Qualitative tests of Carbohydrates

Carbohydrates Overview

• Carbohydrates serve as a primary energy source for living organisms and contribute to structural elements in cell walls of bacteria and plants.
• Defined as polyhydroxy aldehydes or polyhydroxy ketones, carbohydrates often follow the empirical formula (CH2O)n.
• D-glucose is the main carbohydrate used in the human body.
• Simple sugars characterized by the suffix "-ose."

Classification of Carbohydrates

• Monosaccharides: Single carbohydrate unit.
• Disaccharides: Two monosaccharide units.
• Oligosaccharides: Contain 3-9 monosaccharide units.
• Polysaccharides: More than 9 monosaccharide units.
• Complex carbohydrates can be hydrolyzed into smaller sugar units.

Reducing vs. Non-reducing Sugars

• Reducing sugars have a free anomeric carbon and can act as reducing agents; non-reducing sugars do not.

Solubility of Sugars

• Monosaccharides and disaccharides are soluble in water due to their lower molecular weight and polar characteristics.
• Polysaccharides are mostly insoluble in water, forming colloidal solutions.

Chemical Tests for Carbohydrates

• Molisch's Test: Specific for carbohydrates; positive result indicated by a purple ring formation.
• Benedict's Test: Detects reducing sugars through the formation of a reddish precipitate.
• Barfoed's Test: Distinguishes between mono- and disaccharides; reduces copper (II) ions to form a reddish precipitate faster in monosaccharides.
• Bial's Test: Identifies pentose monosaccharides.
• Seliwanoff's Test: Differentiates between aldoses and ketoses.

Detailed Chemical Test Procedures

• Molisch Test
  • Add Molisch reagent (α-naphthol in ethanol) to a sample, followed by concentrated sulfuric acid to form purple ring.
• Benedict's Test
  • Combine sample with Benedict's reagent and heat. Positive result shown by reddish precipitate indicating presence of reducing sugars.
• Barfoed’s Test
  • Add sample to Barfoed's reagent, heat; monosaccharides react rapidly while disaccharides react slowly.

Observations for Tests

• Molisch Test: Rapid positive for monosaccharides, slower for disaccharides and polysaccharides.
• Benedict's Test: Glucose and lactose yield positive results with reddish precipitate; starch is negative.
• Barfoed’s Test: Monosaccharides yield positive (reddish precipitate) within three minutes; disaccharides react slower or may not indicate positive.