Lecture Notes: Resonance Structures and Formal Charge by Professor Dave

Introduction

• Key topic: Resonance structures in organic chemistry.
• Importance of understanding pi electrons in resonance.

Pi Bonds and Resonance

• Pi Bond Formation:

  • Double/triple carbon-carbon bonds consist of one sigma bond and one/two pi bonds.
  • Example: Carbon-carbon double bond has one sigma and one pi bond.
  • Carbons in a double bond are sp2 hybridized with 3 electron domains.
  • Unhybridized p orbitals overlap to form pi bonds.

• Resonance Participation:

  • Electrons in pi bonds and lone pairs participate in resonance.

Formal Charge

• Determining Formal Charge:

  • Compare number of electrons an atom contributes to a Lewis structure vs. its typical valence electrons.
  • Example:
    • Oxygen typically has 6 valence electrons. If showing 7 in a structure (6 from lone pairs + 1 from a bond), it has a formal negative charge.
    • Nitrogen typically has 5. If contributing 4 (as in nitrate ion), it has a formal positive charge.

• Simple Rule: Compare electrons contributed to typical valence.

Resonance Structures

• Delocalization of Pi Electrons:

  • Pi electrons can move within a molecule to form stable resonance structures.
  • Sigma electrons can't delocalize as they are involved in direct orbital overlap.

• Drawing Resonance Structures:

  • Use brackets and double-headed arrows to denote resonance structures.
  • Electron pushing arrows indicate movement from electron-rich to electron-poor areas.

• Composite Resonance Structure:

  • No individual resonance structure exists.
  • Composite structure represents actual distribution of pi electron density.

Examples and Key Concepts

• Equivalent Resonance Structures:

  • Example with nitrate ion shows three equivalent structures.
  • Composite structure represents delocalized negative charge over the molecule.

• Non-equivalent Resonance Structures:

  • Unequal contribution in resonance structures based on atom's ability to accommodate charge.
  • Electropositive atoms (like carbon) accommodate positive charges better.
  • Electronegative atoms (like oxygen) accommodate negative charges better.

• Neutral Atoms in Resonance:

  • Structures with all neutral atoms contribute more strongly than those with charges.

Validity of Resonance Structures

• Must be a valid Lewis structure.
• Cannot violate rules like carbon having five bonds.

Conclusion

• Resonance structures aid in understanding molecule behavior.
• Composite resonance structure is the real representation in nature.
• Further tutorials available on the professor's channel.