Overview
This lecture covered key concepts in organic chemistry, focusing on molecular structure, resonance, isomerism, conformational and stereochemical analysis, reaction mechanisms, and reagent application, with emphasis on practical approaches and problem-solving for exams.
Molecular Structure and Polarity
- Molecules with symmetry (e.g., CH4, CO2) lack dipole moments, while NH3 has one due to its lone pair.
- Formal charge formula: valence electrons minus dots minus sticks (bonds).
- Oxygen's valence is 6; calculate charges accordingly (e.g., 6-4-2=0).
Resonance and Molecular Stability
- Resonance structures must have same overall charge and atom positions; only electrons move.
- Single bonds are not broken in resonance.
- For most stable conformations, large groups should be anti (180° apart).
Acid-Base Concepts
- Bases donate electrons and become conjugate acids after proton transfer.
- Strong conjugate bases arise from weak acids and vice versa.
- ARIO (Atom, Resonance, Induction, Orbital) is a guide to base and acid strength.
Nomenclature and Structure Drawing
- Number carbon chains for the longest path and lowest substituent numbers.
- Use IUPAC rules for naming; prioritize double/triple bonds and substituents.
Stereochemistry and Isomerism
- Stereoisomers have same connectivity, different 3D arrangement.
- Enantiomers: all chiral centers inverted; diastereomers: some centers changed.
- Cis/trans (E/Z) isomerism is assigned based on highest atomic numbers across double bonds.
- Chiral (stereogenic) centers require four different groups; lowest priority should be on a dash when assigning R/S.
Conformational Analysis
- Most stable conformation is anti; gauche is less stable due to crowding.
- In cyclohexane chairs, axial positions alternate up/down, and equatorial positions are most stable.
Reaction Mechanisms
- SN1: rate depends on substrate, not nucleophile; favored by polar protic solvents.
- SN2: concerted, inverts stereochemistry; favored by strong nucleophile and polar aprotic solvent.
- E1/E2: E2 occurs with strong bases, produces Zaitsev (more substituted) or Hofmann (less substituted) products.
Reagents and Functional Group Transformations
- Know which reagents give syn (same side) or anti (opposite sides) addition.
- Ozonolysis cleaves double/triple bonds to give aldehydes, ketones, or acids.
- Nucleophilic additions/epoxide openings depend on nucleophile strength and position.
Radical Reactions
- Radical halogenations involve initiation, propagation, and termination steps.
- Selectivity increases with bromination over chlorination.
Key Terms & Definitions
- Dipole moment — a measure of molecular polarity; requires asymmetry.
- Formal charge — calculated charge on an atom in a molecule.
- Resonance structure — alternative electron arrangements; atoms stay in place.
- Enantiomer — mirror-image, non-superimposable isomer.
- Diastereomer — stereoisomer not related as mirror images.
- Anti conformation — groups are 180° apart; most stable orientation.
- Zaitsev product — more substituted alkene in elimination.
- Hofmann product — less substituted alkene in elimination.
- Nucleophile — electron-pair donor; often negative.
- Electrophile — electron-pair acceptor; often positive or partially positive.
Action Items / Next Steps
- Review mechanisms and reagent outcomes in textbook chapters 9–11.
- Practice resonance and formal charge calculations.
- Memorize SN1/SN2/E1/E2 conditions and product preferences.
- Complete suggested problems and practice naming compounds.
- Study stereochemistry, isomerism, and chair conformations with additional exercises.