Chemical Properties of Monosaccharides

Continuation of Carbohydrates

• Focus on chemical properties of monosaccharides.
• Key reaction: Formation of sugar alcohols.
• Sugar alcohols derived from monosaccharides, used as sweeteners in food.
• Example: Hydrogenation of D-glucose to produce D-sorbitol.
  • Hydrogenation Process:
  1. Change aldehyde (D-glucose) to primary alcohol by removing the double bond (aldehyde group).
  2. Split a hydrogen molecule to provide two hydrogen atoms.
  3. Add hydrogen atoms to the resulting carbons and oxygen in the structure.
  4. Use a catalyst (e.g., platinum, palladium, nickel).
• D-sorbitol:
• Contains hydroxy groups on all carbons, classified as an alcohol, no longer a carbohydrate since it lost its aldehyde.
• Properties:
  • Tastes sweet, low caloric intake (not absorbed well in the body).
  • Useful for satisfying sweet cravings without weight gain.

Formation of Cyclic Structures

• Monosaccharides (e.g., D-glucose) typically form cyclic structures in solution.
• Cyclic Formation Process:
  • D-glucose reacts with itself, forming a hexagon-shaped ring (pyranose).
  • Key groups for reaction:
  • C5 alcohol oxygen.
  • C1 aldehyde carbon.
  • Transformation:
  1. Double bond changes to a single bond.
  2. Migrating the hydrogen from the C5 alcohol to the C1 aldehyde oxygen results in a new hydroxy group.
  • Pyranose is a six-membered ring containing an oxygen atom.
• Alpha and Beta Anomers:
• Alpha: New hydroxy group on C1 points down.
• Beta: New hydroxy group on C1 points up.
• Anomeric carbon: C1 carbon that designates alpha or beta.

Equilibrium in D-Glucose Solutions

• A glucose IV bag contains an equilibrium mix of:
• Straight-chain form, alpha anomer, and beta anomer.
• Represents the variability of glucose forms in solution.

Formation of Furanoses

• Ketohexoses (e.g., D-fructose) can form pentagon-shaped rings (furanoses) with similar processes.
• Key players:
  • C5 alcohol oxygen and C2 ketone carbon.
• Formation of furanose:
  1. Change the C2 ketone double bond to a single bond.
  2. Migrate hydrogen from the alcohol group.
  3. Result in C2 having a new hydroxy group, can point up or down for alpha/beta.
• Furanose Structure:
• Resembles furan, a pentagonal organic structure, distinguished by anomeric carbon at C2.

Classification of Monosaccharides

• D-mannose:
• Identify as pyranose or furanose based on shape (hexagonal = pyranose).
• Classify as alpha or beta based on the orientation of the hydroxy group at C1.
• Complete the name (e.g., alpha-D-mannose-pyranose).

Key takeaways:

• Understanding sugar alcohols and ring structures is crucial in organic chemistry and nutrition.
• Recognition of alpha and beta anomers is important for biochemical applications.
• Familiarity with nomenclature and classification helps in identifying and understanding carbohydrate structures effectively.