Overview
This lecture covers formal charges on carbon, nitrogen, and oxygen atoms, how to identify them in structures, rules for drawing resonance structures, and methods for depicting molecules in three dimensions.
Formal Charges on Carbon Atoms
- Carbon normally forms four bonds and has no formal charge.
- Carbocations have three bonds, an empty orbital, an sp2 hybridization, and a +1 formal charge.
- Carbanions have three bonds, a lone pair, are sp3 hybridized, and have a –1 formal charge.
- Formal charge is calculated as group number minus assigned electrons.
Formal Charges on Oxygen and Nitrogen
- Oxygen (group 6A) forms:
- 1 bond/3 lone pairs: –1 charge
- 2 bonds/2 lone pairs: no charge
- 3 bonds/1 lone pair: +1 charge
- Nitrogen (group 5A) forms:
- 3 bonds/1 lone pair: no charge
- 2 bonds/2 lone pairs: –1 charge
- 4 bonds/no lone pairs: +1 charge
- Lone pairs may not be drawn but formal charges must be shown.
Drawing Molecules in 3D
- Solid lines: atoms/bonds in the plane.
- Wedges: bonds coming out of the plane.
- Dashes: bonds going behind the plane.
- Fischer projections show backbone carbons in one plane; Hayworth projections show cyclic structures.
Resonance Structures
- Resonance indicates delocalized electrons, especially in pi systems or with formal charges.
- Resonance is shown with double-headed arrows between valid Lewis structures (brackets included).
- Actual structure is a hybrid, not rapidly alternating forms.
- Delocalization increases stability.
Rules for Drawing Resonance Structures
- Only move pi bonds or lone pairs with curved arrows; never break single (sigma) bonds.
- Do not exceed the octet for 2nd row elements (C, N, O, F, B).
- Always show formal charges in all resonance forms.
- Curved arrows begin at electron sources (pi bonds/lone pairs) and end at electron acceptors.
Key Terms & Definitions
- Formal charge — Group number minus assigned electrons (bonds + lone pairs) on the atom.
- Resonance — Delocalization of electrons represented by multiple valid Lewis structures.
- Curved arrow — Symbol showing electron movement in resonance or reaction mechanisms.
- Carbocation — Carbon atom with three bonds, positive charge.
- Carbanion — Carbon atom with three bonds and a lone pair, negative charge.
- Fischer projection — 2D representation for multiple chiral centers, usually in sugars.
- Hayworth projection — 2D drawing for cyclic (ring) molecules.
Action Items / Next Steps
- Practice drawing lone pairs and formal charges for assigned structures.
- Draw resonance structures using curved arrows and indicate formal charges.
- Review 3D molecule representation (wedges, dashes, Fischer, Hayworth projections).
- Complete assigned homework problems on formal charges and resonance.