SN2

Introduction

The SN2 mechanism is a substitution reaction, hence the S. The N stands for nucleophilic, and the 2 stands for bimolecular.

Reagents

Substrate: methyl>1˚>2˚>>3˚

Solvent: polar aprotic is best, to allow the nucleophile to be unhindered. Polar aprotic solvents solvate the nucleophile's counterion, leaving the nucleophile ready to attack.

Strong nucleophile, which is often negatively charged.

3D Sense

If the electrophilic site is a chirality center, the SN2 reaction will occur with inversion of configuration.

Mechanism

There is one core step (in bold) and two other possible steps in an SN2 reaction:

  1. Proton transfer

  2. Nucleophilic attack and loss of leaving group

  3. Proton transfer

The slow (rate-determining step) is the simultaneous nucleophilic attack and loss of leaving group. This step involves two reactants (the substrate and nucleophile), hence the bimolecular in the reaction name.

The reaction of ethyl bromide with sodium iodide. The transition state is also shown.

This reaction has the following step:

  1. Simultaneous (concerted) nucleophilic attack and loss of leaving group