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what is the mechanism of E1cb?

what is the mechanism of E1cb?


1 Answers

Suraj Prasad IIT Patna
askIITians Faculty 286 Points
6 years ago
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 SN1 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:[E1 elimination Nash 2008, antiperiplanar relationship in blue]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[Scheme 2. E1 reaction mechanism]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 SN1 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

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