Lecture on Stereochemistry: Configurational Isomers

Overview

• Topic: Configurational isomers, stereochemistry, enantiomers, diastereomers
• Key question: What is the relationship between two given molecules?
• Focus: Identifying and distinguishing between different types of isomers

Identifying Isomers

1. Same Molecular Formulas: Check if the molecules have identical molecular formulas.
2. Same Connectivity: Ensure the connectivity of atoms in the molecules is the same.
3. Orientation in Space: Analyze the spatial arrangement to identify stereoisomers.

Types of Stereoisomers

Enantiomers

• Definition: Molecules that are mirror images of each other but are non-superimposable.
• Characteristics:
  • Same molecular formula
  • Same connectivity
  • Non-superimposable mirror images
• Physical Properties: Enantiomers have identical physical properties (boiling point, melting point).

Diastereomers

• Definition: Molecules with the same molecular formula and connectivity but different spatial arrangement, not mirror images.
• Characteristics:
  • Same molecular formula
  • Same connectivity
  • Different orientation in space
  • Not mirror images
  • Non-superimposable
• Physical Properties: Diastereomers have different physical properties (boiling point, melting point).

Examples and Analysis

• Enantiomers Example: Molecules A and B
  • Mirror images
  • Non-superimposable
  • Therefore, enantiomers
• Diastereomers Example: Molecules with same connectivity but differing bond orientations around double bonds
  • Not mirror images
  • Non-superimposable
  • Therefore, diastereomers

Stereocenters and Relationships

• Molecules can have multiple stereocenters, leading to different isomer combinations.
• Example Analysis: Analyzing relationships between molecules with two stereocenters
  • Molecule A and B: Mirror images, enantiomers
  • Molecule A and C: Not mirror images, diastereomers

Physical Properties and Isomers

• Enantiomers: Same physical properties
• Diastereomers: Different physical properties

Importance in Pharmacy and Medicine

• Synthesis Challenges: Difficulty in creating pure isomer due to potential mix of harmful isomers.
• Design and Recognition: Enzymes in the body recognize molecules based on shape, critical for effective drug design.

Formula for Determining Number of Isomers

• Formula: 2^n (n = number of stereocenters)
  • Provides the maximum number of stereoisomers
  • Example: 2 stereocenters = 4 isomers (maximum)
• Considerations: Actual number can be less due to meso compounds

Meso Compounds

• Definition: Molecules with at least two stereocenters and a plane of symmetry, making them achiral
• Impact: Reduces the number of stereoisomers
• Example: Identifying meso compounds within possible isomers

Symmetry in Molecules

• Plane of Symmetry: Achiral
• Rotational Symmetry: Does not determine chirality

Fischer Projections

• Purpose: Simplified representation to quickly view stereochemistry, especially for sugars
• Conventions:
  • Horizontal lines: Wedges (out of the plane)
  • Vertical lines: Dashes (into the plane)

Converting Between Representations

• Zigzag to Fischer: Rotate molecule in 3D space, identify stereocenters, and align accordingly
• Fischer to Zigzag: Reverse process

Importance in Determining Configuration

• RS Configuration: Assign priorities, determine sequence, and identify if configuration matches expected outcome based on stereocenter positioning.

Conclusion

• Visual Learning: Using model kits to build and understand 3D structures
• Getting Help: Importance of seeking assistance early to grasp complex concepts