Lecture on Isomers

Introduction

• Presenter: Iman
• Continuation from previous lecture on isomers
• Today's focus: Configurational Isomers

Chirality

• Chiral Objects: Not superimposable on their mirror images
  • Example: Human hands
• Chiral Center: A carbon with four different groups attached
  • Identification of chiral centers in molecules is crucial
  • Lack of internal plane of symmetry

Types of Stereoisomers

1. Enantiomers
   • Non-superimposable mirror images
   • Same connectivity, differ in configuration at chiral centers
2. Diastereomers
   • Chiral but not mirror images
   • Differ at some but not all chiral centers

R and S Configuration

Steps to Assign R/S Configuration

1. Identify four atoms attached to the chiral center
2. Assign Priority based on atomic number
   • Highest atomic number = Priority 1
   • Lowest atomic number (often H) = Priority 4
3. Resolve ties by moving outwards from the chiral center
4. Rotate molecule so Priority 4 is on a dash
   • Dash = behind the plane
   • Wedge = out of the plane
5. Determine sequence direction:
   • Clockwise: 'R' configuration
   • Counterclockwise: 'S' configuration
6. Special Cases
   • Priority 4 next to a dash: Assign opposite configuration
   • Priority 4 across from a dash: Use direct sequence

Examples

• Example molecules and identification of chiral centers
• Assigning priorities and determining R/S configuration

CIS and TRANS Configuration

E/Z Nomenclature

• E/Z Forms for poly-substituted double bonds
  • E: High-priority groups on opposite sides
  • Z: High-priority groups on same side

Optical Activity

• Optical Rotation:
  • D (dextrorotary): Rotates light clockwise, labeled '+'
  • L (levorotary): Rotates light counterclockwise, labeled '-'
• Racemic Mixtures: Equal concentrations of enantiomers, no optical activity

Fisher Projections

Converting Fisher Projections to Bond Line Notation

1. Identify chains and groups
2. Assign dashes and wedges
3. Adjust structure to bond-line format

Types of Isomers

1. Enantiomers: Opposite R/S configuration at all chiral centers
   • Same chemical/physical properties except optical activity
2. Diastereomers: Mix of same and opposite R/S configurations
   • Different chemical/physical properties
3. Meso Compounds: Achiral, internal plane of symmetry, optically inactive
   • Two or more chiral centers

Summary Points

• Configurational Isomers can only be interchanged by breaking/reforming bonds
• R/S Configuration using priority rules
• E/Z Nomenclature for double bonds
• Enantiomers: Opposite at all chiral centers
• Diastereomers: Mix of opposite/same configurations
• Meso Compounds: Internal symmetry, optically inactive

Practice & Resources

• Next Lecture: Practice Problems
• Additional Resources: Ochem1 playlist (Chapters 4 & 5)

Closing

• Comments, questions, and further practice suggestions
• Good luck and happy studying!