Overview
This lecture covers the HONC 1234 rule for predicting the number of bonds formed by common organic atoms, how to identify formal charges, and methods for drawing molecular structures to represent 3D geometry accurately.
HONC 1234 Rule
- H: Hydrogen and halogens (F, Cl, Br, I) each form 1 bond and have 3 lone pairs (for halogens).
- O: Oxygen forms 2 bonds and has 2 lone pairs to satisfy the octet rule.
- N: Nitrogen forms 3 bonds and has 1 lone pair for a complete octet.
- C: Carbon forms 4 bonds with no lone pairs in neutral molecules.
- The HONC rule applies only to neutral molecules.
Octet Rule and Bonding Examples
- Halogens and hydrogen fulfill their valence with one sigma (covalent) bond; halogens display three lone pairs.
- Oxygen can have two single bonds or one double bond plus two lone pairs.
- Nitrogen typically forms three bonds and one lone pair; can also be triple-bonded with a lone pair.
- Carbon can be found with four single bonds, a double bond and two singles, or a triple bond and a single, always totaling four.
Formal Charges
- Formal charge = valence electrons (from group number) minus (number of bonds + lone pair electrons).
- Neutral carbon: four bonds, zero formal charge.
- Carbocation (positive charge): three bonds, no lone pairs, +1 formal charge.
- Carbanion (negative charge): three bonds, one lone pair, -1 formal charge.
- For oxygen: Six valence electrons minus bonds and lone pairs determines the formal charge.
- For nitrogen: Five valence electrons minus bonds and lone pairs determines the formal charge.
Degrees of Carbocations
- Tertiary carbocation: Central carbon bonded to three other carbons.
- Secondary carbocation: Central carbon bonded to two other carbons.
- Primary carbocation: Central carbon bonded to one other carbon.
- Identifying carbocation degree is essential for predicting reactivity.
Drawing Molecular Geometry
- Molecules are 3D; tetrahedral carbons have bond angles of 109.5°, not 90°.
- Use wedges (coming out of plane) and dashes (going into the plane) to represent 3D structures.
- Proper zigzag structures represent correct tetrahedral angles; incorrect angles can lead to loss of points.
- Wedges and dashes should be placed at the apex of zigzag lines to indicate bonds in and out of the plane.
Key Terms & Definitions
- HONC 1234 Rule — A mnemonic for the typical number of bonds by hydrogen/halogens, oxygen, nitrogen, and carbon.
- Octet Rule — Atoms prefer eight electrons in their valence shell for stability (except hydrogen: 2).
- Formal Charge — Calculated difference between an atom’s valence electrons and electrons assigned in bonds/lone pairs.
- Carbocation — A carbon atom with a positive formal charge.
- Carbanion — A carbon atom with a negative formal charge.
- Tetrahedral Geometry — 3D shape with 109.5° bond angles around a central atom.
Action Items / Next Steps
- Review resonance structures from general chemistry for next class.
- Practice calculating formal charges and drawing wedge-dash structures.
- Read textbook sections on 3D representations and resonance as preparation.