Potential energy profile

ReactantsTSProducts
Step 0
Ready
2-bromobutane is tetrahedral at C-2. OH⁻ is positioned 180° from Br for backside attack.
Step 1
Backside attack
OH⁻ donates its lone pair toward C-2. The C–Br bond begins to weaken simultaneously.
Step 2
Transition state ‡
Pentavalent trigonal bipyramidal TS. Both C–OH and C–Br are partial bonds. Simulation paused.
Step 3
Bond breaking
Br⁻ departs as a stable leaving group. The three groups sweep through, inverting configuration.
Step 4
Walden inversion
Complete inversion at C-2. C–OH bond is fully formed. Product is tetrahedral with opposite stereochemistry.
View:
Speed: 1.0× Ready

What is Sn2?

Bimolecular nucleophilic substitution. The rate depends on both the nucleophile and the substrate — it is a one-step concerted mechanism with no intermediate.

Backside attack

OH⁻ must approach from exactly 180° opposite the C–Br bond. This is a steric requirement — the nucleophile enters the back lobe of the C–Br σ* antibonding orbital.

Transition state

At the TS, carbon is pentavalent and trigonal bipyramidal. Both C–OH (forming) and C–Br (breaking) bonds are partial (δ). This is the energy maximum.

Walden inversion

The three substituents invert through the carbon centre — like an umbrella flipping inside-out. The product always has inverted stereochemistry at the reaction centre.