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Bromomethane to methanol; SN² inversion

Short answer

Bromomethane converts to methanol by SN2: OH⁻ attacks carbon from the backside, bonds form/break simultaneously, yielding methanol with inversion of configuration (umbrella flip); rate = k[CH3Br][OH⁻].

Describe the mechanism for the conversion of Bromomethane into methanol in presence of aq. alkali. Why does SN² reaction take place with stereochemical inversion?

Conversion of bromomethane into methanol (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).

Why SN2 takes place with 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)

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