📘 IB Understanding
SN1 is a two-step mechanism. Step 1 (slow, rate-determining): the leaving group departs to form a planar carbocation. Step 2 (fast): the nucleophile attacks the carbocation.
Key Features
| Feature | SN1 Detail |
|---|---|
| Rate law | Rate = k[substrate] (first order) |
| Steps | Two steps (carbocation intermediate) |
| Stereochemistry | Racemisation (mixture of R and S products) |
| Preferred substrate | Tertiary > secondary >> primary |
| Solvent | Polar protic solvents (e.g. water, ethanol) |
The Two Steps
Step 1 (slow – rate-determining)
The C–X bond breaks heterolytically. The leaving group takes the bonding pair, forming a planar carbocation (C⁺).
R–X → R⁺ + X⁻
Step 2 (fast)
The nucleophile attacks the planar carbocation from either side, forming the product.
R⁺ + Nu⁻ → R–Nu
Why Racemisation?
The carbocation intermediate is planar (sp²), so the nucleophile can attack from either side with equal probability. This gives a 50:50 mixture of R and S enantiomers (a racemic mixture).
SN1 vs SN2 Summary
| SN1 | SN2 | |
|---|---|---|
| Steps | 2 | 1 |
| Rate law | k[substrate] | k[substrate][Nu] |
| Substrate | Tertiary best | Primary best |
| Stereochemistry | Racemisation | Inversion |
📋 Exam Tip
If an exam question says “identify the mechanism”, check whether the substrate is primary (SN2) or tertiary (SN1). Secondary substrates can go either way.