Question
Describe the SN1 mechanism step by step. Why does it proceed through a carbocation intermediate, and what determines whether a substrate follows SN1?
Solution — Step by Step
The leaving group departs on its own, taking the bonding electrons with it. This creates a carbocation intermediate.
This is the slow step — and since it involves only the substrate (not the nucleophile), the rate depends only on substrate concentration:
That is why it is called SN1 — Substitution, Nucleophilic, Unimolecular (one species in the rate-determining step).
The nucleophile attacks the planar carbocation. Since the carbocation is sp2-hybridised and flat, the nucleophile can attack from either side with equal probability.
This is the fast step. Because attack happens from both faces, we get a racemic mixture (50:50 of R and S configurations) — or close to racemic.
If the nucleophile was neutral (like water or an alcohol), the product carries a positive charge that needs to be removed by losing a proton to the solvent.
graph TD
A["R-X (substrate)"] -->|Slow: leaving group departs| B["R+ (carbocation)"]
B --> C{Nucleophile attacks}
C -->|From front face| D[Product with retention]
C -->|From back face| E[Product with inversion]
D --> F[Racemic mixture]
E --> F
F -->|If needed| G[Deprotonation]Why This Works
SN1 works best with tertiary substrates (3-degree carbon) because they form the most stable carbocations. The order of carbocation stability is:
Tertiary carbocations are stabilised by hyperconjugation and inductive effect from three alkyl groups. Primary carbocations are so unstable that SN1 practically never happens at primary carbons.
Polar protic solvents (water, methanol, ethanol) favour SN1 because they stabilise both the carbocation and the leaving group through solvation.
Alternative Method
We can also predict SN1 by checking three quick conditions:
- Substrate: tertiary (or allylic/benzylic with resonance stabilisation)
- Nucleophile: weak nucleophile (water, alcohols)
- Solvent: polar protic
If all three point to SN1, the reaction will almost certainly follow SN1. If even one condition is unfavourable (say, strong nucleophile with tertiary substrate), competition with E1 becomes significant.
Quick exam trick: If the question mentions “aqueous conditions” or “solvolysis” with a tertiary halide, it is SN1. JEE Main 2023 tested exactly this — tert-butyl bromide in water/acetone mixture.
Common Mistake
Students often say SN1 gives “100% racemisation.” In reality, SN1 gives a predominantly racemic product but with slight excess of inversion product. Why? The leaving group, while departing, partially shields the front face — so the nucleophile attacks slightly more from the back. This is called ion-pair effect. For JEE Advanced, this nuance matters.