Stereochemistry, Structural Representations, and Anomeric Forms of Glucose and Fructose

Limitations of Open-Chain and Newman Structures of Glucose

• Open-chain and Newman structures fail to account for specific chemical reactivity patterns:
  • They do not give positive results in the $2,4\text{-DNP}$ Test ($2,4\text{-Dinitrophenylhydrazine}$ test).
  • They do not give positive results in the Schiff Test.
• Open-chain and Newman representations cannot explain or account for the existence of the two distinct cyclic stereoisomers known as the anomeric forms (\n\alpha and $\beta$ forms) of glucose.

Optical Activity and D/L Nomenclature of Carbohydrates

• Optical Rotation Classifications:
  • Dextrorotatory ($d$ or $(+)$): An experimentally determined term indicating that a compound rotates plane-polarized light to the right.
  • Lævorotatory ($l$ or $(-)$): An experimentally determined term indicating that a compound rotates plane-polarized light to the left.
• $D$ and $L$ Structural Configuration System:
  • The assignment of $D$ or $L$ prefixes in carbohydrates is based on the spatial orientation of the hydroxyl group ($-OH$) attached to the second-to-last carbon atom (the penultimate carbon atom) in the Fischer projection.
  • $D\text{-configuration}$: The hydroxyl group ($-OH$) on the second-to-last carbon is located on the Right side.
  • $L\text{-configuration}$: The hydroxyl group ($-OH$) on the second-to-last carbon is located on the Left side.

Stereochemical Configurations of Glucose and Glyceraldehyde

• Configuration of $L\text{-Glyceraldehyde}$:
  • Carbon 1 ($C1$): Aldehyde group ($-CHO$).
  • Carbon 2 ($C2$): Hydroxyl group ($-OH$) on the left side, Hydrogen atom ($-H$) on the right side.
  • Carbon 3 ($C3$): Primary alcohol group ($-CH_2-OH$).
• Configuration of $D\text{-(+)-Glucose}$ (Fischer Projection):
  • Carbon 1 ($C1$): Aldehyde group ($-CHO$).
  • Carbon 2 ($C2$): Hydroxyl group ($-OH$) on the right side, $-H$ on the left side.
  • Carbon 3 ($C3$): Hydroxyl group ($-OH$) on the left side, $-H$ on the right side.
  • Carbon 4 ($C4$): Hydroxyl group ($-OH$) on the right side, $-H$ on the left side.
  • Carbon 5 ($C5$): Hydroxyl group ($-OH$) on the right side (penultimate carbon defining $D$ configuration), $-H$ on the left side.
  • Carbon 6 ($C6$): Primary alcohol group ($-CH_2-OH$).
• Configuration of $L\text{-(+)-Glucose}$ (Enantiomeric Fischer Projection):
  • Carbon 1 ($C1$): Aldehyde group ($-CHO$).
  • Carbon 2 ($C2$): Hydroxyl group ($-OH$) on the left side, $-H$ on the right side.
  • Carbon 3 ($C3$): Hydroxyl group ($-OH$) on the right side, $-H$ on the left side.
  • Carbon 4 ($C4$): Hydroxyl group ($-OH$) on the left side, $-H$ on the right side.
  • Carbon 5 ($C5$): Hydroxyl group ($-OH$) on the left side (penultimate carbon defining $L$ configuration), $-H$ on the right side.
  • Carbon 6 ($C6$): Primary alcohol group ($-CH_2-OH$).

Cyclic and Haworth Structures of Glucose Anomers

• Haworth Cyclization and Anomer Mechanism:
  • According to Haworth, $D\text{-Glucose}$ exists in two cyclical forms due to an intramolecular reaction between the carbonyl group at $C1$ and the hydroxyl group at $C5$.
  • The isomeric forms created by this cyclization, differing in configuration exclusively at the original carbonyl carbon ($C1$), are called Anomers.
  • $\alpha\text{-form}$: Arises when the hydroxyl group ($-OH$) at the carbonyl/anomeric carbon ($C1$) is situated on the Right side in modified Fischer projections (or directed Down in Haworth pyranose rings).
  • $\beta\text{-form}$: Arises when the hydroxyl group ($-OH$) at the carbonyl/anomeric carbon ($C1$) is situated on the Left side in modified Fischer projections (or directed Up in Haworth pyranose rings).
• Modified Fischer Projections for Cyclic Glucose:
  • $\alpha\text{-(D)-(+)-Glucose}$:
    • Carbon 1 ($C1$): $-H$ on left, $-OH$ on right.
    • Carbon 2 ($C2$): $-H$ on left, $-OH$ on right.
    • Carbon 3 ($C3$): $-OH$ on left, $-H$ on right.
    • Carbon 4 ($C4$): $-H$ on left, $-OH$ on right.
    • Carbon 5 ($C5$): $-H$ on left, connected to an intramolecular oxygen bridge leading to $C1$.
    • Carbon 6 ($C6$): $-CH_2-OH$ group.
  • $\beta\text{-D-(+)-Glucose}$:
    • Carbon 1 ($C1$): $-OH$ on left, $-H$ on right.
    • Carbon 2 ($C2$): $-H$ on left, $-OH$ on right.
    • Carbon 3 ($C3$): $-OH$ on left, $-H$ on right.
    • Carbon 4 ($C4$): $-H$ on left, $-OH$ on right.
    • Carbon 5 ($C5$): $-H$ on left, connected to an intramolecular oxygen bridge leading to $C1$.
    • Carbon 6 ($C6$): $-CH_2-OH$ group.
• Haworth Pyranose Ring Representations:
  • $\alpha\text{-D-(+)-Glucopyranose}$ (Six-membered oxygen-containing pyranose ring):
    • Position 1 ($C1$): $-H$ UP, $-OH$ DOWN.
    • Position 2 ($C2$): $-H$ UP, $-OH$ DOWN.
    • Position 3 ($C3$): $-OH$ UP, $-H$ DOWN.
    • Position 4 ($C4$): $-H$ UP, $-OH$ DOWN.
    • Position 5 ($C5$): $-CH_2-OH$ UP, $-H$ DOWN.
  • $\beta\text{-D-(+)-Glucopyranose}$ (Six-membered oxygen-containing pyranose ring):
    • Position 1 ($C1$): $-OH$ UP, $-H$ DOWN.
    • Position 2 ($C2$): $-H$ UP, $-OH$ DOWN.
    • Position 3 ($C3$): $-OH$ UP, $-H$ DOWN.
    • Position 4 ($C4$): $-H$ UP, $-OH$ DOWN.
    • Position 5 ($C5$): $-CH_2-OH$ UP, $-H$ DOWN.

Structural Characterization of Fructose

• Definition and Formula:
  • Fructose is defined as a ketohexose (a six-carbon carbohydrate containing a ketone functional group).
  • General chemical formula: $C_6H_{12}O_6$.
• Fischer Projection of $D\text{-(+)-fructose}$:
  • Carbon 1 ($C1$): Primary alcohol group ($-CH_2-OH$).
  • Carbon 2 ($C2$): Ketone carbonyl group ($C=O$).
  • Carbon 3 ($C3$): Hydroxyl group ($-OH$) on the left side, $-H$ on the right side.
  • Carbon 4 ($C4$): Hydroxyl group ($-OH$) on the right side, $-H$ on the left side.
  • Carbon 5 ($C5$): Hydroxyl group ($-OH$) on the right side (penultimate carbon establishing $D$ configuration), $-H$ on the left side.
  • Carbon 6 ($C6$): Primary alcohol group ($-CH_2-OH$).
• Fischer Projection of $L\text{-(+)-fructose}$:
  • Carbon 1 ($C1$): Primary alcohol group ($-CH_2-OH$).
  • Carbon 2 ($C2$): Ketone carbonyl group ($C=O$).
  • Carbon 3 ($C3$): Hydroxyl group ($-OH$) on the right side, $-H$ on the left side.
  • Carbon 4 ($C4$): Hydroxyl group ($-OH$) on the left side, $-H$ on the right side.
  • Carbon 5 ($C5$): Hydroxyl group ($-OH$) on the left side (penultimate carbon establishing $L$ configuration), $-H$ on the right side.
  • Carbon 6 ($C6$): Primary alcohol group ($-CH_2-OH$).