Overview
This lecture covers how to draw resonance structures, determine major and minor resonance contributors, and understand stability factors such as the octet rule, electronegativity, atom size, and carbocation classification.
Drawing Resonance Structures
- Begin resonance arrows at a double bond or lone pair and point toward a positive charge or electron-deficient site.
- Electrons always move from nucleophilic (electron-rich) to electrophilic (electron-poor) regions.
- When drawing resonance structures, the location of pi bonds and charges shift positions but the atomic skeleton remains unchanged.
Resonance Contributors and Stability
- Allylic carbocations can have multiple resonance structures, with positive charges and pi bonds shifting along the carbon chain.
- If different resonance forms have equal stability, there is no major or minor contributor; the hybrid is a blend.
- Tertiary carbocations are more stable than secondary, which are more stable than primary due to inductive effects and hyperconjugation from alkyl groups.
- Methyl groups donate electron density, stabilizing carbocations via the inductive effect and hyperconjugation.
Resonance in Aromatic and Other Systems
- Benzylic carbocations allow for multiple resonance structures as positive charges and pi bonds move around the ring.
- Among resonance forms, those with greater carbocation stability (tertiary > secondary > primary) are major contributors.
- Benzene itself only rotates pi bonds for resonance; all resonance forms are equivalent.
- In the carboxylate ion, negative charge is delocalized equally between two oxygens, so both resonance forms are equally stable.
Effects of Atom Size and Electronegativity
- A negative charge is better stabilized on a larger atom (e.g., sulfur vs. oxygen), making the form with charge on sulfur the major contributor.
- In alcohols adjacent to carbocations, moving a lone pair from oxygen forms a pi bond, placing a positive charge on oxygen.
- The octet rule takes priority: Structures where all atoms satisfy octet are favored, even if an electronegative atom bears a positive charge.
Key Terms & Definitions
- Resonance Structure — Alternate Lewis structure differing only in electron placement.
- Resonance Hybrid — The actual molecule, averaging all resonance structures.
- Allylic Carbocation — A carbocation adjacent to a double bond, allowing resonance.
- Benzylic Carbocation — A carbocation adjacent to a benzene ring.
- Inductive Effect — Electron donation or withdrawal through sigma bonds.
- Hyperconjugation — Stability from overlap of filled orbitals with empty carbocation p-orbital.
- Octet Rule — Atoms are more stable when surrounded by eight electrons.
- Major Resonance Contributor — The most stable resonance form, contributing most to the hybrid.
Action Items / Next Steps
- Practice drawing resonance structures for given molecules and identify major resonance contributors.
- Review the octet rule and stability order for carbocations: tertiary > secondary > primary.
- Complete any assigned resonance structure problems.