Skip to content
qorganica.es qorganica.comOrganic Chemistry made transparent Course map Español

Unimolecular Elimination

The carbocation produced when the leaving group departs can evolve in an alternative way to the SN1 reaction: elimination of a proton from the adjacent position.

Animation of carbocation formation by departure of the chloride
Carbocation (SN1)
Animation of carbocation formation by departure of the chloride
Carbocation (E1)

The nucleophile, ACTING AS A BASE, can remove a proton from the carbon adjacent to the carbocation, leading to an olefin.

E1 mechanism: the nucleophile acts as a base and removes a proton from the intermediate carbocation to give the alkene

In the case of the methanolysis of tert-butyl chloride, it is the methanol that acts as a base, taking the proton and producing isobutylene.

Animation of the E1 mechanism: the carbocation loses a proton from the adjacent carbon and the alkene is formed

Therefore, once formed, the carbocation can evolve by two different pathways. The qualitative energy diagram is as follows:

Energy diagram: the intermediate carbocation evolves by E1 towards the alkene or by SN1 towards the substitution product

The substitution-to-elimination ratio does not depend on the leaving group because the carbocation is a common intermediate in both pathways, but it does depend on the basicity/nucleophilicity relationship of the attacking base/nucleophile.

Any hydrogen on a carbon adjacent to the one bearing the leaving group can participate in the E1 reaction. Consequently, the carbocation generated from 2-bromooctane can lead to either 1- or 2-octene:

The 2-bromooctane carbocation loses a proton by path a (2-octene) or by path b (1-octene)

We have now explained why our 'experiment' led to the formation of 20% olefins.

We only have to understand why only traces of 1-octene are produced and why trans-2-octene is the major olefin. These facts will be explained in the chapter on alkenes.

Result of the experiment: (R)-2-bromooctane gives 80% substitution (2-octanol with 94% inversion) and 20% elimination (E alkene 95%, Z 5% and traces of terminal olefin)