Lecture on Relative Configuration

Introduction

• Relative configuration describes the spatial position of atoms/groups relative to others in a molecule.
• Different rules apply based on the number of substituents.

Case with Two Substituents

• Focus on substituents around double bonds or rings.
• Example: Two isomers of a double bond where bromines are on opposite or same sides.
  • Trans configuration: Substituents (e.g., bromines) on opposite sides.
  • Cis configuration: Substituents on the same side.
• Applies to rings as well:
  • Trans: Bromine atoms on opposite sides of the ring.
  • Cis: Bromine atoms on the same side of the ring.
• Stability:
  • Trans isomers are generally more stable due to fewer steric interactions.
  • Steric interactions occur when groups' electron clouds overlap, causing repulsion.

Case with More Than Two Substituents

• More complex assignment of relative configuration:
  • Use Cahn-Ingold-Prelog (CIP) priority rules:
    • Rank by atomic number, then atomic mass, then first point of difference.
    • Multiple bonds count as multiple atoms.
  • Assign priorities to substituents around a double bond (e.g., fluorine > carbon > deuterium > hydrogen).

E/Z Nomenclature

• E/Z Assignment rather than Cis/Trans for more than two substituents:
  • E (Entgegen): Highest priority groups on opposite sides (like Trans).
  • Z (Zusammen): Highest priority groups on the same side (like Cis).

Example Analysis

• Example 1:
  • Highest priorities on opposite sides (E configuration).
• Example 2:
  • Highest priorities on the same side (Z configuration).

Conclusion

• Understanding relative configuration involves recognizing spatial arrangements and applying CIP rules to assign E/Z based on priority of substituents.