Elimination Reaction - E1 Mechanism

E1 Mechanism

E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for unimolecular elimination and has the following specificities.

• It is a two-step process of elimination: ionization and deprotonation.
  • Ionization: the carbon-halogen bond breaks to give a carbocation intermediate.
  • Deprotonation of the carbocation.
• E1 typically takes place with tertiary alkyl halides, but is possible with some secondary alkyl halides.
• The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest step aka rate-determining step. Therefore first-order kinetics apply (unimolecular).
• Reaction usually occurs in complete absence of base or presence of only a weak base (acidic conditions and high temperature).
• E1 reactions are in competition with S_N1 reactions because they share a common carbocationic intermediate.
• A secondary deuterium isotope effect of slightly larger than 1 (commonly 1 - 1.5) is observed.
• No antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol:

Only reaction product A results from antiperiplanar elimination, the presence of product B is an indication that an E1 mechanism is occurring.

• Accompanied by carbocationic rearrangement reactions

An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol.

E1 eliminations happen with highly substituted alkyl halides due to 2 main reasons.

• Highly substituted alkyl halides are bulky, limiting the room for the E2 one-step mechanism; therefore, the two-step E1 mechanism is favored.
• Highly substituted carbocations are more stable than methyl or primary substituted cations. Such stability gives time for the two-step E1 mechanism to occur.
• If S_N1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.

Specific features : 1 . Rearrangement possible 2 . Independent of concentration and basicity of base