E2

status: in progress

unverified

Introduction

The E2 mechanism is an elimination reaction , hence the E. The 2 stands for bimolecular.

Reagents/Reaction Conditions

Substrate: 3˚>2˚>>1˚, allylic and benzylic

Solvent: polar protic is best.

Weak base

Neutral to acidic

Heat

3D Sense

Regioselectivity: Zaitsev is preferred unless a sterically hindered (bulky) base is used, such as potassium tert-butoxide.

Stereoselectivity: trans is preferred

Stereospecificity: anti-periplanar geometry is necessary (except for a bicyclic ring). Rotation around a single bond might be necessary. Use a Newman projection to determine whether the E or Z isomer will be formed.

Mechanism

There are two core steps (in bold) and two other possible steps in an E1 reaction:

  1. Proton transfer

  2. Loss of leaving group

  3. Carbocation rearrangement

  4. Proton transfer

The slow (rate-determining step) is the loss of the leaving group, which only has one reactant (the substrate), hence the unimolecular in the reaction name.

The reaction of 1-bromopropane with sodium ethoxide to form 1-propene

This reaction has the following simultaneous steps:

  1. Proton transfer

  2. Loss of leaving group

Formation of Alkynes

The reaction of a geminal dihalide with sodium amide in liquid ammonia to form an alkyne. Alkynes can also be prepared from vicinal dihalides.