SN1 vs SN2 Reactions: Mechanism, Examples, Key Differences

SN1 Mechanism

• Two-step process: Step 1 (slow): substrate ionises to form carbocation + leaving group. Step 2 (fast): nucleophile attacks carbocation
• Rate = k[substrate] — depends only on substrate (unimolecular)
• Carbocation intermediate — can be attacked from both sides → racemisation
• Favoured by: tertiary substrates (stable carbocation), polar protic solvents (stabilise ions), weak nucleophiles
• Example: (CH₃)₃CBr + H₂O → (CH₃)₃COH + HBr

SN2 Mechanism

• One-step (concerted): nucleophile attacks carbon from back side while leaving group departs simultaneously
• Rate = k[substrate][nucleophile] — depends on both (bimolecular)
• No intermediate — transition state with pentacoordinate carbon
• Backside attack causes Walden inversion (configuration change at chiral carbon)
• Favoured by: primary substrates (less steric hindrance), strong nucleophiles, polar aprotic solvents

Key Differences Table

• Steps: SN1 = 2, SN2 = 1
• Rate law: SN1 = first order, SN2 = second order
• Substrate: SN1 = 3° > 2° > 1°, SN2 = 1° > 2° > 3° (reverse)
• Stereochemistry: SN1 = racemisation, SN2 = inversion
• Nucleophile: SN1 = weak OK, SN2 = strong needed
• Solvent: SN1 = polar protic, SN2 = polar aprotic