Overview
This lecture covers the concept of resonance in organic chemistry, including the nature of delocalized electrons, rules for drawing resonance structures, stability trends, and the connection between resonance and hybridization.
Basics of Resonance
- Resonance involves delocalized electrons, typically delocalized π (pi) electrons.
- Resonance structures are different valid Lewis structures for a molecule, not real states but tools for representing electron distribution.
- The actual molecule is best represented by a resonance hybrid, an average of resonance structures.
Drawing Resonance Structures
- Only non-bonding electrons and π electrons can be moved in resonance structures.
- Non-bonding electrons can move to an adjacent bond; π electrons can move to an adjacent bond or atom.
- Use curved double-headed arrows for movement of two electrons and single-headed arrows for one electron.
- Move electrons stepwise—resonance structures must form a continuous, linear sequence (no branching).
- Lone pairs cannot move toward sp³ hybridized atoms.
Resonance Stabilization and Major Contributors
- Resonance delocalizes charges and electrons, lowering molecule energy ("resonance stabilization").
- Major resonance contributors are most stable, often with filled octets and charges on appropriate atoms.
- For positive charges, place them on the least electronegative atom with a full octet preferred.
- For negative charges, place them on the most electronegative atom.
- More resonance structures usually mean greater stability.
Common Types of Resonance Participants
- Allylic: Atom one bond away from a double bond (alkene).
- Benzylic: Atom one bond away from a benzene ring.
- Resonance occurs if a positive charge, lone pair, or radical is found at allylic or benzylic positions.
Resonance Cases: Carbocations, Lone Pairs, and Radicals
- Carbocations (C⁺) are stabilized when one bond away from π electrons or adjacent to lone pairs.
- Lone pairs (carbanions) and radicals also show resonance if adjacent to π systems, with different electron movement rules.
- For lone pair stabilization, typically two curved arrows are drawn: lone pair becomes a π bond, π bond becomes a lone pair.
Trends in Resonance Stability
- Carbocation stability: tertiary > secondary > primary > methyl.
- Carbanion stability: methyl > primary > secondary > tertiary.
- Radical stability: tertiary > secondary > primary > methyl.
Resonance & Hybridization
- Resonance may alter expected hybridization; atom involved in resonance cannot be sp³ even if it appears so in one structure.
- Atoms with lone pairs one bond away from π electrons involved in resonance will be sp² hybridized.
Key Terms & Definitions
- Resonance Structure — Alternative Lewis structures for a molecule showing electron delocalization.
- Resonance Hybrid — The real structure, averaged from all resonance contributors.
- Delocalized Electrons — Electrons spread over more than two atoms, not confined to a single bond.
- Allylic — Position one single bond away from a C=C double bond.
- Benzylic — Position one single bond away from a benzene ring.
- Curved Arrow — Symbolizes electron movement in resonance or reaction mechanisms.
Action Items / Next Steps
- Practice drawing resonance structures and hybrids for various organic ions and molecules.
- Review stability trends for carbocation, carbanion, and radical resonance contributors.
- Complete homework assignments or practice problems as assigned.