CHEMISTRY OF CARBOHYDRATES
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.
Comments
Post a Comment