Lecture Notes on Fischer Projections

Introduction to Fischer Projections

• Fischer projections are a method to depict molecules, particularly used for sugars.
• Converts zigzag molecular structures into a visual format resembling fish bones.
• Useful to distinguish from Newman projections, which are different structural representations.

Understanding Fischer Projections

• Structure:
  • Central carbon is represented with horizontal lines as wedges and vertical lines as dashes.
  • Essential to grasp that horizontal lines (wedges) come out towards the viewer.

Stereochemistry in Fischer Projections

• Configuration determination (R/S) must account for priority of groups:
  • Prioritize groups attached to a central carbon: 1, 2, 3, 4.
  • Clockwise movement indicates R configuration; counterclockwise indicates S.
  • Trick: If the lowest priority group is a wedge, take the opposite configuration.

Converting Between Zigzag and Fischer Projections

• Conversion Process:
  • Visualize the molecule, orient by spinning to achieve a Fischer-like appearance.
  • Ensure two groups point towards and two away from you.
  • Practice with a model kit to reinforce understanding.

Application to More Complex Molecules

• Application to sugars with multiple stereocenters (e.g., a sugar with 6 carbons and 4 stereocenters).
• Number carbons starting from the key functional group (aldehyde, ketone).
• Stereocenters must maintain consistent stereochemistry when switching forms.

Detailed Process for Determining Stereochemistry

• Rotate molecule 90 degrees to redraw without changing stereochemistry.
• Use horizontal lines (wedges) and vertical lines (dashes) to aid visualization.
  • Approach the molecule from the correct angle to determine left/right hand positions ("handlebar analogy").

Practice and Visualization

• Encouragement to build and manipulate model systems for better visualization.
• Practice moving between zigzag and Fischer projections to solidify understanding.

Conclusion

• Emphasize the importance of visual understanding in stereochemistry.
• Encourage use of model kits for practical learning.