SN1
SN1 — Unimolecular
Rate LawRate = k[substrate]
StepsTwo-step (ionization → attack)
IntermediateCarbocation (sp² planar)
Substrate3° > 2° (never 1° / methyl)
NucleophileWeak nucleophile OK (e.g. H₂O)
SolventPolar PROTIC (stabilize C⁺)
StereochemistryRacemization (R+S mix)
CompetingE1 elimination
SN2
SN2 — Bimolecular
Rate LawRate = k[substrate][Nu⁻]
StepsOne-step (concerted)
IntermediateNone (transition state only)
SubstrateMethyl > 1° > 2° (never 3°)
NucleophileStrong nucleophile required (e.g. I⁻, CN⁻)
SolventPolar APROTIC (DMF, DMSO, acetone)
StereochemistryInversion (Walden)
CompetingE2 elimination
⟐SN1 Mechanism (Two-Step)
1Ionization (Rate-Determining Step)
The C–LG bond breaks heterolytically. Leaving group departs with both bonding electrons → carbocation intermediate forms (sp², trigonal planar). This is the slow step — determines rate. Only substrate appears in rate law.
2Nucleophilic Attack (Fast)
Nucleophile attacks the planar carbocation from either face (top or bottom) → produces a racemic mixture (equal R and S enantiomers). If one side is slightly blocked, you may get unequal proportions (partial racemization).
⟐SN2 Mechanism (One-Step Concerted)
1Backside Attack (Single Step)
Nucleophile attacks from the back side (180° opposite the leaving group). Bond formation and bond breaking occur simultaneously → pentacoordinate transition state (not an intermediate!). Result: complete Walden inversion of stereochemistry — like an umbrella flipping in wind.
🔬Decision Flowchart
FactorFavors SN1Favors SN2
Substrate3° carbon (stable C⁺)Methyl, 1° (no steric block)
NucleophileWeak / neutral (H₂O, ROH)Strong / charged (I⁻, CN⁻, RS⁻)
SolventPolar protic (MeOH, H₂O)Polar aprotic (DMSO, DMF)
2° substrate?⚠️ Borderline — depends on nucleophile strength & solvent!
Leaving GroupBoth require good LG: I⁻ > Br⁻ > Cl⁻ > F⁻ (weak base = good LG)