| Feature | SN1 | SN2 | E1 | E2 |
|---|---|---|---|---|
| Steps | 2 (via carbocation) | 1 (concerted) | 2 (via carbocation) | 1 (concerted) |
| Rate law | Rate = k[RX] | Rate = k[RX][Nu-] | Rate = k[RX] | Rate = k[RX][Base] |
| Molecularity | Unimolecular (RDS) | Bimolecular | Unimolecular (RDS) | Bimolecular |
| Intermediate | Carbocation (sp2) | None (TS only) | Carbocation (sp2) | None (TS only) |
| Substrate | 3° (stable C+) | 1° (least steric) | 3° (same as SN1) | 2° or 3° |
| Nucleophile/Base | Weak Nu | Strong Nu | Weak base, high T | Strong, bulky base |
| Solvent | Polar protic (, ROH) | Polar aprotic (DMSO, DMF) | Polar protic | Polar protic or aprotic |
| Stereochemistry | Racemization | Walden inversion | Mixtures (from racemic C+) | anti-periplanar geometry required |
| Rearrangements? | Yes (via C+) | No | Yes (via C+) | No |
| Competition | Competes with E1 | Competes with E2 | Competes with SN1 | Competes with SN2 |
| Temperature effect | Low T favors SN1 over E1 | T not critical | High T favors E1 over SN1 | Moderate-high T |
| Product | Substitution (ROH, ROR, etc.) | Substitution (ROH, RCN, etc.) | Alkene (Saytzeff) | Alkene (Saytzeff; Hofmann with bulky base) |
| Example substrate | ()_{3}CBr | CH_{3}$$CH_{2}Br | ()_{3}CBr (high T) | ()_{2}CHBr + t-BuO- |
| Example reagent | NaOH/DMSO | , heat | KOH/EtOH, heat |
Comparison of Halogen Properties:
| Halide | Bond Length (pm) | Bond Energy (kJ/mol) | Leaving Group | SN2 Reactivity |
|---|---|---|---|---|
| C-F | 135 (shortest) | 485 (strongest) | Worst (F- is strongest base) | Least reactive |
| C-Cl | 177 | 339 | Moderate | Moderate |
| C-Br | 193 | 285 | Good | High |
| C-I | 214 (longest) | 213 (weakest) | Best (I- is weakest base) | Most reactive |
Comparison of Haloalkanes vs Haloarenes:
| Property | Haloalkanes (R-X) | Haloarenes (Ar-X) |
|---|---|---|
| Carbon hybridization | sp3 | sp2 (aromatic ring) |
| C-X bond type | Pure single bond | Partial double bond (resonance) |
| C-X bond length (Cl) | 177 pm (chloroethane) | 169 pm (chlorobenzene) |
| Reactivity toward Nu substitution | High (SN1 or SN2) | Very low (requires harsh conditions) |
| Mechanism of Nu substitution | SN1 or SN2 | Nucleophilic Aromatic Substitution (NAS) |
| Example conditions for Nu sub | NaOH, 25°C, aqueous | 623 K, 300 atm (Dow process) |
| Resonance in C-X bond? | No | Yes (Cl lone pair → ring) |