Overview
This lecture reviews molecular geometry from general chemistry with a focus on hybridization in organic molecules, then begins an in-depth discussion on acids and bases in organic chemistry, including curved arrow notation and the concept of pKa.
Molecular Geometry & Hybridization
- sp³ hybridized atoms have tetrahedral geometry with ~109.5° bond angles (e.g., methane).
- Ammonia (NH₃) has a pyramidal shape with bond angles slightly less than 109° due to lone pairs (~107.3°).
- Water (H₂O) is bent with bond angles ~104.5°, further reduced by two lone pairs.
- VSEPR Theory explains these shapes and lone pair effects on bond angles.
- sp² hybridized atoms have trigonal planar geometry with ~120° bond angles (e.g., ethene, methyl carbocation).
- Methyl carbocation (CH₃⁺) is planar with a vacant p orbital orthogonal to the plane.
- sp hybridized atoms have linear geometry with 180° bond angles (e.g., acetylene, acetonitrile).
- Lone pairs occupy more space and can alter idealized bond angles slightly.
Acids and Bases: Definitions and Reactivity
- Acids and bases are central to organic chemistry; key to generating electrophiles and nucleophiles.
- Brønsted-Lowry acid: proton donor; Brønsted-Lowry base: proton acceptor.
- Acid-base reactions can be shown using curved arrow notation to depict electron flow.
- Example: HCl reacts with water to form H₃O⁺ (hydronium) and Cl⁻ (chloride), with water acting as base.
- Similar acid-base reactions occur with organic molecules (e.g., methanol, methyl ammonium ion).
Curved Arrow Notation
- Arrows start at electron sources (lone pair or bond) and point to where electrons move.
- Used to illustrate bond breaking and formation in acid-base mechanisms.
Measuring Acidity: pKa
- pKa = –log(Ka), where Ka is the acid dissociation constant.
- Lower pKa means a stronger acid; strong acids are fully dissociated, weak acids are partially dissociated.
- The pKa scale is logarithmic; 1 unit difference = 10× difference in acidity.
- Example: HCl pKa ≈ –7; methane pKa ≈ +50 (extremely weak acid).
Key Terms & Definitions
- sp³, sp², sp hybridization — Ways atomic orbitals combine, determining molecular geometry (tetrahedral, planar, linear).
- VSEPR Theory — Valence Shell Electron Pair Repulsion; predicts molecular shapes based on electron pair repulsion.
- Brønsted-Lowry Acid/Base — Acid donates a proton (H⁺), base accepts a proton.
- Conjugate Acid/Base — Species formed after an acid donates or a base accepts a proton.
- pKa — Negative log of acid dissociation constant (Ka), measures acid strength.
- Curved Arrow Notation — Symbolizes electron movement in reaction mechanisms.
Action Items / Next Steps
- Review examples of acid-base reactions using curved arrow notation.
- Prepare for next lecture: focus on factors affecting acid strength and more examples of acids with varying pKa values.