Editing Elimination reaction

{{Short description|Type of organic chemical reaction}}
[[Image:EliminationReactionCyclohexene.svg|frame|right|Elimination reaction of [[cyclohexanol]] to [[cyclohexene]] with [[sulfuric acid]] and heat<ref>{{cite journal|last1=Coleman|first1=G. H.|last2=Johnstone|first2=H. F.|title=Cyclohexene|journal=[[Organic Syntheses]]|date=1925|volume=5|page=33|doi=10.15227/orgsyn.005.0033}}</ref>]] An '''elimination reaction''' is a type of [[organic reaction]] in which two [[substituent]]s are removed from a molecule in either a one- or two-step mechanism.<ref>{{JerryMarch}}</ref> The one-step mechanism is known as the '''E2 reaction''', and the two-step mechanism is known as the '''E1 reaction'''.  The numbers refer not to the number of steps in the mechanism, but rather to the kinetics of the reaction: E2 is bimolecular (second-order) while E1 is unimolecular (first-order). In cases where the molecule is able to stabilize an anion but possesses a poor [[leaving group]], a third type of reaction, [[E1cB-elimination reaction|E1<sub>CB</sub>]],  exists.  Finally, the [[pyrolysis]] of [[xanthate]] and [[acetate]] [[esters]] proceed through an "internal" elimination mechanism, the [[Ei mechanism|E<sub>i</sub> mechanism]].

== 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]].

== 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

== Competition among mechanisms ==
The [[reaction rate]] is influenced by the reactivity of [[halogen]]s, [[iodide]] and [[bromide]] being favored. Fluoride is not a good leaving group, so eliminations with fluoride as the leaving group have slower rates than other halogens .
There is a certain level of competition between the elimination reaction and [[nucleophilic substitution]]. More precisely, there are competitions between E2 and [[SN2 reaction|S<sub>N</sub>2]] and also between E1 and [[SN1 reaction|S<sub>N</sub>1]]. Generally, elimination is favored over substitution when
* [[steric hindrance]] around the α-carbon increases.
* a stronger base is used.
* [[temperature]] increases (increase [[entropy]])
* the base is a poor [[nucleophile]]. Bases with steric bulk, (such as in [[potassium tert-butoxide]]), are often poor nucleophiles.
For example, when a 3° haloalkane is reacts with an alkoxide, due to strong basic character of the alkoxide and unreactivity of 3° group towards S<sub>N</sub>2, only alkene formation by E2 elimination is observed.  Thus, elimination by E2 limits the scope of the [[Williamson ether synthesis]] (an S<sub>N</sub>2 reaction) to essentially only 1° haloalkanes; 2° haloalkanes generally do not give synthetically useful yields, while 3° haloalkanes fail completely.  

With strong base, 3° haloalkanes give elimination by E2.  With weak bases, mixtures of elimination and substitution products form by competing S<sub>N</sub>1 and E1 pathways.    

The case of 2° haloalkanes is relatively complex.  For strongly basic nucleophiles (p''K''<sub>aH</sub> > 11, e.g., hydroxide, alkoxide, acetylide), the result is generally elimination by E2, while weaker bases that are still good nucleophiles (e.g., acetate, azide, cyanide, iodide) will give primarily S<sub>N</sub>2.  Finally, weakly nucleophilic species (e.g., water, alcohols, carboxylic acids) will give a mixture of S<sub>N</sub>1 and E1.

For 1° haloalkanes with β-branching, E2 elimination is still generally preferred over S<sub>N</sub>2 for strongly basic nucleophiles.  Unhindered 1° haloalkanes favor S<sub>N</sub>2 when the nucleophile is also unhindered.  However, strongly basic and hindered nucleophiles favor E2.  

In general, with the exception of reactions in which E2 is impossible because β hydrogens are unavailable (e.g. methyl, allyl, and benzyl halides),<ref>In rare cases in which β hydrogens are unavailable but substitution is disfavored, α-elimination to form a carbene can sometimes occur.  In particular: (1) Trihalomethanes like chloroform can react with NaOH to form dihalocarbenes (substitution is electronically disfavored).  (2) Allyl and benzyl chloride can react with lithium tetramethylpiperide (LiTMP) to form vinylcarbene and phenylcarbene, respectively (substitution is sterically disfavored).    </ref> clean S<sub>N</sub>2 substitution is hard to achieve when strong bases are used, as alkene products arising from elimination are almost always observed to some degree.  On the other hand, clean E2 can be achieved by simply selecting a sterically hindered base (e.g., potassium ''tert''-butoxide).  Similarly, attempts to effect substitution by S<sub>N</sub>1 almost always result in a product mixture contaminated by some E1 product (again, with the exception of cases where the lack of β hydrogens makes elimination impossible).<ref>{{Cite book |last=Carey |first=Francis A. |title=Organic Chemistry |publisher=McGraw-Hill |year=2003 |isbn=0-07-242458-3 |edition=5th |location=New York |pages=350}}</ref>

In one study<ref>{{cite journal | title = Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory |author1=Stephanie M. Villano |author2=Shuji Kato |author3=Veronica M. Bierbaum | journal = [[J. Am. Chem. Soc.]] | year = 2006 | volume = 128 | issue = 3 | pages = 736–737 | doi = 10.1021/ja057491d | pmid = 16417360}}</ref> the [[kinetic isotope effect]] (KIE) was determined for the gas phase reaction of several alkyl halides with the [[chlorate]] ion. In accordance with an E2 elimination the reaction with [[t-butyl chloride]] results in a KIE of 2.3. The [[methyl chloride]] reaction (only S<sub>N</sub>2 possible) on the other hand has a KIE of 0.85 consistent with a S<sub>N</sub>2 reaction because in this reaction type the C-H bonds tighten in the transition state. The KIE's for the ethyl (0.99) and isopropyl (1.72) analogues suggest competition between the two reaction modes.

== Elimination reactions other than β-elimination ==
β-Elimination, with loss of electrofuge and nucleofuge on vicinal carbon atoms, is by far the most common type of elimination.  The ability to form a stable product containing a C=C or C=X bond, as well as orbital alignment considerations, strongly favors β-elimination over other elimination processes.<ref>{{Cite book|title=Modern physical organic chemistry|author=Anslyn, Eric V.|date=2006|publisher=University Science|others=Dougherty, Dennis A., 1952-|isbn=1891389319|location=Sausalito, CA|oclc=55600610}}</ref> However, other types are known, generally for systems where β-elimination cannot occur.
[[File:Eliminationrxns.png|center|frameless|400x400px]]

The next most common type of elimination reaction is α-elimination.  For a carbon center, the result of α-elimination is the formation of a carbene, which includes "stable carbenes" such as [[carbon monoxide]] or [[isocyanide]]s.  For instance, α-elimination the elements of HCl from chloroform (CHCl<sub>3</sub>) in the presence of strong base is a classic approach for the generation of [[dichlorocarbene]], :CCl<sub>2</sub>, as a reactive intermediate.  On the other hand, formic acid undergoes α-elimination to afford the stable products water and carbon monoxide under acidic conditions.  α-Elimination may also occur on a metal center, one particularly common result of which is lowering of both the metal oxidation state and coordination number by 2 units in a process known as [[reductive elimination]].  (Confusingly, in organometallic terminology, the terms ''α-elimination'' and ''α-abstraction'' refer to processes that result in formation of a metal-carbene complex.<ref>{{Cite book|title=The organometallic chemistry of the transition metals|author=Crabtree, Robert H.|date=2009|publisher=Wiley|isbn=9780470257623|edition=5th |location=Hoboken, N.J.|oclc=268790870}}</ref>  In these reactions, it is the carbon adjacent to the metal that undergoes α-elimination.)

In certain special cases, γ- and higher eliminations to form three-membered or larger rings is also possible in both organic and organometallic processes. For instance, certain Pt(II) complexes undergo γ- and δ-elimination to give metallocycles.<ref>{{Cite journal|last=Moore|first=Stephen S.|last2=DiCosimo|first2=Robert|last3=Sowinski|first3=Allan F.|last4=Whitesides|first4=George M.|date=1981-02-01|title=Ring strain in bis(triethylphosphine)-3,3-dimethylplatinacyclobutane is small|journal=Journal of the American Chemical Society|volume=103|issue=4|pages=948–949|doi=10.1021/ja00394a043|issn=0002-7863}}</ref> More recently, γ-silyl elimination of a silylcyclobutyl tosylate has been used to prepare strained bicyclic systems.<ref>{{Cite journal|last=Kelly|first=Christopher B.|last2=Colthart|first2=Allison M.|last3=Constant|first3=Brad D.|last4=Corning|first4=Sean R.|last5=Dubois|first5=Lily N. E.|last6=Genovese|first6=Jacqueline T.|last7=Radziewicz|first7=Julie L.|last8=Sletten|first8=Ellen M.|last9=Whitaker|first9=Katherine R.|date=2011-04-01|title=Enabling the Synthesis of Perfluoroalkyl Bicyclobutanes via 1,3 γ-Silyl Elimination|journal=Organic Letters|volume=13|issue=7|pages=1646–1649|doi=10.1021/ol200121f|pmid=21366262|issn=1523-7060}}</ref>

==History==
Many of the concepts and terminology related to elimination reactions were proposed by [[Christopher Kelk Ingold]] in the 1920s.

== See also ==
* [[E1cB-elimination reaction]]

== References ==
<references />

==External links==
* {{commons category-inline|Elimination reactions}}
* {{wikiquote-inline}}

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[[Category:Elimination reactions| ]]
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