Overview
This lecture completes Chapter 1 by covering VSEPR Theory, molecular geometry, polarity, and intermolecular forces, focusing on core concepts relevant to organic chemistry.
VSEPR Theory and Molecular Geometry
- VSEPR stands for Valence Shell Electron Pair Repulsion theory, which predicts molecular shapes to maximize bond angles and minimize steric hindrance.
- Electron domains (also called steric number) are bonds or lone pairs around a central atom.
- Four electron domains:
- 4 bonded, 0 unbonded = tetrahedral, 109.5° (e.g., CH₄, CCl₄).
- 3 bonded, 1 unbonded = trigonal pyramidal, 107° (e.g., NH₃, H₃O⁺).
- 2 bonded, 2 unbonded = bent, 105° (e.g., H₂O).
- Three electron domains:
- 3 bonded, 0 unbonded = trigonal planar, 120°, flat molecule (e.g., carbocation).
- Two electron domains:
- 2 bonded, 0 unbonded = linear, 180° (e.g., CO₂, triple-bonded carbons).
- Lone pairs reduce bond angles by repelling bonded pairs more strongly.
Molecular Polarity
- A molecule is polar if it has an unequal distribution of electron density, creating partial positive/negative regions.
- Symmetrical molecules (e.g., CO₂) are nonpolar as dipole moments cancel out.
- Polarity increases with unsymmetrical electron distribution (e.g., O, N, or halogens bonded to less electronegative atoms).
- Lone pairs on central atoms tend to increase polarity.
Intermolecular Forces (IMF)
- Intermolecular forces are attractions between molecules and affect boiling point, solubility, etc.
- London dispersion forces: present in all molecules, weakest, increase with molar mass and surface area; branching reduces IMF.
- Dipole-dipole interactions: occur between polar molecules, stronger than dispersion, permanent partial charges.
- Hydrogen bonding: strongest IMF, occurs when H is bonded to N, O, or F; highly polar and significantly raises boiling points.
Comparing Boiling Points
- All molecules exhibit dispersion forces, but only polar molecules have dipole-dipole interactions.
- Hydrogen bonding exists only if H is bonded to N, O, or F.
- Molecules with all three IMFs (dispersion, dipole-dipole, hydrogen bonding) have the highest boiling points.
- Linear/longer carbon chains have higher boiling points than branched/compact ones due to increased surface area for dispersion forces.
Key Terms & Definitions
- VSEPR Theory — Model predicting molecular shape based on electron domain repulsion.
- Electron Domain (Steric Number) — Number of bonds and lone pairs on a central atom.
- Tetrahedral — Four bonded domains; 109.5° bond angles.
- Trigonal Planar — Three bonded domains; flat shape, 120° bond angles.
- Linear — Two bonded domains; 180° bond angles.
- Polarity — Uneven electron distribution leading to partial positive/negative areas.
- London Dispersion Forces — Temporary dipoles causing weak attraction between all molecules.
- Dipole-Dipole Interaction — Attractions between permanent polar molecules.
- Hydrogen Bonding — Strong IMF when H is bonded to N, O, or F.
Action Items / Next Steps
- Study the provided VSEPR chart and practice identifying molecular shapes and bond angles.
- Complete Orion non-graded practice, Wy+ graded assignment, and proctored multiple-choice quiz.
- Attend learning assistant sessions if further help is needed.