Unimolecular Elimination (E1)

The E1 mechanism is a theoretical model that describes a distinct class of chemical elimination reactions. This mechanism is marked by a two-step elimination process, which involves ionization, resulting in the creation of a carbocation intermediate, followed by the deprotonation of this intermediate. This mechanism is commonly associated with tertiary alkyl halides and a few secondary alkyl halides. The reaction rate in the E1 mechanism is solely influenced by the concentration of the alkyl halide, as the generation of the carbocation is the slowest step, thus demonstrating first-order kinetics. The E1 mechanism typically transpires in the absence of a base or under the influence of a weak base, often in acidic conditions and high temperatures. E1 reactions are known to compete with SN1 reactions due to their shared carbocationic intermediate. 

​

E1 eliminations are particularly common with highly substituted alkyl halides for two main reasons: the steric bulk of these halides hinders the E2 single-step mechanism, making the two-step E1 mechanism more favorable; and the increased stability of highly substituted carbocations allows the E1 mechanism the necessary time to proceed. When SN1 and E1 pathways are in competition, the application of heat can favor the E1 pathway. Notable features of the E1 mechanism include the potential for carbocationic rearrangement and its independence from the concentration and basicity of the base.

​

Summary:

• 2-step reaction

• The intermediate is a carbocation

• The reaction rate depends only on the substrate, not the nucleophile

  • The rate depends on how good the leaving group is and how stable the resulting carbocation is

• Questions which require you to decide between SN1 and E1 will often include ‘heat’ to hint towards E1 instead of SN1.