Question
Compare SN1 and SN2 reactions on the basis of: (a) mechanism, (b) rate law, (c) stereochemistry, and (d) substrate preference. Which mechanism does 2-bromo-2-methylpropane follow, and why?
This exact comparison appeared in JEE Main 2024 and shows up almost every year in CBSE Class 12 board exams. High weightage — know every column of this table cold.
Solution — Step by Step
SN1 is a two-step reaction. First the leaving group leaves on its own, forming a carbocation intermediate. Then the nucleophile attacks the carbocation.
SN2 is a one-step (concerted) reaction. The nucleophile attacks the back of the carbon at the same time the leaving group departs — no intermediate forms.
Because SN1’s slow step is just the substrate breaking apart, only the substrate concentration appears in the rate law:
In SN2, both the substrate and nucleophile must collide in the transition state, so both concentrations matter:
This is the fastest way to identify which mechanism is operating — check the rate expression.
SN1 produces a racemic mixture. The carbocation intermediate is planar (sp² hybridised), so the nucleophile can attack from either face with roughly equal probability — roughly 50% retention, 50% inversion.
SN2 gives complete inversion of configuration (Walden inversion). The backside attack flips the molecule like an umbrella in wind. If you start with an R-substrate, you always get the S-product.
SN1 favours tertiary > secondary > primary substrates. More alkyl groups stabilise the carbocation intermediate through hyperconjugation and inductive effects.
SN2 favours primary > secondary > tertiary substrates — the opposite order. Bulky groups around the central carbon create steric hindrance that blocks the nucleophile’s backside approach.
2-bromo-2-methylpropane is a tertiary substrate (three methyl groups on the carbon bearing Br). It follows SN1.
The tertiary carbocation that forms is highly stable — three methyl groups donate electron density via hyperconjugation, lowering the activation energy for the ionisation step.
Why This Works
The key insight connecting mechanism to substrate is carbocation stability. SN1 requires the substrate to ionise on its own — this is only feasible when the resulting carbocation is stable enough to exist as an intermediate. Tertiary carbocations () are the most stable, so tertiary substrates have the lowest activation energy for SN1.
SN2 is governed entirely by steric accessibility. The nucleophile must reach the back lobe of the C–LG sigma bond. A primary carbon has only one alkyl group — the approach is relatively open. Add two or three alkyl groups (secondary, tertiary) and you’ve built a wall the nucleophile cannot pass.
Solvent plays a role too: polar protic solvents (water, alcohols) stabilise the carbocation and favour SN1; polar aprotic solvents (DMSO, acetone) don’t stabilise the ion pair and favour SN2. JEE questions frequently test this angle.
Alternative Method — Comparison Table
For exam revision, memorise this table directly:
| Feature | SN1 | SN2 |
|---|---|---|
| Steps | 2 (stepwise) | 1 (concerted) |
| Rate law | First order | Second order |
| Intermediate | Carbocation | None |
| Stereochemistry | Racemisation | Inversion |
| Best substrate | 3° | 1° |
| Favoured solvent | Polar protic | Polar aprotic |
| Nucleophile strength | Weak OK | Strong needed |
In MCQs, the single fastest filter is rate law. If a question says “doubling nucleophile concentration has no effect on rate” — that’s SN1, guaranteed, regardless of what else is going on.
Common Mistake
Students write that SN1 gives “complete racemisation” — this is wrong. It gives mostly racemisation, but there is slight excess of the inverted product because the leaving group (still hovering nearby as an ion pair) partially blocks one face. The technically correct answer is “predominantly racemic mixture with slight inversion.” CBSE board marking schemes have accepted “racemic mixture” but JEE descriptive questions have penalised “complete racemisation” — use “racemic mixture” and you’re safe for both.