| QUESTIONS / CUES | NOTES |
|---|---|
| What is the C-X bond polarity? | C^δ+—X^δ-. Halogen is more electronegative → withdraws electrons → C is electrophilic. This makes C the site of nucleophilic attack. |
| What are the two SN mechanisms? | SN1: 2 steps, unimolecular, via carbocation (R.D.S = ionization). Rate = k[RX]. SN2: 1 step, concerted, bimolecular, backside attack. Rate = k[RX][Nu-]. |
| SN1 stereochemistry? | Racemization (50:50 R:S). Planar sp2 carbocation → nucleophile attacks from either face equally. |
| SN2 stereochemistry? | Walden inversion (complete inversion). Backside attack → groups flip to opposite side. |
| SN1 substrate preference? | Tertiary (3°) — forms stable 3° carbocation. |
| SN2 substrate preference? | Primary (1°) — minimal steric hindrance for backside attack. Tertiary cannot do SN2 (blocked). |
| SN1 solvent? | Polar protic (, ROH) — solvates carbocation and X-. |
| SN2 solvent? | Polar aprotic (DMSO, DMF, acetone) — does not solvate Nu-, keeping it reactive. |
| SN1 nucleophile? | Weak nucleophile is sufficient (). |
| SN2 nucleophile? | Strong nucleophile required (OH-, CN-, I-). |
| E1 vs E2? | E1: 2 steps (via carbocation), Rate = k[RX], competes with SN1. E2: 1 step concerted, Rate = k[RX][Base], strong bulky base. |
| Saytzeff's rule? | More substituted alkene = major product (more stable by hyperconjugation). |
| Why haloarenes less reactive? | C-X has partial double bond character via p-π resonance (Cl lone pair into ring) → shorter, stronger C-X bond. |
| Dow process conditions? | 623 K, 300 atm. + NaOH → + NaCl. |
| Finkelstein reaction? | RCl + NaI (acetone) → RI + NaCl↓. Driven by NaCl precipitating. |
| Swarts reaction? | RBr + AgF → RF + AgBr↓. Driven by AgBr precipitating. |
| C-X bond order (length)? | C-F (135 pm) < C-Cl (177 pm) < C-Br (193 pm) < C-I (214 pm). Inversely: C-F strongest, C-I weakest. |
| Most reactive haloalkane? | RI — weakest C-X bond (213 kJ/mol) + best leaving group (I-). |
| Phosgene hazard? | + (light) → (phosgene) + HCl. Store in dark amber bottles with ethanol. |
| DDT problem? | Non-biodegradable + lipophilic → biomagnification through food chain. SMILES: Clc1ccc(C(c2ccc(Cl)cc2)Cl)cc1 |
SUMMARY ROW: Haloalkanes (R-X) react mainly via SN1 (3°, polar protic, weak Nu → racemization) or SN2 (1°, polar aprotic, strong Nu → Walden inversion). Haloarenes resist nucleophilic substitution due to resonance (partial C=X bond). Named reactions: Finkelstein (→RI), Swarts (→RF), Dow process (ArCl → ArOH). Key NEET traps: SN2 = inversion (not racemization); haloarenes need extreme conditions; C-F is shortest/strongest; C-I is longest/weakest but most reactive.