Crash Course Organic Chemistry: Stereochemistry and Chirality

Introduction

• Host: Deboki Chakravarti
• Discusses stereochemistry through a story involving Dr. Gilbert Levin's Mars exploration.
• L-glucose vs. D-glucose: L-glucose tastes the same but is indigestible and hence not used for energy or fat storage.

Importance of Molecular Structure

• Structure affects the chemical behavior: D-glucose provides energy while L-glucose is ignored by our bodies due to stereochemistry.

Isomers

• Definition: Molecules with the same parts but small differences.
• Types of Isomers:
  • Constitutional Isomers: Same molecular formula, different atom connections.
  • Geometric Isomers:
    • Molecules with double bonds.
    • Cis Isomers: Groups on the same side.
    • Trans Isomers: Groups on opposite sides.

Stereochemistry

• Stereoisomers: Same atom connections, different spatial arrangements.
• Chirality:
  • Molecules with non-superimposable mirror images.
  • Example: Hands are non-superimposable mirror images.

Chiral Centers

• Simplest example: Carbon with four different groups.
• Naming:
  • Enantiomers: Pair of non-superimposable mirror images.
  • Cahn-Ingold-Prelog Convention:
    • R (right-handed)
    • S (left-handed)

Assigning Priority and Drawing Enantiomers

• Priority Assignment:
  • Assigned based on atomic number of atoms attached to a chiral center.
  • Example of butan-2-ol: Hydrogen is the lowest priority.

Drawing Enantiomers

• Example of albuterol: Chiral center marked and enantiomer drawn using mirror reflection.
• Solid and Dashed Wedges: Indicate spatial orientation of groups.

Naming Enantiomers

• Based on priority and spatial orientation around the chiral center.
• Examples: (2R)-butan-2-ol and (2S)-butan-2-ol.

Dealing with Cyclic Compounds

• Concept of internal planes of symmetry.
• Achiral Compounds: Superimposable mirror images.
• Example: Methylcyclopentane.

Molecules with Multiple Chiral Centers

• Chiral vs. Achiral:
  • Based on superimposability and internal planes of symmetry.
• Example: cis- and trans-1,2-dibromocyclohexane.

Practice Problems

• Determining chirality based on structure and symmetry.
• Examples provided to apply concepts.

Conclusion

• Stereochemistry is crucial in organic chemistry.
• Upcoming episodes will cover properties of enantiomers and their separation.

Supporting Crash Course

• Encouragement to support Crash Course through Patreon to keep content free.