شيت بايو عملي1

Qualitative Analysis of Carbohydrates

  • Author: Amal Alamri

Introduction to Carbohydrates

  • Carbohydrates are polyhydroxy aldehydes or ketones, commonly referred to as sugars.

  • Produced from CO2 and H2O by plants through photosynthesis.

  • Serve as a major food supply and energy source for people.

  • Average adult body reserves approximately 370 g, primarily stored in liver and muscle glycogen.

Classification of Carbohydrates

I. According to Chemical Composition

  • Simple Carbohydrates: Known as monosaccharides or simple sugars; contain one saccharide unit.

  • Complex Carbohydrates: Contain more than one saccharide group.

II. According to Number of Units

  • Monosaccharides: One saccharide unit.

  • Disaccharides: Two monosaccharide units.

  • Oligosaccharides: 3-6 monosaccharide units.

  • Polysaccharides: Over 7 monosaccharide units.

  • Hydrolysis: Complex carbohydrates can be broken down into smaller sugar units through this process.

III. According to Position of Carbonyl Group

  • Aldoses: Contain a terminal aldehyde group and a hydroxyl group.

  • Ketoses: Contain a carbonyl group attached to two R groups, which have hydroxyl groups.

IV. According to Number of Carbon Atoms

  • Common Classes: Pentoses (5 carbons) and hexoses (6 carbons) are most common.

Structural Types of Carbohydrates

  • Ketoses and Aldoses:

    • Triose Sugars: C3H6O3 (e.g., Glyceraldehyde)

    • Pentose Sugars: C5H10O5 (e.g., Ribose)

    • Hexose Sugars: C6H12O6 (e.g., Glucose, Fructose, Galactose)

Physical Properties of Carbohydrates

  • Solubility:

    • Monosaccharides and disaccharides dissolve freely in water due to polar nature.

    • Polysaccharides are less soluble because of high molecular weight and form colloidal solutions.

    • Starch: Does not dissolve easily in water.

Chemical Properties of Carbohydrates

  1. Molisch Test: Specific for carbohydrates.

  2. Benedict's Test: Presence of reducing sugars.

  3. Barfoed's Test: Distinguishes reducing monosaccharides from reducing disaccharides and non-reducing disaccharides.

  4. Bial's Test: Distinguishes between pentose and hexose monosaccharides.

  5. Seliwanoff's Test: Distinguishes between aldoses and ketoses.

1. Molisch Test

  • Objective: Identify carbohydrates from lipids and proteins.

  • Observation: Monosaccharides give rapid positive tests; disaccharides and polysaccharides react slowly.

  • Principle: Reagent dehydrates pentoses to furfural and hexoses to 5-hydroxymethylfurfural, producing a purple product with α-naphthol.

  • Method:

    • 2ml of sample solution in a test tube.

    • Add 4 drops of Molisch reagent.

    • Pour slowly into 2ml concentrated sulfuric acid to form two layers; violet ring appears between layers.

2. Benedict's Test

  • Objective: Distinguish between reducing and non-reducing sugars.

  • Principle: Copper sulfate in Benedict's solution reacts with electrons from aldehyde or ketone group; reducing sugars oxidized to carboxylic acids, forming reddish precipitate of copper(I) oxide.

  • Method:

    • 1ml sample solution in a test tube.

    • Add 2ml of Benedict's reagent and heat for 3 minutes.

    • Positive test: formation of reddish precipitate.

3. Barfoed's Test

  • Objective: Distinguish between reducing monosaccharides and disaccharides.

  • Principle: Copper (II) ions oxidize reducing sugars to carboxylic acids forming reddish precipitate; reducing disaccharides react slower.

  • Method:

    • 1ml of sample solution in a test tube.

    • Add 3ml of Barfoed's reagent and heat for 3 minutes.

4. Bial's Test

  • Objective: Distinguish between pentose and hexose monosaccharides.

  • Principle: Uses conc. HCl and orcinol; pentoses form furfural, which reacts to produce bluish/green color; hexoses yield brown products.

  • Method:

    • 2ml sample solution in a test tube.

    • Add 2ml Bial's reagent and gently heat.

    • Positive test: bluish product.

5. Seliwanoff's Test

  • Objective: Distinguish between aldoses (like glucose) and ketoses (like fructose).

  • Principle: Uses 6M HCl and resorcinol; ketohexoses form cherry red product, aldohexoses produce yellow/faint pink color.

  • Method:

    • 1ml sample solution in a test tube.

    • Add 2.5ml Seliwanoff's reagent; heat for 5 minutes.

Conclusion

  • Comprehensive tests for distinguishing between different types of carbohydrates are essential for qualitatively analyzing carbohydrate compositions in various samples.