Carbohydrate Experiments تجارب الكربوهيدرات

Qualitative Analysis Of Carbohydrates

Carbohydrate Experiments

Molisch's test

This reaction is a general for given by all carbohydrates (sugar).Carbohydrates when treated with concentrated H₂SO₄ undergo Dehydrates to give furfural derivatives .this compound condense with   α-naphthol (in alcoholic solution) gives red- violet ring.

Procedure:-

1 ml of sugar solution + 3 drops of α-naphthol   (Molisch's reagent) And mix +2ml of concentrated H₂SO₄ →                         violet ring that spreads with shaking

Benedict's Test

Reducing sugars as Monosaccharides and disaccharides –Exception of sucrose –to gives in medium alkaline intermediate compound (enedioles).This indicates the sugars of strong reducing agents reduce cupric (Cu²⁺)ions (Blue colour) to cuprous (Cu⁺)ions(Red colour) by heating and oxidize them to sugar acids

Procedure

Solution of sugar +5 ml of Benedict's Reagent –shake well and heat on Boiling water both for 2-3min       →  Red or yellow precipitate. Indicates the presence of reducing sugars

Fehling Test

This is another reduction test to detect the presence of reducing sugar .principle: as Benedict's test .

Procedure

1 ml of sugar Solution + 2 ml of Fehling Solution (FA+FB)

Shake well +bath water for 3-5min → reddish –brown ppt

Indicates the presence of reducing sugar

Barfoed's test

This is used to distinguish reducing monsaccharides from a reducing disaccharides by controlling PH and time of heating monosaccharides react very test whereas reaction with disaccharides is slow Procedure:-

1 ml of sugar solution +2ml of Barfoed's reagent –shake well + water bath for 5 min + cool under running water →Red ppt (Cu₂O) .

Indicates the presence of monosaccharides .

Seliwanoff's test :-

This test is positive for Ketohexose only fructose Ketohexoses on treatment with hydrochloric acid form 5-hydroxyl methyl furfural which on condensation with resorcinol gives a cherry red coloured complex .

And used in distinguish between Glucose and Fructose where gives positive result with Fructose only.

Procedure:-

0.5ml of sugar solution +2 ml of Seliwanoff's solution + water bath for 10 min → cherry –Red colour .

Iodine test :-

This is used to identify on polysaccharides "Starch"

Procedure:-

 1ml of Starch Solution in hot water +1-2 drops of Iodine  

                Solution →blue Colour

Osazone test

Reaction of phenyl hydrazine with reducing derivatives of sugars

It is a useful means of preparing crystalline derivatives of sugars

Procedure

Dry 0.5 g of mixture from (( phenyl hydrazine chloride +sodium Acetate  )) + 4 ml of sugar Solution then shake the Solution bath water 0.5 hour +cold the Solution crystals are formed .A coverslip preparation is mode on a clean slide and seen under the microscope

 The shape of osazone Crystals are

Note

The sugar which different in the structural formula on first and second atoms (as Glucose ,fructose ,Mannose ) give the some Osazone crystals at reacts with phenyl hydrazine