TESTS FOR CARBOHYDRATESDr.S.Sethupathy

•Conc.Sulfuric acid dehydrates carbohydrates into furfural derivatives.

•Pentoses yield furfurals•Hexoses yield 5-Hydroxy methyl furfurals

•Furfurals condense with α –naphthol to form colored products

Molisch test

Importance

•General test for carbohydrates•Nucleic acids , Glycoproteins and glycolipids give a positive test due to their carbohydrate content.

Benedict’s test• Reducing sugars give positive Benedict’s test due to their

free aldehyde or keto group.•

Glucose + 2 Cu++ + 2 H2O Gluconic acid + Cu2+O +4H+

• Cu2+O - red precipitate

The principle of Benedict's test• When reducing sugars are heated in the presence of an

alkali, they become powerful reducing compounds known as enediols.

• Enediols reduce the cupric ions (Cu2+) present in the Benedict's reagent to cuprous ions (Cu+) which get precipitated as insoluble red Cuprous oxide(Cu2O).

Benedict’s reagent•Copper sulfate- Cupric ions•Sodium carbonate – Alkaline Medium for enediol formation

•Sodium citrate – prevents precipitation and keeps cupric ions in solution

Semiquantitative• Reagent – blue color• A greenish precipitate indicates about 0.5 g%

concentration; • yellow precipitate indicates 1 g% concentration;• orange indicates 1.5 g% • red indicates 2 g% or higher concentration.• Spectrum of light- VIBGYOR

Blue,Green,Yellow,Orange,Red

Bial’s rest for pentoses• HCl in the Bial’s reagent dehydrates pentoses to furfural

derivatives• Furfural reacts with orcinol and ferric ions in the reagent to

form blue color product• Procedure: 2 ml solution is treated with 2 ml Bial’s reagent and gently heated• Formation of bluish color indicates the presence of

pentoses • Hexoses give different colors such as brown, yellow , red

etc.

Bial’ test – blue color is positive

Osazones• Reducing sugar solution when heated with phenyl

hydrazine , characteristic crystalline yellow colored oazones are formed

• Needle shaped crystals by glucose, sunflower shaped crystals by maltose and powder puff shaped crystals by lactose are formed.

• Sugar phenyl hydrazine Osazone

Osazones

Mucic acid test for galactose• The mucic acid test is used to identify the presence of the sugar galactose 

• When galactose solution is heated with concentrated nitric acid, a dicarboxylic acid called mucic acid  or galactaric acid is formed as a white precipitate, which counts as a positive result.

Mucic acid test

Barfoed’s test• Monosaccharides react faster with Barfoed’s reagent than disaccharides and form reddish precipitate within three minutes of heating whereas disaccharides reacts slowly

• Barfoed's reagent consists of a 0.3 m  solution of neutral copper acetate in 1% acetic acid solution.

Barfoed’s test Reagent Positive

Iodine test for starch• Addition of a few drops of iodine solution to starch, gives a

dark blue color.

Iodine test – dark blue - positive

Thank you