Overview
This lecture continued the foundation course in organic chemistry, focusing on the primary and inductive effects, especially electronegativity, atomic size, and the inductive effect's application in acidity and stability.
Primary Effects: Electronegativity & Atomic Size
- Electronegativity is the tendency of an atom to attract electron density towards itself.
- Across a period (left to right), electronegativity increases; down a group, it decreases.
- Anion stability increases with greater electronegativity.
- Acidity increases as the stability of the resulting anion increases; lower pKa indicates higher acidity.
- Atomic size increases down a group; larger anions better disperse negative charge and are more stable.
- In halogen acids (HF, HCl, HBr, HI), acidity increases down the group due to increasing anion size and bond weakness.
Inductive Effect: Concept & Application
- The inductive effect is the shift of electrons through sigma bonds due to differences in electronegativity.
- It operates only through sigma bonds and is permanent but weak and distance-dependent (diminishes over 2-4 carbons).
- Electron withdrawing groups (EWG, -I effect) pull electrons, increasing stability of electron-rich sites.
- Electron donating groups (EDG, +I effect) push electrons, stabilizing electron-deficient sites.
Classification of Groups by Inductive Effect
- +I (electron donating): all alkyl groups, anions like CH2-, NH-, O-, carboxylate.
- -I (electron withdrawing): cations, carbonyl, NO2, halogens, NR3+, SO3R, CN.
Inductive Effect in Stability of Intermediates
- Carbocation stability increases with more +I groups (electron donating).
- Carbanion stability increases with more -I groups (electron withdrawing).
- Free radical stability trends similar to carbocations: more +I groups increase stability.
Inductive Effect in Acidity
- Acidity increases with the presence of -I groups near the acidic hydrogen (anion is stabilized).
- +I groups decrease acidity by destabilizing the anion.
- In carboxylic acids with halogen substituents, stronger inductive effect (-I) increases acidity (F > Cl > Br > I).
Application: Order & Trends
- For alkyl groups: Tert-butyl > isopropyl > ethyl > methyl in +I effect strength.
- More substituted carbocations are more stable; less substituted carbanions are more stable.
Key Terms & Definitions
- Electronegativity — Tendency of an atom to attract electrons toward itself.
- Inductive Effect — Electron shift through sigma bonds due to electronegativity differences.
- +I Effect — Electron donating effect, stabilizes electron-deficient centers.
- -I Effect — Electron withdrawing effect, stabilizes electron-rich centers.
- Carbocation — Positively charged carbon intermediate (electron-deficient).
- Carbanion — Negatively charged carbon intermediate (electron-rich).
Action Items / Next Steps
- Homework: Compare the acidity order between H3O+ and H3S+ and provide reasoning (refer to Jerry March's book if possible).
- Review lecture notes, especially group classifications and trends in stability and acidity.
- Prepare for further discussion on applications of inductive effect in the next class.