The characterization of Carbohydrates

CHEM 5 - BSN

• Carbohydrate is detected by Molisch’s Test

Molisch’s Test

• Reducing sugars are detected by Fehling’s and Benedict’s Test

Benedict’s test

Benedict's reagent• Benedict's reagent (also called Benedict's solution) is

a reagent used as a test for reducing sugars including glucose, lactose, and fructose.

• Benedict's reagent contains blue copper sulfate (CuSO4.5H2O) in a strong alkaline solution. The alkaline conditions of the test cause isomeric transformations of ketoses to aldoses (reducing sugars) that result in the Cu2+ of the copper sulfate being reduced to red copper oxide.

Benedict's reagent

• Benedict's reagent can also be used to test for the presence of glucose in urine.

• If glucose is found to be present in urine then this is an indication of diabetes.

• Once sugar is detected in urine, further tests have to be completed in order to ascertain which sugar is present. Only glucose is indicative of diabetes.

Benedict's reagent

• When Benedict's reagent is used as a test for the presence of reducing sugars in food, the food sample is dissolved in water and about 5 mL of the sample solution is added to 5 mL of Benedict's qualitative reagent. The mixture is placed in a boiling water bath for 5 minutes and any precipitate formed is recorded as a positive for the presence of sugar in the food.

Reducing sugars

• Carbohydrates that contain aldehydes or a-hydroxymethyl ketones can be oxidized by Cu(II) ion and are classified as reducing sugars. They reduce the Cu(II) ion in Benedict’s reagent to Cu(I).

• The copper oxide (Cu2O(s)) that forms is insoluble in water and therefore is a precipitate. The color of the precipitate ranges from green to brick red depending on how many of the copper II ions are present

• When 1 mL of reagent is heated with 5 drops of sample in a boiling water bath, a positive test for monosaccharides is formation of a brick-red precipitate within five minutes.

Barfoed’s test

Barfoed's Reagent

• Barfoed's Reagent looks like Benedict's reagent but differs somewhat.

• Barfoed’s reagent is capable of detecting monosaccharides in the presence of disaccharides and therefore provides a test to differentiate the two.

Barfoed’s reagent

• The reagent is prepared by dissolving copper acetate monohydrate and glacial acetic acid in water. The milder acidic conditions allow for the oxidation of monosaccharides while not affecting disaccharides which are much weaker reducing agents and thus much slower to oxidize. Also, ketoses don’t isomerize with this reagent.

Barfoed’s reagent

• Disaccharides generally don't give any reaction for as long as ten minutes. The precipitate isn't nearly as voluminous as that seen with Benedict's test and tends to adhere to the walls of the test tube.

• The formation of a bluish product.

• All other colors indicate a negative result for pentoses.

• Note that hexoses generally react to form green, red, or brown products.

Bial’s test

• Aldoses and Ketoses are detected by Seliwanoff’s Test

Seliwanoff’s test

Tollen’s Test

• Lactose and Maltose is distinguished by Tollen’s Test

Hydrolysis of sucrose

• Sucrose (common name: table sugar, also called saccharose) is a disaccharide (glucose + fructose) with the molecular formula C12H22O11.

• Sucrose is confirmed by inversion test

Hydrolysis of Sucrose