CHEMCA
EXAM MASTER REVIEW SHEET
Organic Reaction Mechanisms
1. Reagents & Bond Fission
Electron deficient species (Lewis Acids).
Examples: $H^+, Cl^+, NO_2^+, AlCl_3, BF_3$
Electron rich species (Lewis Bases).
Examples: $OH^-, CN^-, NH_3, H_2O$
Bond Fission Types
$A-B \to A^\bullet + B^\bullet$ (Forms Free Radicals)
Favored by: Light ($h\nu$), Heat, Peroxide.
$A-B \to A^+ + B^-$ (Forms Ions)
Favored by: Polar Solvents.
2. Nucleophilic Substitution ($S_N1$ vs $S_N2$)
| Feature | $S_N1$ | $S_N2$ |
|---|---|---|
| Kinetics | Unimolecular, Rate $= k[RX]$ | Bimolecular, Rate $= k[RX][Nu^-]$ |
| Mechanism | 2-step (via Carbocation) | 1-step (Transition State) |
| Stereochemistry | Racemization | Walden Inversion |
| Substrate Reactivity | $3^\circ > 2^\circ > 1^\circ > \text{methyl}$ | $\text{methyl} > 1^\circ > 2^\circ > 3^\circ$ |
| Solvent | Polar Protic ($H_2O, EtOH$) | Polar Aprotic ($DMSO, DMF$) |
3. Elimination Mechanisms ($E1, E2$)
The major product is the more substituted (more stable) alkene.
Favored by: Small bases, standard leaving groups.
The major product is the less substituted alkene.
Favored by: Bulky bases ($t$-BuOK), Poor leaving groups ($F^-$).
High Temperature + Strong/Bulky Base $\implies$ Elimination
Low Temperature + Strong Nucleophile $\implies$ Substitution
4. Electrophilic Addition ($Ad_E$)
Markovnikov's Rule
Negative part of the addendum goes to the carbon with fewer hydrogen atoms.
Reason: Formation of more stable carbocation.
Kharasch Effect (Anti-Mk)
Only for HBr in presence of Peroxide.
Mechanism: Free Radical Addition.
5. Electrophilic Aromatic Substitution ($S_EAr$)
| Reaction | Reagents | Electrophile |
|---|---|---|
| Nitration | Conc. $HNO_3$ + Conc. $H_2SO_4$ | $NO_2^+$ |
| Sulphonation | Fuming $H_2SO_4$ (Oleum) | $SO_3$ (Neutral) |
| Friedel-Crafts Alkylation | $R-Cl$ + anhyd. $AlCl_3$ | $R^+$ |
$-OH, -NH_2, -OR, -R$ (Alkyl)
$-NO_2, -CN, -CHO, -COOH$
*Halogens are deactivating but $o/p$ directing.*
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