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E1 Mechanism

Overview:

The general form of the E1 mechanism is as follows:

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

In the E1 mechanism, the the first step is the loss of the leaving group, which leaves in a very slow step, resulting in the formation of a carbocation. The base then attacks a neighboring hydrogen, forcing the electrons from the hydrogen-carbon bond to make the double bond. Since this mechanism involves the formation of a carbocation, rearangements can occur.

An example of the E1 reaction:

Base Strength: A strong base not required, since it is not 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.

Rearangements: Since the mechanism goes through a carbocation intermediate, rearangements can occur.

E2 Mechanism

Overview:

The general form of the E2 mechanism is as follows:

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:

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.

more at : http://www.chemhelper.com/mechanisms.html

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