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SN reaction; inversion of configuration; SN1

Short answer

SN reactions replace a leaving group with a nucleophile. SN2 proceeds with inversion: backside attack of nucleophile, one-step concerted, Walden inversion (e.g., CH₃Br + OH⁻). In SN1, the planar carbocation can be attacked from either face, but the leaving group ion pair partially shields the front, making rear-side attack slightly easier, so inversion predominates.

What is meant by SN reaction? Explain the mechanism for the nucleophilic reaction that proceeds with an inversion of configuration. Why inversion product predominates more than retention product in SN₁ reaction.

SN reaction

A nucleophilic substitution (SN) reaction is a reaction in which a nucleophile (an electron-rich species with a lone pair, e.g. OH⁻, CN⁻, NH₃) replaces the leaving group (e.g. a halide) of a substrate: R–X + Nu⁻ → R–Nu + X⁻. The two mechanisms are SN1 (unimolecular) and SN2 (bimolecular).

Mechanism with inversion of configuration (SN2)

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

  • Rate = k[CH₃Br][OH⁻]: second order (bimolecular).
  • The nucleophile attacks from the side opposite to the leaving group; the three H atoms flip over like an umbrella in the wind (Walden inversion).
  • Reactivity order: CH₃X > 1° > 2° > 3° (steric hindrance).

For a chiral halide, e.g. (R)-2-bromobutane + OH⁻ → (S)-butan-2-ol, the product has the opposite configuration.

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).

Why inversion predominates in SN1

In SN1 the carbocation formed is planar (sp²), so the nucleophile can attack from either face: attack from the face opposite to the leaving group gives inversion, and attack from the same face gives retention. This gives a mixture (racemisation). Inversion slightly predominates because the departing leaving group stays near the front face as an ion pair and partly blocks it, so attack from the rear is easier.

✓ Answer checked by Claude Opus 5.5 (high effort)

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