Chemistry

Eliminations: Introduction and Mechanisms


Eliminations: Mechanism of E.1-Elimination

Characteristic of the E.1-Elimination is the process via a carbenium ion as an intermediate. This mechanism will therefore preferentially take place on molecules that can easily form carbenium ions.

In the first step of the reaction, the leaving group X is split off with formation of the carbenium ion. This dissociation is also the rate-determining step. Since the base is not involved in this step, its concentration does not play a role in the speed of the overall reaction.

In the second step, the base attacks a proton on the neighboring carbon atom. The C-H bond is cleaved and an olefin is formed.

Good (easily ionizable) or stable neutral leaving groups (e.g. iodide or water) favor the formation of the carbenium ion. Cation-stabilizing substituents on the neighboring carbon atoms (e.g. Ph, CH3, ...). Thus, dehydration (removal of water) from tertiary alcohols is much easier than from primary alcohols. Furthermore, polar, protic solvents such as methanol or water facilitate the reaction because they stabilize the carbenium ion through solvation.

The carbenium ion is not only able to form various elimination products, but also substitution reactions or rearrangements are possible. That's why it is E.1-Elimination of little importance in preparative organic chemistry. By choosing the reaction conditions (e.g. by using a bulky base), the reaction in the laboratory can be controlled in such a way that it proceeds according to the E2 mechanism.

2D animation of the E.1-Elimination

3D animation of the E.1-Elimination


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