Organic Chemistry Lecture: Resonance and Line Angle Formula

Line Angle Formula

• Introduced to aid in drawing resonance structures quickly.
• A line represents a bond between carbons.
  • Starting and ending points of lines indicate carbon atoms.
  • Angles in lines also represent carbon atoms.
• Hydrogens attached to carbons are not shown in this representation.
• Example: Butane can be drawn with lines, ignoring hydrogen atoms.
• Key points:
  • Carbons are represented by endpoints and angles.
  • Hydrogen on carbon is not shown.
  • Makes drawing and visualizing organic molecules faster.

Drawing Practice

• Line angle formula helps in rapidly converting structures.
• Example: A molecule with three carbons and a chlorine is quickly represented.
• Questions to consider:
  • Total number of carbons, hydrogens, oxygens in a structure.
  • Account for hidden hydrogens.

Resonance

• Definition: Movement of electrons without forming or breaking sigma bonds.
• Important concepts:
  • Sigma bonds remain intact; pi bonds can move.
  • Electrophile: Electron-deficient species seeking electrons.
• Example: Oxygen in molecule forms sigma bonds via lone pair attack.

Resonance Structures

• Demonstrated using carboxylic acids.
• Electrons move to spread charge, improving stability.
• Resonance Hybrid: Shows potential electron movement.
• Analogy: Spreading debt over many people to lessen individual burden.

Molecular Orbital Perspective

• Electrons move between atomic orbitals.
• Changes in hybridization occur due to resonance.
• Double-edged arrows indicate reversible electron movement.

Examples of Resonance and Non-Resonance

• Valid Resonance:
  • Lone pairs or negative charges interacting with adjacent pi bonds.
• Invalid Resonance:
  • Lone pairs on sp3 carbon; no p orbitals available for interaction.

Stabilizing Effects of Resonance

• More resonance = more stability.
• Non-stabilizing resonance raises molecule's energy.

Deconstructive vs. Constructive Resonance

• Deconstructive Resonance:
  • Results in high energy, less stability, often non-existent.
• Constructive Resonance:
  • Stabilizes molecules without creating extreme charges.

Special Case: Aromaticity

• Highly stabilizing form of resonance.
• Found in DNA bases for stability.

Resonance Practice

• Example with a double bond and nitrogen.
• Resonance involves lone pair donation and breaking pi bonds.
• Constructive due to partial charges.

Conclusion

• Resonance is crucial for chemical reactivity and stability.
• Understanding both line angle formulas and resonance can aid in organic chemistry comprehension.
• Review the concepts frequently and ask questions if needed.