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SN² mechanism and difference from SN¹

Short answer

SN2: one-step concerted backside attack; rate = k[RX][Nu⁻]; inversion (Walden); reactivity order CH₃X > 1° > 2° > 3°. Example: CH₃Br + NaOH → CH₃OH. SN1: two-step with carbocation; rate = k[RX] only; racemization; rearrangement possible; order 3° > 2°; (CH₃)₃CBr + aq. NaOH → (CH₃)₃COH.

Write the reaction mechanism of SN² reaction with a suitable example. How does it differ from SN¹ reaction?

SN2 mechanism

Example: reaction of methyl bromide with aqueous NaOH.

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).

Difference from SN1

BasisSN1SN2
StepsTwo steps; carbocation intermediateOne step; transition state
Rate lawRate = k[RX] (first order)Rate = k[RX][Nu⁻] (second order)
Substrate order3° > 2° > 1°CH₃ > 1° > 2° > 3°
NucleophileStrength unimportantStrong nucleophile needed
SolventPolar protic (water, alcohol)Polar aprotic (acetone, DMF, DMSO)
StereochemistryRacemisation (inversion + retention)Complete inversion (Walden)
RearrangementPossibleNot possible
Example(CH₃)₃CBr + aq. NaOHCH₃Br + aq. NaOH

✓ Answer checked by Claude Opus 5.5 (high effort)

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