CHEMCA
EXAM MASTER FORMULA SHEET
Organic Reaction Mechanisms
1. Reagents & Bond Fission
Electron-deficient species (Lewis Acids). Attack at electron-rich sites.
Charged: $H^+, Cl^+, NO_2^+, CH_3^+$
Neutral (Incomplete octet): $AlCl_3, BF_3, SO_3, :CH_2$ (Carbene)
Electron-rich species (Lewis Bases). Attack at electron-deficient sites.
Charged: $OH^-, CN^-, X^-, RO^-$
Neutral (Lone pairs): $NH_3, H_2O, R-OH$
Bond Fission Types
Symmetrical cleavage forming Free Radicals.
Favored by: Light ($h\nu$), Heat ($\Delta$), Peroxides, Radicals (HELP).
Unsymmetrical cleavage forming Ions (Carbocation/Carbanion).
Favored by: Polar Solvents, High EN difference, Acids/Bases.
2. Aliphatic Nucleophilic Substitution
| Feature | $S_N1$ (Unimolecular) | $S_N2$ (Bimolecular) |
|---|---|---|
| Kinetics & Rate | Rate $= k[R-X]$ | Rate $= k[R-X][Nu^-]$ |
| Mechanism | 2-step (Carbocation intermediate formed). Rearrangements possible! | 1-step concerted (Pentacoordinate Transition State). No rearrangements. |
| Stereochemistry | Racemization (slight net inversion due to ion-pair) |
100% Walden Inversion (Backside attack) |
| Substrate Reactivity | Benz./Allyl > $3^\circ > 2^\circ > 1^\circ > CH_3$ | $CH_3 > 1^\circ > 2^\circ > 3^\circ$ (Steric hindrance dominates) |
| Favored Solvent | Polar Protic ($H_2O, EtOH, HCOOH$) Stabilizes both carbocation and leaving group. |
Polar Aprotic (Acetone, DMF, DMSO) Leaves nucleophile naked and reactive. |
- In Polar Protic Solvents: $I^- > Br^- > Cl^- > F^-$
(Smaller ions get heavily solvated/blocked). - In Polar Aprotic Solvents: $F^- > Cl^- > Br^- > I^-$
(True basicity order dominates).
Reaction of alcohols with $SOCl_2$ (Darzen's process).
- • Without Pyridine: Retention of configuration ($S_Ni$).
- • With Pyridine: Inversion of configuration ($S_N2$).
3. Elimination Mechanisms ($E1, E2, E1cb$)
- • Rate: $k[Substrate]$
- • Intermediate: Carbocation (Rearrangements possible!).
- • Product: Saytzeff alkene (most stable).
- • Reagents: Weak base, high heat (e.g., Conc. $H_2SO_4/\Delta$).
- • Rate: $k[Substrate][Base]$
- • Intermediate: None (Concerted Transition State).
- • Stereochem: Anti-periplanar (H and LG must be anti).
- • Reagents: Strong base (e.g., Alc. $KOH$).
- • Intermediate: Carbanion.
- • Condition: Requires a very poor leaving group (like $-F, -NR_3^+$) and a strong Electron Withdrawing Group to stabilize the carbanion.
- • Product: Hofmann alkene (less substituted).
Saytzeff (Zaitsev) Rule: Major product is the more substituted alkene (thermodynamically more stable).
Favored by: Standard leaving groups (Cl, Br, I) and small bases ($EtO^-$).
Hofmann Rule: Major product is the less substituted alkene (kinetic control).
Favored by: Bulky bases ($t-BuO^-$), Poor leaving groups ($F^-$), or Steric hindrance in substrate.
$\implies$ Elimination ($E2/E1$)
$\implies$ Substitution ($S_N2$)
4. Electrophilic Addition ($Ad_E$) to Alkenes
Characteristic reaction of alkenes and alkynes. The $\pi$-bond breaks, forming a carbocation intermediate (which can rearrange!).
Markovnikov's Rule
When an unsymmetrical reagent ($HX, H_2O/H^+$) adds to an unsymmetrical alkene, the negative part goes to the carbon with fewer hydrogen atoms.
Underlying Reality: The reaction proceeds via the most stable Carbocation. Rearrangements (Hydride or Alkyl shifts) WILL occur if a more stable carbocation can be formed!
Kharasch (Peroxide / Anti-Mk) Effect
The negative part goes to the carbon with more hydrogen atoms.
- Valid ONLY for $HBr$ in the presence of Peroxides ($R_2O_2$).
- Fails for $HCl$ and $HI$ due to endothermic propagation steps.
- Mechanism: Free Radical Addition (No carbocations, NO rearrangements).
Mechanism involves a cyclic bromonium ion intermediate leading to Anti-Addition (Trans product).
5. Electrophilic Aromatic Substitution ($S_EAr$)
Aromatic rings undergo substitution rather than addition to preserve resonance stabilization (aromaticity).
| Reaction Name | Standard Reagents | Active Electrophile |
|---|---|---|
| Nitration | Conc. $HNO_3$ + Conc. $H_2SO_4$ ($\Delta$) | $NO_2^+$ (Nitronium) |
| Halogenation | $Cl_2$ or $Br_2$ + Anhyd. $FeCl_3$ / $AlCl_3$ | $Cl^+$ or $Br^+$ |
| Sulphonation | Fuming $H_2SO_4$ (Oleum) | $SO_3$ (Neutral) |
| F.C. Alkylation | $R-X$ + Anhyd. $AlCl_3$ | $R^+$ (Can rearrange!) |
| F.C. Acylation | $R-COCl$ + Anhyd. $AlCl_3$ | $R-C^+=O$ (No rearrangement) |
Increase electron density on the ring (via $+M$ or $+H$). Direct incoming $E^+$ to ortho and para positions.
$-O^-, -NH_2, -OH, -OR, -NHCOR, -R$
Para is usually the major product due to less steric hindrance at the ortho position.
Decrease electron density on the ring (via $-M$). Direct incoming $E^+$ to the meta position.
$-NO_2, -CN, -SO_3H, -CHO, -COOH$
Halogens ($-Cl, -Br$) are Deactivating ($-I > +M$) but are Ortho/Para directing.
6. Free Radical Substitution (FRS)
Alkane Halogenation
Occurs in presence of UV light ($h\nu$) or heat.
- 1. Initiation: $Cl_2 \xrightarrow{h\nu} 2Cl^\bullet$
- 2. Propagation:
$CH_4 + Cl^\bullet \to CH_3^\bullet + HCl$
$CH_3^\bullet + Cl_2 \to CH_3Cl + Cl^\bullet$ - 3. Termination: $Cl^\bullet + Cl^\bullet \to Cl_2$
$CH_3^\bullet + CH_3^\bullet \to C_2H_6$
Allylic / Benzylic Bromination
Reagent: NBS (N-Bromosuccinimide) with $h\nu$ or Peroxide.
Specifically targets the allylic ($C=C-CH_3$) or benzylic ($Ph-CH_3$) position due to the high stability of resonance-stabilized allylic/benzylic free radicals.
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