Skip to main content

Mechanism and kinetics of SN2; stereochemical inversion [partly illegible]

Short answer

SN2 is a concerted single-step displacement: nucleophile attacks backside, bonds to leaving group break simultaneously; rate = k[RX][Nu⁻], second order. Stereochemical inversion occurs because the nucleophile approaches from the side opposite the leaving group, causing substituents to flip (Walden inversion).

Describe mechanism and kinetics of nucleophilic bimolecular substitution reaction. Why does SN2 reaction take place with stereochemical inversion?

Mechanism

Mechanism: SN2 (one step, concerted).

HO⁻ + H₃C—Br  →  [HO···CH₃···Br]⁻‡  →  HO—CH₃ + Br⁻
                 (transition state)

        H                 H                  H
         \                |                 /
 HO⁻ +  H—C—Br  →  [HO····C····Br]⁻  →  HO—C—H  +  Br⁻
         /               / \                \
        H               H   H                H
 backside attack      transition state    inverted product

Overall: CH₃Br + NaOH(aq) → CH₃OH + NaBr

Kinetics

The reaction takes place in one step, and both reactants take part in the rate-determining step, so: Rate = k [R–X] [Nu⁻] It is second order overall (first order in each reactant). Doubling either concentration doubles the rate.

Stereochemical inversion

In SN2 the nucleophile attacks the carbon from the backside, 180° from the leaving group. This is the position of least repulsion between the lone pair of the incoming nucleophile and the electron-rich leaving group, and it gives the best overlap with the C–X σ* orbital. As the new bond forms, the other three groups flatten and then flip to the opposite side, so the configuration is inverted (Walden inversion).

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Engineering Chemistry chapter 10 questions →