Crash Course Organic Chemistry: Stereochemistry and Chirality

Introduction

• Host: Deboki Chakravarti
• Episode Focus: Importance of molecular structure and stereochemistry in organic chemistry.

Martian Life Experiment

• Dr. Gilbert Levin's Experiment: Sent D-glucose and L-glucose to Mars on Viking lander.
• Findings:
  • L-glucose tastes similar to D-glucose but isn’t digestible by humans.
  • It cannot be used as a sugar substitute due to production costs.

Importance of Structure in Chemistry

• Stereochemistry: Key to understanding molecular function.
• Isomers:
  • Constitutional Isomers: Same molecular formula, different atom connectivity.
  • Geometric Isomers: Related to double bonds; cis (same side) and trans (opposite sides).
  • Stereoisomers: Same bonds, different spatial arrangements.

Chirality and Enantiomers

• Chirality: Non-superimposable mirror images.
  • Example: Human hands.
• Chiral Center: Carbon bonded to four different groups.
• Enantiomers: A pair of chiral molecules (opposite mirror images).
  • Example: butan-2-ol has a chiral center.

Drawing Enantiomers

• Mirror Images: Draw reflection across a line (mirror) or using wedges and dashes.
• Example: Albuterol, a common asthma medication.
  • Stereochemistry: Only one carbon with four different groups.
  • Naming Enantiomers: Cahn-Ingold-Prelog convention.

Naming Enantiomers

• Priority Assignment: Based on atomic number.
  • Highest priority to atom with highest atomic number.
• R/S Naming:
  • R (Right-handed): Clockwise priority order.
  • S (Left-handed): Counterclockwise priority order.

Practice with Enantiomers

• 3D Model Method: Hold lowest priority group; twist molecule to assign R/S.
• Example Molecule: albuterol.

Cyclic Compounds and Chirality

• Achiral Compounds: Have internal plane of symmetry.
  • Example: Methylcyclopentane.
• Chiral Compounds: No internal plane of symmetry.
  • Example: 2-methylcyclopent-1-ene.

Molecules with Multiple Chiral Centers

• Enantiomers with Two Chiral Centers: Named by specifying R/S for each center.
  • Example: (3R,4S)-4 bromohexan-3-ol.
• Achiral Molecules with Two Centers: Internal symmetry or superimposable images.
  • Example: cis-1,2 dibromocyclohexane is achiral.

Chirality Assessment

• Flowchart for Chirality: Steps to determine if a molecule has enantiomers.
• Practice Problems: Determine chiral or achiral status.

Conclusion

• Upcoming Topics: Properties of enantiomers, their interaction with light, and separation techniques.
• Support: Crash Course available on Patreon.

Additional Resources

• Crash Course app: Available on Android and iOS for further learning.