SN1 vs SN2: what's the difference?
A-Level Chemistry
Two nucleophilic substitution mechanisms of haloalkanes. The number is the molecularity of the rate-determining step.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
SN1 vs SN2 — side by side
| SN1 | SN2 | |
|---|---|---|
| Mechanism | Two steps, via a carbocation intermediate | One step, concerted |
| Rate-determining step | Unimolecular (rate depends only on haloalkane) | Bimolecular (rate depends on haloalkane AND nucleophile) |
| Favoured by | Tertiary haloalkanes (stable carbocation) | Primary haloalkanes (less steric hindrance) |
| Stereochemistry | Racemic mixture (planar carbocation) | Inversion of configuration (backside attack) |
Exam tip — how to tell them apart: Tertiary → SN1 (stable 3° carbocation). Primary → SN2 (nucleophile can reach the carbon). Secondary can do either.
Common trap: The "1" and "2" are the order of the rate equation, not the number of steps — SN1 actually has two steps, SN2 has one.
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