Editing Elimination reaction (section)

== E1 mechanism ==<!-- This section is linked from [[E1]] -->
E1 is a model to explain a particular type of chemical elimination reaction. E1 stands for '''unimolecular elimination''' and has the following specifications

* 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 carbon atom|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, as known as the [[rate-determining step]]. Therefore, [[Rate equation#First-order reactions|first-order kinetics]] apply (unimolecular).
* The reaction usually occurs in the complete absence of a base or the presence of only a weak base (acidic conditions and high temperature).
* E1 reactions are in competition with [[SN1 reaction|S<sub>N</sub>1 reaction]]s because they share a common carbocationic intermediate.
* A secondary [[deuterium isotope effect]] of slightly larger than 1 (commonly 1 - 1.5) is observed.
* There is no antiperiplanar requirement. An example is the [[pyrolysis]] of a certain [[sulfonate ester]] of [[menthol]]:

[[File:E1-eliminationNash2008.svg|center|500px|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.<ref>{{cite journal |title= Pyrolysis of Aryl Sulfonate Esters in the Absence of Solvent: E1 or E2? A Puzzle for the Organic Laboratory |journal= [[Journal of Chemical Education]] |volume= 85 |issue= 4 |date=April 2008 |pages= 552 |doi= 10.1021/ed085p552 |author1=Nash, J. J. |author2=Leininger, M. A. |author3=Keyes, K. |bibcode = 2008JChEd..85..552N}}</ref>
* It is accompanied by carbocationic [[rearrangement reaction]]s

[[File:E1 Elimination Reaction.png|center|600px|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 for two 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<sub>N</sub>1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat. 

Specific features : 
# Rearrangement possible
# Independent of concentration and basicity of base