E2 Mechanism

Overview:

The general form of the E2 mechanism is as follows:

General form of the E2 mechanism

B: = base
X = leaving group (usually halide or tosylate)

In the E2 mechanism, a base abstracts a proton neighboring the leaving group, forcing the electrons down to make a double bond, and, in so doing, forcing off the leaving group. When numerous things happen simultaneously in a mechanism, such as the E2 reaction, it is called a concerted step.

An example of the E2 reaction:

Example of the E2 mechanism

Base Strength: A strong base is required since the base is involved in the rate-determining step.

Leaving groups: A good leaving group is required, such as a halide or a tosylate, since it is involved in the rate-determining step.

Stereochemistry requirements: Must occur with antiperiplanar stereochemistry.