Overview
This lecture covers the five main resonance bonding patterns, strategies for recognizing resonance, and rules for determining major versus minor resonance structures in organic molecules.
The Five Resonance Bonding Patterns
- There are five general patterns where resonance can occur in molecules.
- Pattern 1: Allylic lone pair—lone pair on an atom adjacent to a double bond.
- Pattern 2: Allylic carbocation—positive charge adjacent to a double bond.
- Pattern 3: Lone pair adjacent to a carbocation—lone pair next to a positively charged atom.
- Pattern 4: Pi bond between atoms of different electronegativity—double bond between, e.g., C and O or N.
- Pattern 5: Conjugated pi bonds in a ring—alternating double/single bonds in cyclic structures (e.g., benzene).
Recognizing Resonance Patterns
- Identify double bonds and look for adjacent positions (allylic) to spot lone pairs or carbocations for resonance.
- Only move one set of electrons at a time when drawing resonance structures.
- Always account for formal charges after moving electrons.
Drawing and Evaluating Resonance Structures
- When moving electrons, ensure octet rule is followed for higher stability.
- Show all possible valid resonance forms by shifting electrons as allowed by patterns.
- Assign formal charges after electron movement.
- In rings like benzene, shifting double bonds produces equivalent resonance forms.
Determining Major vs. Minor Contributors
- The most significant resonance structure has the most filled octets and fewest formal charges.
- Octet rule violations (e.g., empty orbitals or extra bonds) make a structure less significant.
- Negative charges should be on more electronegative atoms; positive charges on less electronegative atoms.
- The resonance hybrid is a blend of all valid contributors—major contributors influence the structure most.
Delocalized vs. Localized Electrons
- Delocalized electrons are involved in resonance; localized electrons are not.
- Lone pairs adjacent to unhybridized p orbitals can be delocalized.
- In aromatic rings, certain lone pairs remain localized if resonance would break aromaticity.
Key Terms & Definitions
- Resonance — Delocalization of electrons across adjacent atoms through overlapping orbitals.
- Allylic position — Atom next to the atoms involved in a double bond.
- Carbocation — Carbon atom with a positive charge.
- Formal charge — The charge assigned to an atom based on valence electrons minus assigned electrons.
- Conjugated system — Alternating double and single bonds, enabling electron delocalization.
- Delocalized electrons — Electrons that participate in resonance across multiple atoms.
- Localized electrons — Electrons restricted to a single atom or bond; do not participate in resonance.
Action Items / Next Steps
- Practice identifying the five resonance patterns in various molecules.
- Draw all valid resonance structures for assigned homework molecules, showing proper electron movement and formal charges.
- Memorize the rules for determining major/minor resonance contributors.
- Complete Chapter 2 homework on resonance structures.