Editing Elimination reaction (section)

== E2 mechanism ==<!-- This section is linked from [[E2]] -->
The E2 mechanism, where E2 stands for '''bimolecular elimination''', involves a one-step mechanism in which ''carbon-hydrogen'' and ''carbon-halogen'' bonds break to form a double bond (''C=C [[molecular geometry|Pi bond]]'').

The specifics of the reaction are as follows:
* E2 is a single step elimination, with a single [[transition state]].
* It is typically undergone by primary substituted alkyl halides, but is possible with some secondary alkyl halides and other compounds.
* The [[reaction rate]] is [[second order reaction|second order]], because it's influenced by both the alkyl halide and the base (bimolecular).
* Because the E2 mechanism results in the formation of a pi bond, the two leaving groups (often a hydrogen and a [[halogen]]) need to be [[antiperiplanar]]. An antiperiplanar [[transition state]] has [[staggered conformation]] with lower energy than a [[synperiplanar]] transition state which is in [[eclipsed]] conformation with higher energy. The reaction mechanism involving staggered conformation is more favorable for E2 reactions (unlike E1 reactions).
* E2 typically uses a strong [[base (chemistry)|base]]. It must be strong enough to remove a weakly acidic hydrogen.
* In order for the pi bond to be created, the [[orbital hybridization|hybridization]] of carbons needs to be lowered from ''sp<sup>3</sup>'' to ''sp<sup>2</sup>''.
* The C-H bond is weakened in the [[rate determining step]] and therefore a primary [[deuterium isotope effect]]  much larger than 1 (commonly 2-6) is observed.
* E2 competes with the [[SN2 reaction|S<sub>N</sub>2 reaction]] mechanism if the base can also act as a nucleophile (true for many common bases).

[[File:E2 elimination reaction.svg|center|500px|Scheme 1: E2 reaction mechanism]]

An example of this type of reaction in ''scheme 1'' is the reaction of [[isobutyl]]bromide with [[potassium ethoxide]] in [[ethanol]]. The reaction products are [[isobutene]], ethanol and [[potassium bromide]].