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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.
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