SN2 Reaction: Backside Attack & Walden Inversion

Substrate (steric bulk)

Leaving group

Nucleophile

Temperature 298 K (25°C)

Ready. Set conditions and press Run reaction.

Kinetics (Hughes–Ingold)

Activation energy Eₐ

— kJ/mol

Rate constant k

—

Reaction coordinate

How it works

This models a bimolecular nucleophilic substitution, S_N2: Nu⁻ + R–LG → R–Nu + LG⁻, a single concerted step with no intermediate.

rate = k[Nu⁻][R-LG]
k = A · exp(−Eₐ / RT)   (Arrhenius equation)

The nucleophile attacks the electrophilic carbon from the side directly opposite the leaving group (backside attack), because that is where the carbon's empty σ* (C–LG) antibonding orbital has its largest lobe. At the transition state the carbon is roughly trigonal-bipyramidal, with the three other substituents momentarily flattened into a plane. As the leaving group finishes departing, the substituents snap through to the opposite side — a stereochemical inversion known as Walden inversion (a chiral R-configured carbon becomes S, and vice versa).

Two factors set Eₐ in this simplified model, both textbook S_N2 trends:

Steric bulk — more substituents crowd the backside approach path. Methyl reacts fastest; tertiary carbons are so hindered that S_N2 essentially does not compete with other pathways.
Leaving-group ability — a weaker C–LG bond and a more stable anion lower Eₐ. Iodide (large, polarizable, stable as I⁻) is an excellent leaving group; fluoride is a poor one because the C–F bond is strong and F⁻ is a poor leaving anion.
Nucleophile strength — a stronger nucleophile (e.g. OH⁻ vs. neutral H₂O) donates electron density into σ*(C–LG) more effectively, lowering Eₐ further.

The Temperature slider changes k through the Arrhenius exponential, which sets how fast the animation plays out — exactly the same exponential sensitivity that makes real reaction rates roughly double for every ~10 K rise.

Central carbon Spectator groups Nucleophile Leaving group

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