CHEMISTRY OF CARBOHYDRATES

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CHEMISTRY OF CARBOHYDRATES

REACTIONS OF MONOSACCHARIDES,

 

Carbohyrates are polyhydroxy aldehydes or ketones. Monosaccharides are the basic compounds in this series having one single carbon chain. The common monosaccharides that are used in the biochemistry laboratory are Glucose & fructose. Both Glucose and fructose are present in honey and fruits glucose is also the sugar of blood.

 

Perform the following tests with solution (1) 1% Glucose and (ii) 1% fructose

 

MOLISCH TEST:

                In a clean and dry test tube take 2 ml of the given solution and add 2 drops of molisch reagent mix and add carefully down the sides of the test tube 2 ml of concentrated Sulphuric acid, so that the acid & aqueous solution do not mix but form a separate layer. Formation of the purple colour indicates the presence of carbohydrate.

 

EXPLANATION:

                Molisch reagent consists of 1% solution of alpha naphthol in ethanol. This reaction is due to the formation of furfural and furfural derivatives. The furfural formed by the dehydration of the acid condenses with the alpha naphthol, given a purple rings. This test is general for all carbohydrates it is not specific for carbohydrates since all furfural yielding substances answer this test. Appearance of a black, brown or green coloured ring is not a positive reaction.

 

SELIWANOFF’S TEST:

                Take 3 ml of cherry’s reagent and .1ml of the given solution and heat for 30sec boiling. The rapid development of red colour indicates the presence of ketoses like fructose, glucose does not give the colour or gives it on prolonged boiling.

 

EXPLANATION:

                Seliwanoff’s reagent contains resorcinol in concentrated HC1 acts on Ketosugar to form a derivative of furfural-dehyde which gives a red coloured compound with rescorcinol. Glucose also gives red colour due to the transformation into fructose by the catalytic action of HCl but after prolonged heating. This test distinguishes between fructose and glucose.

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