R3.4.9 SN1 and SN2 mechanisms

SN1 Mechanism

Two-step reaction: forms a carbocation intermediate.

Nucleophile bonds to the carbocation.

Common in tertiary halogenoalkanes.

SN2 Mechanism

One-step reaction: nucleophile attacks opposite to leaving group.

Forms a transition state.

Common in primary halogenoalkanes.

SN1 Step 1

C–X bond undergoes heterolytic fission.

Forms carbocation and halide ion.

Carbocation stabilized by inductive effect.

SN1 Step 2

Nucleophile donates a lone pair to carbocation.

Forms new C–Nu bond. Leads to alcohol product.

SN1 Geometry

Carbocation is trigonal planar.

Nucleophile attacks from either side.

Can lead to racemic mixtures.

Steric Hindrance in SN1

Bulky alkyl groups prevent backside attack.

Promotes SN1 over SN2.

Seen in tertiary halogenoalkanes.

SN2 Mechanism Details

Backside attack by nucleophile.

Nucleophile and leaving group weakly bond in transition state.

Results in inversion of configuration.

SN1 Kinetics

Rate depends on halogenoalkane concentration.

Unimolecular rate-determining step.

First-order kinetics.

SN2 Kinetics

Rate depends on halogenoalkane and nucleophile.

Bimolecular reaction.

First-order in both reactants.

SN1 Energy Profile

Two peaks for two steps.

First step has higher activation energy.

Carbocation intermediate forms.

SN2 Energy Profile

Single transition state.

One activation energy peak.

Involves simultaneous bond breaking and forming.