Inductive effect (I): Permanent polarity along σ-bonds due to electronegativity difference. Electron-withdrawing groups (-I): -F, -Cl, -Br, -I, -NO₂, -CN, -COOH, -OH, -OR (halogens, electronegative atoms). Electron-donating groups (+I): alkyl groups (CH₃ > C₂H₅ > n-C₃H₇...).
Mesomeric/Resonance effect (M): Delocalization through π bonds or lone pairs. -M groups (withdraw electrons by resonance): -NO₂, -CHO, -COOH, -CN, -COR. +M groups (donate electrons by resonance): -OH, -OR, -NH₂, -NR₂, -halogens (lone pairs into ring).
Hyperconjugation: Delocalization involving C-H σ bonds adjacent to π bond or carbocation. No H, no hyperconjugation. Stability of alkenes: greater substitution = more hyperconjugation = more stable. Carbocation stability: 3° > 2° > 1° (due to hyperconjugation + inductive effect).
Electrophilic Addition (EA): Unsaturated compounds (alkenes, alkynes). Electrophile attacks π-rich double bond. Markovnikov's rule: H adds to C with more H already (positive charge on more substituted C). HBr + alkene → anti-Markovnikov with peroxide (free radical mechanism — Kharash effect).
Electrophilic Aromatic Substitution (EAS): Benzene reactions. Mechanism: electrophile attacks → σ complex (arenium ion, Wheland intermediate) → loss of H⁺ → restored aromaticity.
Nucleophilic Substitution: SN2 (inversion of configuration — Walden inversion, one step, 2nd order, prefers primary), SN1 (carbocation intermediate, two steps, 1st order, racemisation, prefers tertiary).
Alkanes: Free radical substitution. Cl₂ in UV light. Reactivity: F₂ > Cl₂ > Br₂ > I₂. Cl₂ is most useful for selective halogenation.
Alkenes: Electrophilic addition. Ozonolysis (O₃ → O₃ complex → cleave with Zn/H₂O → aldehydes/ketones). Hydroxylation with cold KMnO₄ (Baeyer's reagent, turns from purple → colourless = positive test for unsaturation).
Alkynes: More reactive than alkenes toward nucleophiles (linear molecule, exposed π). Acidic H on terminal alkyne (pKa ~25) — forms acetylides with NaNH₂ or Na. Used in Lindlar hydrogenation (cis-alkene) vs Na/liquid NH₃ (trans-alkene).
Benzene: Delocalized π electrons (6 electrons in ring). Resonance energy = 150 kJ/mol (Kekulé structures). Prefers EAS over EA to preserve aromaticity.
Wurtz reaction: 2RX + 2Na → R-R + 2NaX. Couples two alkyl halides. Only alkanes with even carbon number from same halide.
Sandmeyer reaction: ArN₂⁺ + CuCl/CuBr/CuCN → ArCl/ArBr/ArCN + N₂. Converts diazonium salt to aryl halide.
Friedel-Crafts reactions: Alkylation (AlCl₃ + RX → R-Ar), Acylation (AlCl₃ + RCOCl → RCO-Ar). Cannot use on deactivated rings (NO₂, SO₃H groups).
Ozonolysis: Alkene + O₃ → ozonide → Zn/H₂O → carbonyl compounds. Products identify the structure of the original alkene.
+I (donate): C(CH₃)₃ > CH(CH₃)₂ > C₂H₅ > CH₃
Acidity increases with -I groups
Basicity increases with +I groups
Allylic ≈ 3° carbocation (resonance)
Benzylic carbocation: very stable
Vinyl carbocation: very unstable
Phenyl carbocation: very unstable
SN2: 1° substrate, polar aprotic, inversion (Walden), bimolecular
Strong Nu: favours SN2
Polar protic solvent: favours SN1
Purple → colourless = alkene/alkyne present
Product: diol (cis-dihydroxylation)
Hot KMnO₄: cleaves double bond
Ozonolysis: O₃ then Zn/H₂O
Named reactions: 1 Q/year
Reaction mechanism: 1 Q/year
Expected: SN1 vs SN2 condition
Watch: Ozonolysis product identification