Question
Compare the SN1 and SN2 mechanisms with respect to:
- Rate of reaction
- Stereochemical outcome
- Effect of substrate structure
This is a 3-5 mark question type that appeared in JEE Main 2022 and is a consistent CBSE 12 board favourite.
Solution — Step by Step
SN2 is a one-step, concerted process — the nucleophile attacks as the leaving group departs, simultaneously. SN1 is a two-step process — the leaving group leaves first to form a carbocation, then the nucleophile attacks.
The carbocation formation step is rate-determining in SN1. There is no carbocation involved at any point in SN2.
SN2 is bimolecular — both substrate and nucleophile appear in the rate law. SN1 is unimolecular — only the substrate concentration matters because the slow step happens before the nucleophile gets involved.
| Substrate | Favours |
|---|---|
| Methyl, Primary | SN2 |
| Secondary | Both possible (solvent/nucleophile dependent) |
| Tertiary | SN1 |
The reasoning: SN2 requires backside attack, so steric hindrance kills it for tertiary substrates. Tertiary carbocations are stabilised by hyperconjugation and inductive effect, making SN1 viable.
SN2 gives inversion of configuration (Walden inversion). The nucleophile attacks from the back, flipping the molecule like an umbrella in wind. The product always has the opposite configuration at the chiral centre.
SN1 gives a racemic mixture (approximately). The carbocation intermediate is sp² hybridised and planar — the nucleophile can attack from either face with roughly equal probability, giving ~50:50 R and S products.
SN1 needs a polar protic solvent (water, ethanol) to stabilise the developing carbocation and the leaving group via solvation. SN2 works better in polar aprotic solvents (DMSO, acetone) because these solvents don’t solvate the nucleophile, keeping it “naked” and highly reactive.
Why This Works
The entire SN1 vs SN2 comparison comes down to one question: can the transition state or intermediate handle the steric and electronic demands?
In SN2, the transition state has five groups around the central carbon — that’s why tertiary substrates simply cannot accommodate it. For methyl and primary substrates, backside attack is geometrically accessible and the reaction proceeds cleanly.
SN1 is about carbocation stability. Tertiary carbocations are stabilised by three alkyl groups donating electron density via hyperconjugation. That’s why tertiary substrates prefer SN1 — the intermediate is low enough in energy to actually form.
Stereochemistry follows mechanistically: a concerted backside attack must invert; a free planar carbocation must give racemisation. These aren’t arbitrary rules to memorise — they’re consequences of the mechanism.
Alternative Method — Comparison via Energy Profile
Instead of listing properties, think about the energy diagrams:
SN2: One energy hill (transition state). One step, one hump. Reaction coordinate shows a single maximum.
SN1: Two energy hills with a valley (the carbocation) between them. The first hill is higher — that’s your rate-determining step. The nucleophile attack happens after the valley, which is why nucleophile concentration doesn’t appear in the rate law.
In CBSE board exams, if a question asks “why does SN1 give racemic product,” full marks require you to mention the planar carbocation intermediate specifically. Just writing “attack from both sides” without explaining why both sides are accessible loses you a mark.
This energy-profile approach helps when a problem gives you an unknown substrate and asks you to predict the mechanism — you’re asking “which intermediate/transition state is more feasible here?”
Common Mistake
Students write that SN1 gives “complete racemisation.” In reality, SN1 gives predominantly racemic mixture but often with slight inversion excess. This is because the leaving group is still hovering near one face of the carbocation immediately after it departs, partially blocking that face. The JEE phrasing to use is “racemisation with partial inversion” for a nuanced answer. Writing “complete racemisation” in JEE Advanced can cost you marks.
A second mistake: confusing the substrate effect with the nucleophile effect. The nucleophile strength matters enormously for SN2 (strong nucleophile → faster SN2) but has no effect on SN1 rate. Students often write “strong nucleophile favours SN1” — this is incorrect. Strong nucleophiles favour SN2.
Final Summary Table:
| Property | SN1 | SN2 |
|---|---|---|
| Steps | 2 | 1 |
| Rate law | Rate = k[RX] | Rate = k[RX][Nu⁻] |
| Best substrate | 3° | 1° / methyl |
| Stereochemistry | Racemisation | Inversion |
| Best solvent | Polar protic | Polar aprotic |
| Intermediate | Carbocation | None (concerted) |