Qualitative tests of Carbohydrates

Study Notes

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.

Description

Test your knowledge on qualitative tests for carbohydrates, which are essential sources of energy for living organisms. Learn about the structure, classification, and functions of carbohydrates, including simple sugars like D-glucose.