Stereochemistry Lecture Notes

Introduction

• Topic: Stereochemistry, R-S configuration assignment
• Focus includes Fischer projections and various examples

Chiral Centers

• Definition: A chiral center, or chiral carbon, is a carbon atom with four different groups attached.

R-S Configuration Assignment

1. Con Ingall Pre-Log Process:

   • Rank the four groups by atomic number (highest to lowest).
   • Example: Bromine (highest), Chlorine, Carbon, Hydrogen (lowest).
   • Priority Assignment:
     • Group 1: Highest priority
     • Group 4: Lowest priority, must be in the back (hatch wedge)
   • Rotation and Configuration:
     • Clockwise rotation: R configuration
     • Counterclockwise rotation: S configuration

2. Example Assignments:

   • Bromine: Group 1, Chlorine: Group 2, Methyl: Group 3, Hydrogen: Group 4
   • If counterclockwise with H in the back: S configuration
   • If clockwise: R configuration

Relationship Between Molecules

• Enantiomers:
  • Mirror images, opposite R-S configurations
  • Example: One molecule S, the other R

Additional Practice Problems

• Assign R-S configurations to various chiral centers
• Example groups: Ethyl, Hydrogen, Bromine, Hydroxyl
• Assign priorities and determine configurations using the same principles

Naming Stereoisomers

1. Identify Chiral Centers
   • Assign R-S configuration
   • Count carbons in the parent chain
2. Example:
   • Find bromine, chlorine groups
   • Assign configurations
   • Name using IUPAC system (e.g., S-2-bromo-butane)

Number of Stereoisomers

• One chiral center: 2 possibilities (R or S)
• Two chiral centers: 4 possibilities
• General formula: (2^n) (where n = number of chiral centers)

Fischer Projections

• Understanding Fischer Projections:
  • Horizontal lines: Groups in the front
  • Vertical lines: Groups in the back
• Assign configurations similarly, reversing when H is in front

Advanced Problem Solving

• Dealing with Complex Structures:
  • Identify chiral centers in compounds like cholesterol
  • Determine possible stereoisomers using (2^n)

Special Techniques

• Assigning Configurations When H is Neither Front nor Back:
  • Use triangle and circle technique for visualization
  • Example provided: Assigning groups into a structured format for evaluation

These notes should serve as a comprehensive guide for understanding stereochemistry, R-S configurations, and practical application in problem-solving and nomenclature.