Overview
This lecture covers resonance theory in chemistry, focusing on when and how to apply resonance structures, the notation involved, and examples illustrating electron delocalization.
Resonance Theory Basics
- Resonance theory is used when a single Lewis structure cannot accurately represent a molecule or ion.
- Multiple resonance structures are drawn in brackets and connected by double-headed arrows.
- The real molecule is a hybrid (blend) of all resonance structures; individual structures do not exist in reality.
- Resonance is not the same as equilibrium; resonance structures do not coexist independently.
When to Use Resonance
- Resonance applies when three or more adjacent (next to each other) p-orbitals or sp² atoms are properly aligned.
- Delocalization occurs in systems with at least three adjacent sp² or p-orbital atoms.
- Assign hybridization to all atoms (not just carbons) to identify sp² centers for resonance.
- Lone pairs next to double bonds or positive charges often indicate resonance possibilities.
Drawing Resonance Structures
- Only π (pi) electrons and lone pairs move in resonance; sigma bonds and atoms remain fixed.
- Use curved arrows to indicate electron movement in resonance structures.
- Arrows start at electron sources (lone pairs or π bonds) and end at electron sinks (atoms or bonds).
- All resonance structures must have the same net charge.
Key Examples & Concepts
- Delocalized systems: multiple resonance contributors share electron deficiency (positive charge) or excess.
- Polarity and electron distribution can be shown using resonance, such as in carbonyl groups.
- Functional groups like amides and carbocations (carbocations = carbons with a positive charge) are analyzed for resonance by assigning hybridization to each atom.
- Not all resonance structures contribute equally to the resonance hybrid.
Key Terms & Definitions
- Resonance Hybrid — the real structure, representing an average of all resonance contributors.
- Resonance Structure — alternate Lewis structures showing possible delocalizations.
- Curved Arrow Notation — arrows used to show electron flow in resonance.
- SP² Hybridization — atom with a double bond or lone pair next to π systems.
- Delocalization — spreading of electrons across multiple atoms.
- Carbocation — a carbon atom bearing a positive charge.
Action Items / Next Steps
- Complete assigned reading on resonance theory.
- Practice drawing resonance structures and assigning hybridizations for organic and inorganic molecules with provided problems.