SN1 vs SN2 Reactions
SN1 vs SN2
Two major mechanisms for nucleophilic substitution reactions in organic chemistry. SN1 is a two-step process via carbocation intermediate, while SN2 is a one-step concerted mechanism with backside attack.
Comparison Table
| Feature | SN1 | SN2 |
|---|---|---|
| Mechanism | Two-step via carbocation | One-step concerted |
| Rate Law | Rate = k[substrate] | Rate = k[substrate][nucleophile] |
| Stereochemistry | Racemization | Inversion (Walden inversion) |
| Substrate Preference | Tertiary > Secondary > Primary | Primary > Secondary > Tertiary |
| Nucleophile | Weak nucleophile okay | Strong nucleophile required |
| Solvent | Polar protic (stabilizes carbocation) | Polar aprotic (activates nucleophile) |
| Carbocation Rearrangement | Possible | Not possible |
| Steric Effects | Less affected | Highly affected (backside attack) |
Key Differences
- →SN1 forms a planar carbocation intermediate; SN2 has a pentavalent transition state
- →SN1 rate depends only on substrate; SN2 rate depends on both substrate and nucleophile
- →SN1 loses stereochemistry (racemization); SN2 inverts stereochemistry
- →Tertiary substrates only do SN1 due to steric hindrance blocking backside attack
- →Primary substrates favor SN2 because primary carbocations are too unstable
When to Use SN1
- ✓Tertiary alkyl halides (no SN2 possible)
- ✓Weak nucleophiles are present
- ✓Polar protic solvents (water, alcohols)
- ✓Carbocation stability is high (tertiary, allylic, benzylic)
When to Use SN2
- ✓Primary alkyl halides
- ✓Strong nucleophiles (CN-, RS-, I-)
- ✓Polar aprotic solvents (DMSO, DMF, acetone)
- ✓Stereospecific reactions needed (inversion)
Common Confusions
- !Thinking substrate determines everything (solvent and nucleophile matter for secondary substrates)
- !Forgetting that SN1 can have carbocation rearrangements
- !Not recognizing that tertiary substrates cannot do SN2
- !Confusing the stereochemical outcomes (inversion vs racemization)
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Common questions about this comparison
Yes! Secondary substrates are borderline cases. The dominant pathway depends on nucleophile strength and solvent. Strong nucleophile + polar aprotic solvent favors SN2. Weak nucleophile + polar protic solvent favors SN1.
In SN2, the nucleophile attacks from the backside (opposite the leaving group). As the leaving group departs and the nucleophile bonds, the other three groups invert their positions, like an umbrella flipping in the wind.