Qualitative Identification of Carbohydrates

Study Notes

Physical Properties: Color, shape, and solubility are physical characteristics used to identify carbohydrates.
Chemical Properties (Specific Tests): Various chemical tests are used to confirm the presence and type of carbohydrate.

General Test for Carbohydrates: A test for almost all types of carbohydrates.
Reaction Process: Concentrated sulfuric acid converts the carbohydrate into furfural or its derivative. This product then reacts with α-naphthol to produce a purple-colored compound.
Procedure:
- Add 2mL of the sample solution to a clean test tube.
- Add 2-3 drops of Molisch reagent slowly.
- Carefully add concentrated sulfuric acid down the side of the test tube.
- Observe the formation of a violet ring at the junction of the two layers. A violet ring confirms the presence of carbohydrate.

Caramelization: When sugars are heated, they undergo caramelization, a process that breaks them down and forms new compounds, changing the color and flavor of the food.
Observation: Heating solid sugars will produce a change in appearance, with an odour of burnt sugar.

Detects Monosaccharides: Specifically designed to distinguish between reducing monosaccharides (which are sugars containing an aldehyde or ketone group) and disaccharides.
Reagent: A mixture of ethanoic (acetic) acid and copper(II) acetate is used.
Procedure: Test solution is mixed with Barfoed's reagent and boiled. The presence of a reducing monosaccharide is confirmed by the formation of a red copper(II) oxide precipitate. Disaccharides do not produce this precipitate as quickly.

Reaction Time: Monosaccharides react relatively quickly (within 1-2 minutes), while disaccharides take longer (7-8 minutes).
Hydrolyzation: Disaccharides first need to undergo hydrolysis (breaking down) into monosaccharides before the reaction takes place.
Precipitate Appearance: The appearance of a red precipitate confirms the presence of reducing monosaccharide.

Differentiates Ketoses and Aldoses: A timed color reaction to distinguish sugars with a ketone group (ketoses) from those with an aldehyde group (aldoses). The test is specific for ketohexoses (ketoses with six carbon atoms).
Resorcinol and HCl: The test reagent contains resorcinol and concentrated HCl.
Dehydration: Ketoses undergo dehydration faster than aldoses in the presence of the concentrated acid, resulting in a deeper cherry-red color produced.
Procedure: Add 1mL of the test sample and 1mL of distilled water to separate tubes. React each solution with Seliwanoff's Reagent. Observe the formation of color in a water bath.

Identifying Reducing Sugars: Used to differentiate reducing sugars (containing a free aldehyde or ketone group) from non-reducing sugars.
Fehling's Solution: Contains copper(II) sulfate, potassium sodium tartrate and strong alkali (usually sodium hydroxide)
Reaction with Reducing Sugars: Reducing sugars react with Fehling's solution, causing the copper ions to reduce to copper(I) oxide (Cu2O) which appears as a red precipitate.