Overview
This lecture covers the foundational concepts of general organic chemistry, focusing on types of bond cleavage, electronic effects (inductive, resonance, hyperconjugation), and their influence on molecular stability, acidity, and basicity.
Types of Bond Cleavage and Intermediates
- Bond cleavage in organic molecules can be homolytic (equal electron sharing) or heterolytic (unequal electron sharing).
- Homolytic cleavage produces free radicals, which are intermediates with unpaired electrons.
- Heterolytic cleavage leads to carbocations (positive charge on carbon) or carbanions (negative charge on carbon).
- Conditions for homolytic cleavage: high temperature, presence of photons (light), nonpolar solvents, or radical initiators (e.g., peroxides).
- Conditions for heterolytic cleavage: significant electronegativity difference, low temperature, or polar solvents.
Inductive Effect
- Inductive effect is electron displacement along sigma bonds due to electronegativity differences, without actual electron transfer.
- Electronegative atoms (like Cl) pull electron density toward themselves, causing partial charges (δ+, δ−).
- Groups are classified as electron-withdrawing (−I effect) or electron-donating (+I effect).
- Inductive effect influences acidity, basicity, and stability of carbocations and radicals.
- The effect diminishes with distance; order of impact: distance > number of groups > strength.
- Acid strength increases with −I effect, decreases with +I effect; basicity trends are reversed.
Resonance Effect
- Resonance involves the delocalization of electrons (mainly pi electrons) across adjacent parallel p orbitals.
- Only electrons move during resonance; atoms/nuclei remain stationary.
- Resonance hybrid is more stable than individual Lewis structures.
- Stability of resonating structures depends on completed octets, maximum covalent bonds, minimal charge separation, and negative charge on electronegative atoms.
- Resonance has a dominant influence on acidity, basicity, and stability compared to the inductive effect.
Hyperconjugation
- Hyperconjugation refers to delocalization of sigma electrons (especially from C–H bonds) adjacent to a pi system or a carbocation.
- It stabilizes carbocations, free radicals, and alkenes—the greater the number of alpha hydrogens, the more hyperconjugation structures, hence more stability.
- Stability order: tertiary > secondary > primary carbocations/radicals, due to increasing hyperconjugation.
Aromatic, Nonaromatic, and Antiaromatic Compounds
- Aromatic compounds are cyclic, planar, conjugated systems satisfying Hückel's rule (4n+2 π electrons).
- Antiaromatic compounds are cyclic and conjugated but have 4n π electrons, making them less stable.
- Nonaromatic systems lack cyclic conjugation or planarity.
- Stability: aromatic > nonaromatic > antiaromatic.
Special & Steric Effects
- Steric hindrance can prevent resonance by misaligning orbitals (e.g., bulky ortho substituents in benzoic acid derivatives).
- Ortho-substituted benzoic acids are more acidic due to steric inhibition of resonance.
- Inductive and resonance effects combine to influence observed acidity/basicity trends.
Key Terms & Definitions
- Homolytic Cleavage — Bond breakage where each atom takes one electron, forming free radicals.
- Heterolytic Cleavage — Bond breakage where both electrons go to one atom, forming ions.
- Free Radical — Intermediate with an unpaired electron.
- Inductive Effect (I effect) — Electron shift along sigma bonds due to electronegativity differences.
- Resonance Effect — Electron delocalization across multiple atoms via pi bonds.
- Hyperconjugation — Delocalization of sigma electrons (C–H) to stabilize adjacent positive/negative centers.
- Carbocation — Positively charged carbon intermediate.
- Carbanion — Negatively charged carbon intermediate.
- Aromaticity — Enhanced stability in cyclic, conjugated systems that follow Hückel's rule.
Action Items / Next Steps
- Review examples of bond cleavage, inductive and resonance effects in assigned textbook chapters.
- Practice drawing resonating structures and identifying types of intermediates for common organic reactions.
- Prepare for quiz on acidity, basicity, and stability trends using discussed electronic effects.