Overview
This lecture covers Fischer projections, a method for representing molecules with multiple chiral centers, focusing on their use, interpretation, and conversion to bond-line structures.
Introduction to Fischer Projections
- Fischer projections are used to represent molecules, especially sugars, with multiple chiral centers.
- Named after Emil Fischer, these diagrams simplify the visualization of tetrahedral chiral centers.
Drawing Fischer Projections
- In a Fischer projection, horizontal lines represent bonds coming out of the plane (wedges), and vertical lines represent bonds going behind the plane (dashes).
- This method provides a clear, consistent way to depict stereochemistry without in-plane bonds.
Assigning R and S Configuration
- Assign priorities to four groups at the chiral center using Cahn-Ingold-Prelog rules.
- If the lowest priority group is on a vertical line (dash), a right-handed turn is R and left-handed is S.
- If the lowest priority group is on a horizontal line (wedge), the assignment is the opposite (right-handed is S, left-handed is R).
- Fischer projections eliminate the issue of the lowest priority group being "in the plane."
Example: D-Glucose Fischer Projection
- D-glucose has four chiral centers commonly shown in Fischer projections for quick R/S assignments.
- The method for converting Fischer projections to bond-line structures involves matching R/S configurations for each chiral center.
Converting Between Representations
- Bond-line structures provide a true 3D representation; Fischer projections are only accurate for one chiral center at a time.
- When converting, assign R/S to each center and arrange wedge/dash bonds to match.
- Swapping two groups inverts configuration (R ↔ S).
Rotations and Manipulating Projections
- Bond-line structures can be rotated freely without changing identity.
- Fischer projections can only be rotated 180° in the plane of the board; any other rotation alters the stereochemistry.
Key Terms & Definitions
- Fischer Projection — a 2D representation of a molecule showing the spatial arrangement of groups around chiral centers.
- Chiral Center — a carbon atom bonded to four different groups.
- Wedge — a bond projecting out of the plane towards the viewer.
- Dash — a bond projecting behind the plane away from the viewer.
- R/S Configuration — labels for absolute configuration assigned using Cahn-Ingold-Prelog rules.
Action Items / Next Steps
- Practice converting Fischer projections to bond-line structures and assigning R/S configurations.
- Review provided study guides and attempt practice problems on Fischer projections and stereochemistry.