Lecture Notes on Chirality

Introduction to Chirality

• Chiral Molecules: A type of stereoisomer with non-superimposable mirror images.
  • Example: Right and left hands.
• Achiral Molecules: Have superimposable mirror images.
  • Feature: Plane of symmetry.

Stereoisomers

• Stereoisomers: Same bonding sequence but different atomic orientation in space.
  • Types previously discussed: Geometric (cis-trans) isomers.
  • Chiral Molecules: Lack internal mirror planes, leading to non-superimposable images.
  • Achiral Molecules: Contain internal mirror planes, leading to superimposable images.

Identifying Chirality

• Mirror Plane of Symmetry: Tool for identifying achiral molecules.
  • Example: Spooning an object along its axis.
  • Molecules similar: cis-1,2-dichlorocyclopentane (achiral), trans-1,2-dichlorocyclopentane (chiral).
• Exercise: Practice identifying mirror planes using molecules and modeling kits.

Chiral Centers

• Chiral Center: Typically an sp3 hybridized atom bonded to four different groups.
  • Organic chemistry focus: Often a carbon atom.
  • Example: 2-bromobutane with a chiral center.
  • Enantiomers: Mirror image isomers derived from chiral molecules.

Stereocenters vs. Chiral Centers

• Stereocenter: An atom where the interchange of groups creates a stereoisomer.
  • All chiral centers are stereocenters, not vice versa.
  • Example Molecules:
    • Two stereocenters but no chiral centers (sp2 hybridized). Example: Cis/trans isomers.
    • Molecule with a chiral center (sp3 hybridized, four different groups).
    • Molecule with no chiral or stereocenters (symmetric).

Additional Examples

• Practice with molecules to understand stereocenters and chiral centers.
• Importance of internal mirror planes in determining chirality.

Conclusion

• Chirality is determined by presence/absence of a mirror plane and chiral centers.
• Practice identifying features in molecular models to build intuition on molecular chirality and symmetry.