction specific to ketohexoses. Objectives of Seliwanoff’s test To distinguish ketoses from aldoses. Principle of Seliwanoff’s test The reagent of this test consists of resorcinol and concentrated HCl. The acid hydrolysis of polysaccharides and oligosaccharides yields simpler sugars. Ketoses are more rapidly dehydrated than aldoses. Ketoses undergo dehydration in the presence of concentrated acid to yield 5- hydroxymethyl furfural. The dehydrated ketose reacts with two equivalents of resorcinol in a series of condensation reactions to produce deep cherry red color. Aldoses may react slightly to produce a faint pink to cherry red color if the test is prolonged. Procedure of Seliwanoff’s test 1. Take two clean, dry test tubes and add 1 ml of the test sample in one test tube and 1 ml of distilled water in another as blank. 2. Add 2 ml of Seliwanoffs’ reagent to both the test tubes. 3. Keep both the test tubes in a water bath for 1 min. 4. Observe the formation of color and note it down. 4 (5) Fehling’s Test Definition Fehling’s test is a chemical test used to differentiate between reducing and non- reducing sugars. Objectives of Fehling’s Test To differentiate between reducing and non-reducing sugars. Principle of Fehling’s Test The carbohydrates having free or potentially free carbonyl groups (aldehyde or ketone) can act as reducing sugars. The Fehling’s solution appears deep blue in color and consists of copper sulfate mixed with potassium sodium tartrate and strong alkali, which is usually sodium hydroxide. Procedure of Fehling’s Test 1. Take 1 ml of a given sample in a clean, dry test tube. 2. Take control of 1 ml of distilled water in another tube. 3. Add about 2-3 drops of Fehling’s reagent to both the tubes and mix them in a vortex. 4. Keep the test tubes in the water bath for 1-2 minutes. 5. Observe the appearance of color in the test tubes. 6. Note down the appearance of color seen in the test tubes. 5