Overview
This lecture covers key concepts in organic chemistry including molecular polarity, formal charges, resonance, acid-base reactions, isomerism, stereochemistry, reaction mechanisms, and synthetic strategies, with examples and key definitions throughout.
Molecular Polarity & Formal Charge
- Symmetrical molecules like CH₄ and CO₂ do not have dipole moments.
- Formal charge formula: valence electrons – dots (lone pairs) – sticks (bonds).
- Oxygen's formal charge varies based on its bonding and lone pairs.
Resonance & Lewis Structures
- Atoms never move during resonance, only electrons do.
- Keep the same overall charge in all resonance structures.
- Single bonds never break in resonance structures.
Acids, Bases & Conjugates
- Acids donate protons (H⁺); bases donate electron pairs.
- Strong acids have weak conjugate bases and vice versa.
- Inductive effects and electronegativity influence acid/base strength.
Nomenclature & Structural Analysis
- Number the longest carbon chain and identify substituents.
- Assign lowest possible numbers to substituents and double/triple bonds.
- Bridgehead carbons and substituent positions are key in bicyclic structures.
Conformation & Stability
- Anti conformation (substituents 180° apart) is most stable.
- Gauche interactions (substituents close together) are less stable, especially with larger groups.
- In chair conformations, equatorial positions are more stable than axial.
Stereochemistry & Isomerism
- Stereoisomers: same connectivity, different spatial arrangement.
- Enantiomers: all chiral centers inverted; diastereomers: at least one chiral center inverted, not all.
- Cis (Z) = groups on same side; trans (E) = groups on opposite sides.
- Assign R/S by atomic number priority; lowest priority on a dash before determining configuration.
Reaction Mechanisms
- Nucleophile: electron-rich, attacks electrophile (electron-poor).
- SN1: two-step, rate depends only on substrate.
- SN2: one-step, rate depends on substrate and nucleophile.
- E1/E2: elimination reactions depend on base strength and substrate substitution.
Addition, Elimination & Reagents
- Anti (opposite side) and syn (same side) addition depend on reagents.
- Ozonolysis splits double bonds, forming carbonyl compounds (aldehydes, ketones, acids).
- Know which reagents produce cis/trans products and Markovnikov/anti-Markovnikov addition.
Radicals & Stability
- Radical reactions proceed via initiation, propagation, and termination steps.
- More substituted radicals are more stable.
- Reaction selectivity can be enhanced using specific halogens and conditions.
Synthesis Strategies
- Use excess NaNH₂ to form alkynes from dihalides.
- Ozonolysis and hydration reactions create carboxylic acids or ketones from alkynes.
- Epoxide opening depends on nucleophile strength; strong nucleophiles attack less substituted carbon unless tertiary is available.
Key Terms & Definitions
- Formal Charge — calculated as valence electrons minus lone pairs minus bonds.
- Resonance — delocalization of electrons without moving atoms.
- Enantiomer — mirror-image, non-superimposable isomer.
- Diastereomer — stereoisomers not related as mirror images.
- Nucleophile — electron-rich species seeking positive centers.
- Electrophile — electron-poor species seeking electrons.
- Markovnikov — addition to more substituted carbon.
- Ozonolysis — oxidative cleavage of double bonds by ozone.
- Anti/Syn Addition — addition of groups to opposite/same sides of a double bond.
Action Items / Next Steps
- Review mechanisms and reagents for addition/elimination reactions.
- Practice drawing chair conformations and assigning R/S configurations.
- Complete assigned problems on synthesis and mechanism prediction.
- Study key reagent outcomes and stereochemistry rules for exams.