Overview
This lecture explains how to systematically draw resonance forms in organic molecules by recognizing key three-atom groupings with p orbitals, using examples and practice problems to illustrate the process.
Recognizing Resonance Groupings
- Any three-atom sequence, each with a p orbital (often C, N, O, P, or S), can form two resonance structures.
- Resonance forms differ by moving a double bond and electron set (lone pair, vacant, or unpaired electron) between atoms in the grouping.
- Identifying these groupings in larger molecules helps to systematically draw all possible resonance structures.
Example: 2,4-Pentanedione Anion
- Removing H+ from 2,4-pentanedione creates a central carbon with a negative charge and lone pair next to C=O bonds.
- Three resonance forms can be drawn by shifting the lone pair and double bonds across the O=CC groupings on both sides.
Worked Example: Carbonate Ion (CO₃²⁻)
- In carbonate, each singly bonded O with a negative charge is next to a C=O double bond, creating a three-atom grouping.
- Exchanging the double bond and lone pair positions across the groupings generates three resonance structures for CO₃²⁻.
Worked Example: Pentadienyl Radical
- A radical has a single unpaired electron; in pentadienyl, this electron is on a carbon next to a C=C bond.
- Two resonance forms are drawn by moving the double bond and radical position; repeating this, a third form is found, totaling three resonance structures.
Practice Problems
- Identify if given pairs of structures are resonance forms.
- Draw the indicated number of resonance forms for:
- Methyl phosphate anion (3 forms)
- Nitrate anion (3 forms)
- Allyl cation (2 forms)
- Benzoate anion (2 forms)
Key Terms & Definitions
- Resonance form — Alternative Lewis structures of a molecule/ion differing only in electron placement.
- p orbital — A dumbbell-shaped region around a nucleus where an electron can be found.
- Radical — A species with an unpaired electron (shown as a dot).
- Three-atom grouping — A sequence of three atoms, each with a p orbital, involved in delocalized bonding.
Action Items / Next Steps
- Practice drawing resonance forms for the carbonate ion, pentadienyl radical, methyl phosphate anion, nitrate anion, allyl cation, and benzoate anion.
- Review and be able to identify three-atom p-orbital groupings in larger molecules for resonance analysis.