Lecture on Carbohydrates and Ring Formation

Overview of Carbohydrates

• Structure: Chains of carbon atoms with an aldehyde or ketone functional group.
• Ratio: Typically a 1:2:1 ratio of carbon, hydrogen, and oxygen (e.g., glucose).
• Terminology: Often referred to as polyhydroxylated due to multiple hydroxyl groups.

Intramolecular Reactions

• Potential for Reaction: Carbohydrates can undergo internal reactions between the carbonyl carbon and one of the hydroxyl groups.
  • Reaction Type: Carbinol and alcohol chemical reaction.
  • Outcome:
    • Hemiacetal/Hemiketal: Formed with a single nucleophilic attack by an alcohol.
    • Acetal/Ketal: Formed if there's an excess of alcohol.

Glucose Ring Formation

• Nucleophilic Attack Process:
  • Hydroxyl oxygen acts as the nucleophile after deprotonation.
  • Targets the carbonyl carbon (partial positive charge due to electron density).
• Product Formation:
  • Electrons move to form a new hydroxyl group.
  • Six-membered ring (pyranose) is more stable than the straight chain.

Naming and Structure

• Pyranose: Six-membered sugar rings (e.g., glucose).
• Furanose: Five-membered sugar rings (e.g., ribose).
• Anomeric Carbon: The only carbon bonded to two oxygen atoms, significant in ring structure.

Diagrammatic Representations

• Haworth Diagram

  • Simplified representation showing substituents above or below the ring.
  • Convention: "Downright up lefting" helps track substituents.

• Chair Conformation

  • More accurate depiction of six-membered ring stability.
  • Follows Haworth projection for substituents' above/below positions.

Anomeric Configurations

• Alpha vs. Beta Anomers:
  • Alpha Anomer: Hydroxyl group in the axial position (trans to last carbon).
  • Beta Anomer: Hydroxyl group in the equatorial position (cis to last carbon).
  • Mnemonic: "Fishes are down in the sea (alpha), birds are up in the air (beta)."

Mutarotation

• Process: Ring opens and closes in water, allowing rotation and formation of alpha and beta products.
• Equilibrium:
  • Glucose: 36% alpha and 64% beta due to steric hindrance effects.
  • General rule: The beta anomer is more stable with the anomeric oxygen in the cis position.