Monosaccharides 

Definition: Monosaccharides are the simplest form of carbohydrates that cannot be hydrolyzed further into simpler sugars. They are classified based on their functional group (aldoses or ketoses) and number of carbon atoms (trioses, tetroses, pentoses, hexoses, etc.).

General Formula: (CH₂O)n  Where n usually ranges from 3 to 7.

Types of Monosaccharides

1. Based on Functional Groups

• Aldoses: Contain an aldehyde (-CHO) group at carbon 1. Example: Glucose, Galactose, Ribose
• Ketoses: Contain a ketone (-C=O) group at carbon 2. Example: Fructose, Ribulose

2. Based on Number of Carbon Atoms

Cyclic Structures of Monosaccharides: Monosaccharides exist in two structural forms:

1. Open-chain (linear) form
2. Cyclic form (more stable in aqueous solution)

Hemiacetal and Hemiketal Formation

• Aldoses (e.g., Glucose): Form hemiacetals when the -OH group reacts with the aldehyde group.
• Ketoses (e.g., Fructose): Form hemiketals when the -OH group reacts with the ketone group. The formation of hemiacetals/hemiketals creates a new asymmetric carbon called the anomeric carbon.

Anomers and Anomeric Carbon

• The anomeric carbon is the carbon derived from the carbonyl carbon in the cyclic form.
• α-Anomer: The OH group at the anomeric carbon is below the plane.
• β-Anomer: The OH group at the anomeric carbon is above the plane.

Example: D-Glucose Forms

• α-D-Glucose: Anomeric OH is down.
• β-D-Glucose: Anomeric OH is up.

Mutarotation of Monosaccharides

Definition: Mutarotation is the change in the optical rotation of a monosaccharide solution due to the interconversion between α and β anomers via the open-chain form.

Mechanism of Mutarotation

1. α-D-glucose (rotation: +112°) dissolves in water.
2. It undergoes equilibrium by opening up into its open-chain form.
3. The open-chain form then re-cyclizes, forming a mixture of α- and β-anomers.
4. β-D-glucose (rotation: +19°) is more stable and predominates.
5. The final equilibrium optical rotation is +52.7°.

Example

• D-Glucose mutarotates to a final equilibrium mixture:
  • 36% α-D-glucose (rotation: +112°)
  • 64% β-D-glucose (rotation: +19°)
  • Final rotation: +52.7°

Mutarotation occurs in all reducing sugars, including glucose, fructose, lactose, and maltose.

Reducing and Oxidizing Properties of Monosaccharides

Reducing Properties: A sugar is a reducing sugar if it has a free aldehyde (-CHO) or ketone (-C=O) group that can donate electrons to a reducing agent.

• All aldoses (e.g., glucose, galactose) are reducing sugars.
• Ketoses (e.g., fructose) can also be reducing after tautomerization to aldoses under alkaline conditions.

Tests for Reducing Sugars

1. Benedict’s Test

• Reagent: Benedict’s solution (Cu²⁺ sulfate, sodium carbonate, sodium citrate)
• Reaction: The aldehyde group reduces Cu²⁺ to Cu₂O, forming a red precipitate.
• Result Interpretation:
  • Blue: No reducing sugar
  • Green: Low concentration
  • Yellow: Moderate concentration
  • Brick-red: High concentration
• Example: Glucose, fructose, and lactose give positive results.

2. Fehling’s Test

• Reagent: Fehling’s solution (Cu²⁺ ions)
• Reaction: Aldehyde group reduces Cu²⁺ to Cu₂O (red precipitate).
• Result: Red precipitate confirms reducing sugar presence.

3. Tollens’ Test (Silver Mirror Test)

• Reagent: Tollens’ reagent (ammoniacal silver nitrate)
• Reaction: Aldehyde reduces Ag⁺ to metallic silver.
• Result: Silver mirror forms inside the test tube.
• Example: Glucose gives a positive result.

4. Barfoed’s Test (Distinguishes Monosaccharides from Disaccharides)

• Reagent: Copper(II) acetate in acidic medium.
• Reaction: Monosaccharides react faster than disaccharides.
• Result: Red precipitate appears within 2-5 minutes for monosaccharides.

Oxidizing Properties: Monosaccharides can be oxidized to carboxylic acids by strong oxidizing agents.

Types of Oxidation

1. Mild Oxidation (Using Bromine Water): Oxidizes only the aldehyde (-CHO) to carboxyl (-COOH). Example: D-Glucose → D-Gluconic Acid
2. Strong Oxidation (Using Nitric Acid, HNO₃) Oxidizes both the aldehyde (-CHO) and the terminal -CH₂OH group to carboxyl (-COOH). Example: D-Glucose → D-Glucaric Acid
3. Oxidation of Ketoses: Ketoses (e.g., Fructose) do not have a free aldehyde. Under alkaline conditions, they tautomerize to aldoses, which then undergo oxidation.

Summary Table