Organic Chemistry: Resonance and Line Angle Formula

Introduction

• This lecture covers the line angle formula and resonance in organic chemistry.
• Understanding the line angle formula is crucial for drawing resonance structures quickly.

The Line Angle Formula

• Basics:

  • Represents carbon atoms at the ends and angles of lines.
  • Hydrogens bonded to carbons are not shown.
  • Simplifies the drawing of organic molecules.

• Example of Butane:

  • A simple line with endpoints and angles represents the carbon atoms.
  • The line angle formula does not depict the carbon and hydrogen atoms explicitly.

• Additional Practice:

  • The molecule with three carbons and a double bond (possible chlorine) can be quickly represented using the line angle formula.
  • Counts: 3 Carbons, 8 Hydrogens, 1 Oxygen.

Understanding Resonance

• Definition:

  • Movement of electrons without forming/destroying sigma bonds.
  • Uses pi bonds as a tool for electron movement.

• Example with Electrophile and Water:

  • Demonstrates a reaction, not resonance, as it involves forming a sigma bond.

• Resonance in Carboxylic Acid:

  • Distributes negative charge across atoms for stability.
  • Movement occurs without altering sigma bonds.

• Resonance Hybrid:

  • Illustrates possible electron distribution using dashed lines.

Mechanism of Resonance

• Electron Movement:

  • Electrons in p orbitals are redistributed.
  • Hybridization states can change during resonance.

• Example of Movement:

  • Electrons move between atoms, forming new pi bonds and shifting hybridization states.
  • Resonance hybrids represent an average of all resonance structures.

Conditions for Resonance

• Basic Scenario:

  • Atoms with lone pairs or negative charges adjacent to double bonds can show resonance.

• Resonance Exceptions:

  • Resonance doesn’t occur if carbon forms more than four bonds, or if adjacent atoms lack double bonds.

Stability Through Resonance

• Resonance Stability:

  • Spreading electron density lowers energy.
  • More resonance structures lead to greater stability.

• Deconstructive vs. Constructive Resonance:

  • Deconstructive builds unfavorable charges, less stable.
  • Constructive spreads charges, enhancing stability.
  • Aromaticity is a highly stable form of constructive resonance.

Examples

• Resonance with Amines and Double Bonds:
  • Lone pairs can donate and form resonance structures, showing partial charges.
• Example with Single and Double Bonds:
  • Resonance fails if it leads to unstable charges, like a negative charge on carbon.

Importance

• Resonance plays a crucial role throughout organic chemistry.
• It affects chemical reactivity and stability of compounds.

Conclusion

• Understanding line angle formula and resonance is essential in organic chemistry.
• Revisit the material and practice for better comprehension.
• Reach out with questions during office hours or via email.