Overview
This lecture outlines the five key rules for understanding and drawing resonance forms in organic chemistry, emphasizing their nature, electron movement, equivalency, valency, and stability implications.
Rules for Resonance Forms
Rule 1: Resonance Forms Are Imaginary
- Individual resonance forms are not real; the actual molecule is a resonance hybrid of these forms.
- Molecules do not alternate between resonance structures; the true structure is a composite depicted by all forms.
Rule 2: Only Electrons Move in Resonance
- Resonance forms differ only in the arrangement of π (pi) or nonbonding (lone pair) electrons.
- Atoms do not change position or hybridization between resonance forms.
- Curved arrows in diagrams show electron movement, never atom movement.
Rule 3: Resonance Forms Need Not Be Equivalent
- Resonance forms can have different stabilities and are not always equivalent.
- The resonance hybrid structure resembles the most stable resonance form.
- For example, in an anion, the form placing a negative charge on a more electronegative atom contributes more.
Rule 4: Resonance Forms Must Obey Valency Rules
- Valid resonance forms must follow normal rules of valency, such as the octet rule for second-row elements.
- A structure violating the octet rule (e.g., carbon with five bonds) is not a valid resonance form.
Rule 5: Resonance Increases Stability
- The resonance hybrid is more stable than any individual resonance form.
- The greater the number of valid resonance forms, the more stable the molecule, as electrons are delocalized over more atoms.
Key Terms & Definitions
- Resonance Form — A possible Lewis structure representing different electron arrangements for a molecule.
- Resonance Hybrid — The actual structure of a molecule, combining all resonance forms.
- Curved Arrow — Symbol in diagrams showing the movement of electron pairs during resonance.
- Valency — The number of bonds an atom can form, adhering to rules like the octet rule.
Action Items / Next Steps
- Practice drawing resonance forms for molecules like acetate, benzene, and acetone anion.
- Review the octet rule and apply it when checking resonance structures.
- Read the next section on drawing resonance forms for further practice.