Lecture on Diastereomers

Definition

• Diastereomers: Stereoisomers that are not mirror images.
  • Broad definition.
  • Includes geometric isomers (cis-trans isomers).
  • Includes molecules with more than one chiral center that are not enantiomers.

Types of Isomers

1. Constitutional (Structural) Isomers
   • Different order of attachment of atoms.
2. Stereoisomers
   • Same order of attachment, different 3D arrangement.

Types of Stereoisomers

• Enantiomers
  • Mirror image isomers.
  • Identical physical properties, except optical activity (rotation of plane-polarized light).
• Diastereomers
  • Non-mirror image isomers.
  • Different physical properties.
  • Can be separated by methods like recrystallization, distillation, and chromatography.

Geometric (Cis-Trans) Isomers

• Exist due to lack of rotation around double bonds or rings.
• Are diastereomers because they are non-mirror image stereoisomers.

Other Diastereomers

• Molecules with more than one chiral center that are not mirror images.

Separation Techniques

• Diastereomers have different physical properties, allowing separation by lab techniques:
  • Recrystallization
  • Distillation
  • Chromatography
• Separating enantiomers is more difficult and will be discussed later.

Review of Isomers

• Isomers: Structures with the same molecular formula but are different compounds.
• Types of Isomers:
  • Constitutional Isomers: Different atom attachment.
  • Stereoisomers: Same atom attachment, different spatial arrangement.

Key Points

• Be specific when determining the relationship between molecules.
• Key distinction: Enantiomers vs. Diastereomers in terms of mirror image relationship and physical properties.