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SN² mechanism for hydrolysis; solvent; no carbocation

Short answer

In SN2 hydrolysis, OH- attacks the alkyl halide from the opposite side of the leaving group in one concerted step with Walden inversion; rate = k[RX][OH-]. Polar aprotic solvents are favored. No carbocation forms because the reaction is bimolecular, gives complete inversion, and reactivity is methyl > 1° > 2° > 3°, opposite to carbocation stability.

Explain the SN2 reaction mechanism with reference to hydrolysis of alkylhalide. What type of solvent favors this type of path? How can you say that carbocation is not formed during SN2 reaction?

SN2 hydrolysis of an alkyl halide

Mechanism: SN2 (one step, concerted).

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

 HO⁻  →  C ← Br      backside attack, 180° from Br
         /|\
        H H H        inverted like an umbrella in the wind

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 (Walden inversion).
  • Reactivity order: CH₃X > 1° > 2° > 3° (steric hindrance).

Solvent

Solvent: polar aprotic solvents (acetone, DMF, DMSO, acetonitrile). They dissolve ionic nucleophiles but do not hydrogen-bond to the anion, so the nucleophile remains "naked" and more reactive. Polar protic solvents like water solvate and weaken the nucleophile, slowing SN2.

No carbocation in SN2

No carbocation is formed in SN2 because:

  1. The reaction is a single concerted step (bond formation and bond breaking at the same time), with a transition state, not an intermediate.
  2. The rate depends on both the substrate and the nucleophile (second order), whereas carbocation formation would give first-order kinetics.
  3. A chiral substrate gives complete inversion; a planar carbocation would give racemisation.
  4. Methyl halides react fastest, then primary halides, even though they would form the least stable carbocations; tertiary halides, which form the most stable carbocations, react slowest.

✓ Answer checked by Claude Opus 5.5 (high effort)

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