Stereochemistry Lecture Notes

Introduction to Stereochemistry

• Stereochemistry Definition: Study of the spatial arrangement of atoms in molecules.
• Chiral Center: A carbon atom bonded to four different groups.
• R-S Configuration: Method for naming the orientation of chiral centers.

Assigning R-S Configuration

• Priority Ranking: Use the Cahn-Ingold-Prelog priority rules.

  • Based on atomic number: Higher atomic number = higher priority.
  • Example: Br > Cl > C > H.

• Configuration Steps:

  1. Assign priorities (1 to 4) to substituents around chiral carbon.
  2. Ensure group 4 is positioned at the back.
  3. Determine rotation direction:
     • Counterclockwise = S
     • Clockwise = R

Example Analysis

• Example Molecules: Assign R/S to given chiral centers and describe relationships between molecules.
• Enantiomers:
  • Mirror images with opposite configurations (R/S).
  • Not the same in spatial arrangement but same connectivity.

Practice Assignments

• Practice Molecule 1:
  • Ethyl group, Bromine, Hydroxyl group, Hydrogen.
  • Assign R/S following priority rules.
• Example Results: Correct placement of H in spatial diagrams (wedge/dash).

Naming Stereoisomers

• IUPAC Naming with R-S system:
  • Identify chiral centers and assign configurations.
  • Follow the numbering of carbon chains and substituents.
• Example: 2R-3S-3-bromo-2-butanol.

Fischer Projections

• Concept: Vertical lines go back, horizontal lines come forward.
• Configuration Assignment: Must reverse configuration when H is horizontal.
• Example: Convert Fischer projection to wedge-dash representation.

Stereoisomer Possibilities

• Stereoisomer Calculation:
  • Formula: 2^n, where n = number of chiral centers.
  • Example: 1 chiral center = 2 isomers, 2 chiral centers = 4 isomers.

Advanced Problems

• Complex Molecules:
  • Cholesterol with multiple chiral centers.
  • Calculation results in 256 possible stereoisomers.

Conclusion

• Key Techniques:
  • Use Cahn-Ingold-Prelog rules for priority.
  • Correct spatial representation for assigning configurations.
  • Use of Fischer projections and problem-solving strategies for complex configurations.
• Final Tips: Practice determining configurations with and without Fischer projections and identifying stereoisomer relationships.