Enantiomers and Diastereomers Lecture Notes

Key Concepts

• Stereochemical Relationships: Enantiomers and diastereomers are the only stereochemical relationships between stereoisomers.
  • If two molecules have the same molecular formula and connectivity, they are either enantiomers or diastereomers, or the same molecule.
  • Descriptions like trans, R, S, E, Z are properties of individual molecules, not relationships.

Enantiomers

• Definition: Two molecules that are mirror images and non-superimposable in space.
  • Example: Molecules A and B are mirror images but cannot be superimposed.
• Chiral Atoms:
  • Stereo descriptors (R, S) switch between enantiomers.

Diastereomers

• Definition: Molecules that are not mirror images and are non-superimposable in space.
  • Example: Molecules C and D are neither mirror images nor superimposable.
• Chiral Atoms:
  • Only some stereo descriptors change.

Mirror Images Techniques

• Creating Mirror Images:
  • Vertical Plane: Reflects like butterfly wings.
  • Horizontal Plane: Flips molecule over the line.
  • Page as Mirror: Flips dashes to wedges and vice versa.

Example of Mirror Imaging

• Five-membered ring example:
  • Bromine on carbon, mirrored through vertical and horizontal planes.
• Open chain example:
  • Bromine and OH group, mirrored in vertical and horizontal planes.

Reflections vs Rotations

• Reflections: Create a new mirror image molecule.
• Rotations: Do not create a new molecule, simply reorient the existing one.
  • Three types of rotations: In-plane, horizontal, and vertical axis.

Misconceptions

• Chiral Atoms Unnecessary:
  • Enantiomers and diastereomers do not require chiral atoms.
  • Example: Cyclohexanes with identical sides (no chiral atoms) can be diastereomers.
• Enantiomers in Non-Chiral Environment:
  • Indistinguishable physically/chemically unless in a chiral environment.
  • Example: Body as chiral environment affecting drug reactions.

Practical Implications

• Separation of Diastereomers:
  • Can be separated via physical methods like chromatography.
• Optical Activity:
  • The only distinguishing property of enantiomers in achiral environments.

Importance in Chemistry

• Stereochemistry in Reactions:
  • Many reactions produce or destroy stereochemistry, important to track.

Conclusion

• Enantiomers and diastereomers are critical in understanding molecular relationships and reactions.
• Real-world implications in drug design and chemical synthesis.

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