Lesson 2: Resonance and Lewis Structures (Copy)

Resonance and Lewis Structures for Organic Chemistry Students

Introduction

• Review of Previous Lesson:

  • Quick recap of key points from Lesson 1, including the basics of organic chemistry, resonance, and formal charge.

Resonance Structures

• Concept of Resonance:

  • Define resonance in organic chemistry.

  • Emphasis on electron delocalization and its impact on molecular stability.

• Rules for Drawing Resonance Structures:

  • Ensure the same number of valence electrons in all structures.

  • Maintain the positions of nuclei.

  • Use only valid Lewis structures (obey the octet rule where applicable).

• Steps for Drawing Resonance Structures:

  • Identify possible resonance contributors.

  • Use arrows to indicate electron movement.

  • Draw and compare all possible resonance structures.

• Examples:

  • Simple molecules (e.g., ozone, nitrate ion).

  • Complex molecules (e.g., benzene, allylic cations).

Evaluating Resonance Structures

• Criteria for Stability:

  • Structures with full octets are more stable.

  • Minimize formal charges; structures where formal charges are closest to zero are more favorable.

  • Negative charges on more electronegative atoms increase stability.

  • Positive charges on less electronegative atoms or more substitutable positions are preferred.

• Resonance Hybrid:

  • Concept of a resonance hybrid representing the true structure of a molecule.

  • Visualizing the contribution of each resonance structure to the hybrid.

Lewis Structures

• Introduction to Lewis Structures:

  • Definition and importance in predicting molecular geometry, reactivity, and physical properties.

• Steps for Drawing Lewis Structures:

  • Count total valence electrons.

  • Connect atoms using single bonds.

  • Distribute remaining electrons to satisfy the octet rule (or duet rule for hydrogen).

  • Use double or triple bonds if necessary to satisfy the octet rule.

• Practice Examples:

  • Simple molecules (e.g., water, carbon dioxide).

  • Polyatomic ions (e.g., ammonium ion, sulfate ion).

Combining Resonance and Lewis Structures

• Application of Formal Charge:

  • Revisit the concept of formal charge and its calculation.

  • Use formal charges to determine the most stable Lewis structure.

• Practice Problems:

  • Drawing Lewis structures and identifying resonance contributors.

  • Evaluating the stability of different resonance structures using formal charge and other stability criteria.

Summary and Conclusion

• Review of Key Concepts:

  • Importance of resonance in understanding molecular stability and reactivity.

  • Steps and rules for drawing both resonance and Lewis structures.

  • Criteria for evaluating the stability of resonance structures.

• Looking Ahead:

  • Brief introduction to upcoming topics, such as isomerism, stereochemistry, and organic reactions.

Practice Problems and Exercises

• Exercises and Practice Problems:

  • Drawing resonance structures for a variety of molecules and ions.

  • Determining the most stable resonance structure using formal charges