Overview
This lecture focused on foundational concepts needed for organic chemistry, covering primary effects, especially electronegativity and atomic size, and providing a comprehensive introduction to the inductive effect and its applications.
Review of Primary Effects: Electronegativity & Size
- Electronegativity is the tendency of an atom to attract electron density towards itself.
- Electronegativity increases from left to right across a period, increasing anion stability and acidity.
- Acidity increases as pKa decreases; a low pKa means stronger acid.
- Size factor matters up to down a group in the periodic table: atomic size increases downwards.
- Larger anions disperse negative charge more, stabilizing the anion and increasing acidity (especially in columns).
- Bond length increases down a group, leading to weaker bonds that break more easily and increase acidity (e.g., HI > HBr > HCl > HF).
Inductive Effect: Concept & Mechanism
- The inductive effect is the transmission of charge through sigma bonds due to differences in electronegativity.
- It is a permanent effect, operating through the polarization of sigma bonds.
- It is distance-dependent and becomes negligible after 3–4 carbons.
- +I effect: Groups that donate electrons via sigma bonds (alkyl groups, anions) stabilize electron-deficient species.
- –I effect: Electron-withdrawing groups (like NO₂, halogens, carbonyls, cations) stabilize electron-rich species.
- The inductive effect is additive in nature—multiple groups increase the overall effect.
Application of Inductive Effect: Stability of Intermediates
- Organic intermediates: carbocations (positive, electron-deficient), free radicals (neutral, single unpaired electron), and carbanions (negative, electron-rich).
- Carbocation stability: increased by +I groups, decreased by –I groups.
- Carbanion stability: increased by –I groups, decreased by +I groups.
- Radical stability: generally follows the same trend as carbocations (+I stabilizes).
- Acidity is directly proportional to anion stability; stabilized by –I groups and higher electronegativity, destabilized by +I groups.
Orders and Trends
- +I order (alkyl groups): tertiary butyl > isopropyl > ethyl > methyl.
- Among isotopes: CT₃ (tritium) > CD₃ (deuterium) > CH₃ (hydrogen) in +I effect due to bond length.
- For carboxylic acids, –I effect (e.g., F > Cl > Br > I) increases acidity; do not apply the size factor here.
Key Terms & Definitions
- Electronegativity — Atom’s tendency to attract electrons in a bond.
- Inductive Effect — Transmission of charge via sigma bonds due to electronegativity differences.
- +I Effect — Electron-donating effect via sigma bonds.
- –I Effect — Electron-withdrawing effect via sigma bonds.
- Carbocation — Positively charged, electron-deficient organic intermediate.
- Carbanion — Negatively charged, electron-rich organic intermediate.
- Free Radical — Neutral species with one unpaired electron.
- Anion Stability — Tendency of a negatively charged ion to be stabilized by delocalization or electron-withdrawing effects.
- Resonance Effect — Delocalization of electrons across π bonds (mentioned for context, details not covered).
Action Items / Next Steps
- Homework: Compare acidity between H₃O⁺ and H₃S⁺, find reference in Jerry March’s book and provide page number.
- Review and memorize the orders of +I and –I effects.
- Prepare for next session on further applications and effects in organic chemistry.