Lecture Notes: Fischer Projections and Stereochemistry

Key Concepts

• Fischer Projections: A method to represent molecules, particularly sugars, in a two-dimensional form.

  • Visualized as resembling fish bones.
  • Used to simplify complex 3D molecular structures.

• Structure Conversion:

  • Converting between zigzag and Fischer projections.
  • Importance of understanding the representation of bonds in Fischer projections:
    • Horizontal lines represent wedges (atoms coming out of the plane).
    • Vertical lines represent dashes (atoms going into the plane).

Understanding Stereochemistry in Fischer Projections

• Prioritizing Groups:
  • Assign priorities to the groups attached to a chiral center (typically numbered 1, 2, 3, and 4).
  • Determine the configuration (R or S) by the sequence of priorities around the chiral center:
    • Clockwise sequence is usually R.
    • Counterclockwise sequence is usually S.
  • Important Rule: If the lowest priority group is a wedge (horizontal line in Fischer), take the opposite of the calculated configuration.

Practical Steps

• Modeling Molecular Structures:
  • Encouraged to use molecular model kits to physically build molecules.
  • Helps visualize how Fischer projections relate to actual 3D structures.

Application to Sugars

• Analyzing Sugar Molecules:

  • Typical sugar molecules have multiple stereocenters (e.g., 6-carbon sugars may have 4 stereocenters).
  • Number carbons starting from the carbonyl group.
  • Ensure stereochemistry in projections matches throughout the molecule.

• Rotating and Matching Configurations:

  • Rotate molecules 90 degrees to visualize stereochemistry correctly.
  • Match OH and H positions on Fischer projections to ensure accuracy in stereochemical representation.

Tips and Advice

• Visual Learning:
  • Building models aids comprehension of molecular structures beyond 2D representations.
  • Recommended to draw and model molecules for better understanding and retention.

The lecture emphasized the importance of visual aids and practice in mastering the concepts of Fischer projections and stereochemistry. The use of model kits is highly suggested to better grasp the three-dimensional nature of molecules.