SN2 mechanism: bromoethane and hydroxide ions

In an SN2 reaction the nucleophile attacks and the leaving group departs in a single step. Bromoethane is a primary halogenoalkane, so there's little crowding around the carbon and backside attack is easy.

Try it in the builder →  Change the reactants or conditions and generate your own mechanism.

Nucleophilic substitution (SN2)

Hydroxide ions replace the bromine atom of bromoethane in one concerted step, giving ethanol and bromide ions. Bond making and bond breaking happen at the same time, so the rate depends on both the halogenoalkane and the hydroxide concentration.

Overall reaction

1. 1Backside attack and loss of bromideconcerted (SN2)

   hydroxide ion

   +

   bromoethane

   ethanol

   +

   bromide ion

   Electron movement

   1. A lone pair on the hydroxide oxygen attacks the carbon bonded to bromine, from the side opposite the bromine.
   2. At the same time the C–Br bond breaks and both electrons go to bromine, which leaves as Br⁻.

   The C–Br bond is polar (Cδ+–Brδ−), so the electron-rich hydroxide is attracted to the carbon. It approaches from the back, opposite the bromine, passing through a single transition state where the C–O bond is half-formed and the C–Br bond is half-broken.