Overview
This lecture covers foundational concepts in chemical structure including Lewis structures, bonding, hybridization, resonance, formal charge, IMFs, functional groups, and IR spectroscopy.
Lewis Structures & Formal Charge
- Count total valence electrons for all atoms in a molecule.
- Add electrons for negative charges, subtract for positive charges.
- Arrange atoms with the least electronegative (except H) in the center.
- Distribute remaining electrons to fulfill octet rule (8 electrons per atom, except H).
- Assign leftover electrons to the central atom.
- Calculate formal charge: (valence electrons) – (non-bonding electrons + ½ bonding electrons).
- The most stable Lewis structure has the fewest atoms with formal charge, with negative charges on more electronegative atoms.
Octet Rule & Exceptions
- Most atoms aim for 8 electrons in their valence shell (octet), except hydrogen (2 electrons).
- Some atoms may violate the octet rule in resonance structures or unstable intermediates.
Hybridization & Molecular Geometry
- 2 domains: sp, linear geometry.
- 3 domains: sp2, trigonal planar or bent.
- 4 domains: sp3, tetrahedral, trigonal pyramidal, or bent.
- Lone pairs count as electron domains.
Resonance Structures
- Draw all valid resonance structures by moving π bonds or lone pairs.
- Most stable resonance structures have complete octets and charges on appropriate atoms.
- Total charge and atom connectivity must not change between resonance forms.
Constitutional Isomers
- Molecules with the same molecular formula but different connectivity.
- Example: C6H12 can form different structural isomers.
Intermolecular Forces (IMFs)
- Strength order: dispersion (weakest) < dipole-dipole < hydrogen bonding (strongest).
- Hydrogen bonding occurs between H and N, O, or F.
IR Spectroscopy & Functional Groups
- O-H (alcohol): 3200–3600 cm⁻¹ (broad).
- N-H (amine): 3300–3500 cm⁻¹.
- C=O (carbonyl): 1670–1820 cm⁻¹.
- C≡C (alkyne): 2100–2260 cm⁻¹.
- C-H (alkene): 3010–3100 cm⁻¹.
- Carboxylic acid O-H: 2500–3200 cm⁻¹.
Primary, Secondary, Tertiary Carbons & Nitrogens
- Primary (1°): attached to one other carbon.
- Secondary (2°): attached to two carbons.
- Tertiary (3°): attached to three carbons.
- For nitrogen, count the number of carbons bonded directly to N.
Key Terms & Definitions
- Valence Electrons — Electrons in the outermost shell involved in bonding.
- Octet Rule — Atoms tend to have 8 electrons in their valence shell.
- Formal Charge — Charge assigned to an atom in a molecule relative to its valence electrons.
- Hybridization — Mixing of atomic orbitals to form new orbitals (sp, sp², sp³).
- Electron Domain — A region with shared or unshared electron pairs around a central atom.
- Resonance — Delocalization of electrons across adjacent atoms, represented by multiple structures.
- Constitutional Isomers — Compounds with the same formula but different structures.
- Intermolecular Forces (IMFs) — Forces between molecules affecting physical properties.
- Functional Group — A group of atoms responsible for characteristic chemical reactions.
Action Items / Next Steps
- Practice drawing Lewis structures with formal charge and resonance forms.
- Memorize key IR absorption ranges for major functional groups.
- Complete assigned homework on drawing and identifying isomers, hybridizations, and predicting IMFs.