Qualitative tests of functional groups in carbohydrates

                                Qualitative tests of functional groups in Carbohydrate 

Carbohydrates are abundantly found in the plants and as glycogen in the animals. In animals they are found in free stored state as glycogen or in combination with proteins as glycoprotein. The name carbohydrate is given because they are composed of carbon, hydrogen and oxygen atoms.

Many carbohydrates are of low molecular weight and having sweet taste. Carbohydrates are classified into 3 groups : (1) Monosaccharides or simple sugars (C6H1206), (2) Di- and tri-saccharides or compound sugars, (3) Polysaccharides.

For experiment make 2% solutions of the Dextrose-D or Fructose and perform the following experiments in order to identify the reducing action of glucose and fructose

Sr. No.	Test	Observation	Inference
1	Reduction test. In a test tube take 3 ml  of distilled water, then add a drop of methylene blue (1%). The water becomes blue coloured. Add 0.5 cc of 40% NaOH. Boil the solution. Colour is not discharged, blue colour remains. Add 1 ml  of 0.2% OS and boil.	Decoloration	Carbohydrate may be present.
2	Fehling's test. Procedure: Take 5ml  of Fehling's solution and boil. Add 1 ml  of OS  and boil again.	Yellow or brick-red precipitate	Glucose is  present.
3	Benedict's test. In a test tube 5 ml  of Benedict's reagent, then add 0.5 ml  of glucose solution and heat to boiling. Boil for 2 minutes. Cool the solution under tap water.	Green, yellow or red precipitate
4	Picric acid test. In a test tube take 3 ml  of 2% O.S. then add 1 ml  of picric acid saturated solution and then add 1 ml  of 40% NaOH.	Formation of  red colour.	Glucose is present
5	Rapid furfural test. Take 1 ml of 2% fructose solution, add 6 drops at a-naphthol, then add 5 ml of conc. HCI in a test tube and boil.	Deep purple colour appears.	Fructose is present

 

Result: The given  sample contains  Glucose / Fructose