| QUESTIONS / CUES | NOTES |
|---|---|
| Rate law difference? | SN1: Rate = k[RX] — only substrate. SN2: Rate = k[RX][Nu-] — both substrate and nucleophile. |
| Intermediate in SN1? | Carbocation (R+) — planar, sp2, trigonal planar geometry. |
| Intermediate in SN2? | NONE — single transition state only. Pentacoordinate transition state (5 groups on C). |
| Can SN1 give rearrangement? | YES — carbocation can undergo 1,2-hydride or 1,2-methyl shift to more stable carbocation before Nu attack. |
| Can SN2 give rearrangement? | NO — concerted single step, no intermediate → no opportunity for rearrangement. |
| Effect of [Nu] on SN1? | No effect — Nu not involved in rate-determining step (RDS). Doubling [Nu] → rate unchanged. |
| Effect of [Nu] on SN2? | Proportional — Rate = k[RX][Nu]. Doubling [Nu] → rate doubles. |
| How to choose between SN2 and E2? | Strong non-bulky Nu → SN2. Strong bulky base (t-BuO-) → E2 (can't access C-X, abstracts β-H instead). |
| Example: SN1 product | (){3}CBr + → (){3}COH + HBr. Product: tert-butanol SMILES: CC(C)(C)O. |
| Example: SN2 product | (R)-CH_{3}$$CHBrCH_{2}$$CH_{3} + NaOH (DMSO) → (S)-CH(OH)CH_{2}$$CH_{3}. Complete R→S inversion. |
| Competition at 2° substrate? | 2° can undergo either SN1 or SN2. Strong Nu + aprotic → SN2. Weak Nu + protic → SN1. |
| What is "common ion effect" in SN1? | Adding excess X- (same leaving group) shifts SN1 equilibrium backward → slows product formation. |
| Solvent effect on nucleophile? | Protic: solvates Nu- (cage effect) → weaker Nu → SN2 slower. Aprotic: Nu- is naked → stronger → SN2 faster. |
| Rate order for SN2 substrates? | Methyl > Primary > Secondary > Tertiary (inverse of SN1 substrate order). |
SUMMARY ROW: SN1 and SN2 are distinguished by: (1) rate law (mono vs bimolecular), (2) substrate type (3° vs 1°), (3) nucleophile (weak vs strong), (4) solvent (protic vs aprotic), (5) stereochemistry (racemization vs inversion), (6) rearrangement possibility (yes in SN1, no in SN2). The most NEET-tested distinction: SN2 always gives Walden inversion; SN1 always gives racemization for chiral substrates.