3D SN2 Walden Inversion | mysimulator.uk

SN2 Walden Inversion

Leaving-group bond strength 0.5

Steric bulk of substituents 0.4

Nucleophile approach speed 1.0×

Inversion progress

Backside angle

180°

Transition state E

–

Stage

Approach

How it works

SN2 substitution is a single concerted step: the nucleophile (blue) attacks the electrophilic sp3 carbon from the side directly opposite the leaving group (red), at a 180° backside angle. As the new bond forms, the carbon passes through a trigonal-bipyramidal transition state where all three other substituents (grey/green/yellow) are momentarily coplanar, then flip to the opposite side as the leaving group departs — exactly like an umbrella turning inside-out in the wind. This is Walden inversion: the product's stereochemistry at that carbon is the mirror image of the starting material's.

Nu: + R₃C–X  →  [Nu···C···X]‡  →  R₃C–Nu + X:
backside attack angle = 180° (Nu–C–X)
configuration inverts: R → S (or S → R)

Leaving-group bond strength — a weaker C–X bond (poor bond strength, good leaving group like I⁻) departs faster and lowers the transition-state energy, speeding the whole inversion.
Steric bulk of substituents — bulkier groups crowd the backside approach path, slowing the nucleophile and raising the transition-state energy (this is why SN2 favors primary and methyl carbons, not tertiary).
Nucleophile approach speed — how fast the nucleophile closes in from 180°; watch how the three spectator substituents stay locked in the trigonal-bipyramidal plane until the leaving group is far enough to let them snap through.

Unlike a 2D reaction-coordinate plot, this view shows the actual 3D consequence of the mechanism: the molecule that comes out the other side is genuinely a different stereoisomer from the one that went in.

Nucleophile (Nu) Central carbon Leaving group (X) Substituents

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