Comparison Between SN1 and SN2 Reactions
1. Basic Definition
SN1 Reaction (Unimolecular Nucleophilic Substitution)
SN1 is a two-step nucleophilic substitution reaction where the rate depends only on the concentration of the substrate.
SN2 Reaction (Bimolecular Nucleophilic Substitution)
SN2 is a one-step nucleophilic substitution reaction where the rate depends on both the nucleophile and the substrate.
2. Mechanism
SN1 Mechanism
- Step 1: Formation of a carbocation (rate-determining step)
- Step 2: Nucleophile attacks carbocation
- Intermediate: Carbocation forms
SN2 Mechanism
- Single-step mechanism
- Nucleophile attacks from the backside of the leaving group
- No intermediate, only transition state
3. Kinetics
SN1: First-order kinetics → Rate = k[substrate]
SN2: Second-order kinetics → Rate = k[substrate][nucleophile]
4. Substrate Preference
- SN1: 3° > 2° > 1° (stable carbocation)
- SN2: 1° > 2° > 3° (less steric hindrance)
5. Stereochemistry
- SN1: Racemization occurs due to planar carbocation
- SN2: Inversion of configuration (Walden inversion)
6. Nucleophile Requirement
- SN1: Weak nucleophile is sufficient
- SN2: Strong nucleophile required
7. Solvent Effect
- SN1: Favors polar protic solvents
- SN2: Favors polar aprotic solvents
8. Leaving Group Ability
Both SN1 and SN2 require a good leaving group, but SN1 is more sensitive to leaving group stability because carbocation formation is key.
9. Summary Table
| Feature | SN1 | SN2 |
|---|---|---|
| Reaction Order | First-order | Second-order |
| Mechanism | Two-step (carbocation) | One-step (backside attack) |
| Substrate | 3° > 2° > 1° | 1° > 2° > 3° |
| Stereochemistry | Racemization | Inversion |
| Nucleophile | Weak | Strong |
| Solvent | Polar Protic | Polar Aprotic |
Conclusion
SN1 and SN2 reactions differ in mechanism, kinetics, stereochemistry, and substrate preference. SN1 involves carbocation formation and racemization, while SN2 proceeds in one step with inversion of configuration. Understanding these differences helps in predicting reaction outcomes in organic chemistry.
