Understanding Fischer Projections and Stereochemistry

Aug 14, 2024

Lecture on Fischer Projections

Introduction to Fischer Projections

  • Visualization of molecules in three dimensions.
  • Example molecule: Lactic Acid.
    • Has a carboxylic acid group.
    • Contains one chirality center (sp3 hybridized carbon).
    • Results in two stereoisomers (enantiomers).
    • Discussed the R enantiomer of lactic acid:
      • Oxygen gets the highest priority for absolute configuration.
      • Assigns R configuration (clockwise sequence of priorities).

Drawing Fischer Projections

  • To draw a Fischer projection:
    • Look down at the chirality center.
    • Flat representation:
      • Horizontal lines represent bonds coming out of the plane.
      • Vertical lines represent bonds going behind the plane.
    • Example:
      • Carboxylic acid and CH3 below the plane.
      • OH on the right; H on the left (coming out of the plane).
      • Introduced by Emil Fischer for carbohydrates.

From R to S Lactic Acid

  • Reflect the molecule in a mirror to obtain the S enantiomer.
  • S-lactic acid is found in muscles post-exercise.
  • Lactic acid from milk is a racemic mixture (50% R and 50% S).

Fischer Projections in Carbohydrates

  • Example: 4-carbon carbohydrate.
    • Two chirality centers (carbon 2 and carbon 3).
    • Formula for stereoisomers: 2^n (n = number of chirality centers).
    • Results in 4 possible stereoisomers.

Sawhorse to Fischer Projection

  • Convert sawhorse projection to Fischer:
    • Look down at specific bonds.
    • Assign directions for groups (e.g., OH and H).
    • Example given for aldehyde group.

Identifying Stereoisomers and Enantiomers

  • Draw and label four stereoisomers (A, B, C, D):
    • C and D are enantiomers.
    • A and B are enantiomers.
    • A and C are diastereomers (non-mirror images).

Absolute Configuration Assignment

  • Assign configurations in Fischer projections:
    • Use priorities (assigned numbers based on atomic numbers).
    • Example given for carbon 2 chirality center.
    • Reverse configuration if hydrogen is coming out of the plane.
    • Example: Carbon 2 and 3 both R (2R, 3R configuration).

Conclusion

  • Fischer projections simplify working with stereochemistry, especially in carbohydrates.
  • Recommended practice with molecular model sets for better visualization.