Carbohydrates Overview

Carbohydrates

• Carbohydrates are polyhydroxy aldehydes, polyhydroxy ketones, or compounds that yield polyhydroxy aldehydes and polyhydroxy ketones upon hydrolysis.
• They are commonly known as sugars because most of them have a sweet taste.
• They serve as a major source of energy.
• They furnish the carbon chains of compound synthesis by living cells.

Classification of Carbohydrates

• Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides.

Monosaccharides

• They are highly soluble in water.
• They are less soluble in ethanol.
• They are insoluble in ether.
• They are either aldoses or ketoses.
• Free monosaccharides are all reducing sugars.

Disaccharides

• They are formed by two molecules of monosaccharides.

Polysaccharides

• They are classified as structural polysaccharides and storage polysaccharides.
• Structural polysaccharides: Cellulose
• Storage polysaccharides: Starch, Dextrin, Glycogen, Insulin

Test Based on the Formation of Furfural & its Derivatives

• The principle of this test is that hot H2SO4 will dehydrate sugars into furfural and its derivatives.
• Furfural / Hydroxymethylfurfural + phenolic compounds (𝛂-naphthol, orcinol, resorcinol) → colored condensation products.
• A positive reaction is indicated by the presence of color.

Molisch Test

• This test is used to detect the presence of carbohydrates.
• Procedure:
  • Add 4 mL of solution to each tube and 2 drops of Molisch reagent.
  • Incline the test tube and slowly add about 2 mL of concentrated H2SO4.
  • Sulfuric acid is denser than water and will form a lower layer.
• Results:
  • Distilled H2O: Presence of green ring
  • 3% Glucose: Presence of purple ring
  • 3% Lactose: Presence of purple ring
  • 3% Starch: Presence of purple ring
  • 3% Xylose: Presence of purple ring