- Elimination reaction
An elimination reaction is a type of
organic reactionin which two substituents are removed from a molecule in either a one or two-step mechanism [JerryMarch] . Either the unsaturation of the molecule increases (as in most organic elimination reactions) or the valence of an atom in the molecule decreases by two, a process known as reductive elimination. An important class of elimination reactions are those involving alkyl halides, or alkanes in general, with good leaving groups, reacting with a Lewis baseto form an alkenein the reverse of an addition reaction. When the substrate is asymmetric, regioselectivityis determined by Zaitsev's rule. The one and two-step mechanisms are named and known as E2 reaction and E1 reaction, respectively.
In the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. E2 stands for bimolecular elimination and has the following specificities.
* It is a one-step process of elimination with a single
* Typical of secondary or tertiary substituted alkyl halides. It is also observable with primary alkyl halides if a hindered base is used.
reaction rate, influenced by both the alkyl halide and the base, is second order.
* Because E2 mechanism results in formation of a Pi bond, the two leaving groups (often a hydrogen and a
halogen) need to be coplanar. An antiperiplanar transition statehas staggeredconformation with lower energy and a synperiplanartransition state is in eclipsedconformation with higher energy. The reaction mechanism involving staggered conformation is more favourable for E2 reactions.
* Reaction often present with strong base.
* In order for the pi bond to be created, the hybridization of carbons need to be lowered from "sp3" to "sp2".
* The C-H bond is weakened in the
rate determining stepand therefore the deuterium isotope effectis larger than 1.
* This reaction type has similarities with the SN2 reaction mechanism.
The reaction fundamental elements are
* Breaking of the "carbon-hydrogen" and "carbon-halogen" bonds in one step.
* Formation of a "C=C Pi bond".
An example of this type of reaction in "scheme 1" is the reaction of isobutylbromide with
potassiumethoxide in ethanol. The reaction products are isobutylene, ethanoland potassium bromide.
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, Carbon-halogen breaks to give a carbocationintermediate.
Deprotonationof the carbocation.
* Typical of tertiary and some secondary substituted alkyl halides.
reaction rateis influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step. Therefore first order kinetics apply.
* Reaction mostly occurs in complete absence of base or presence of only a weak base.
* E1 reactions are in competition with SN1 reactions because they share a common carbocationic intermediate.
deuterium isotope effectis absent.
* No antiperiplanar requirement. An example is the
pyrolysisof a certain sulfonate esterof menthol:
:Only reaction product A results from antiperiplanar elimination, the presence of product B is an indicator for a E1 mechanism ["Pyrolysis of Aryl Sulfonate Esters in the Absence of Solvent: E1 or E2? A Puzzle for the Organic Laboratory" John J. Nash, Marnie A. Leininger, and Kurt Keyes
Journal of Chemical Education• Vol. 85 No. 4 April 2008 552] .
* Accompanied by carbocationic
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. 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.
E2 and E1 elimination final notes
reaction rateis influenced by halogen's reactivity; iodideand bromidebeing favored. Fluoride is not a good leaving group.There is a certain level of competition between elimination reaction and nucleophilic substitution. More precisely, there are competitions between E2 and SN2 and also between E1 and SN1. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when
* basicity increases
* the steric bulk of the base increases for example
In one study [cite journal | title = Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory | author = Stephanie M. Villano, Shuji Kato, and Veronica M. Bierbaum | journal =
J. Am. Chem. Soc.| year = 2006 | volume = 128 | issue = 3 | pages = 736–737 | doi = 10.1021/ja057491d] the kinetic isotope effect(KIE) was determined for the gas phase reaction of several alkyl halides with the chlorateion. In accordance with a E2 elimination the reaction with t-butyl chlorideresults in a KIE of 2.3. The methyl chloridereaction (only SN2 possible) on the other hand has a KIE of 0.85 consistent with a SN2 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.
Specific elimination reactions
E1cB elimination reactionis a special type of elimination reaction involving carbanions. In an addition-elimination reactionelimination takes place after an initial addition reaction and in the Ei mechanismboth substituents leave simultaneously in a syn addition.
In each of these elimination reactions the reactants have specific
dehydration reactionis one where the leaving group is water.
Bamford-Stevens reactionwith a tosyl hydrazone leaving group assisted by alkoxide
Cope reactionwith an amine oxide leaving group
Hofmann eliminationwith quaternary amine leaving group
Chugaev reactionwith a methyl xanthate leaving group
Grieco eliminationwith a selenoxide leaving group
Shapiro reactionwith a tosyl hydrazone leaving group assisted by alkyllithium
Hydrazone iodinationwith a hydrazone leaving group assisted by iodine
Grob fragmentationwith degree of unsaturation increasing in one of the leaving groups.
Kornblum–DeLaMare rearrangement(elimination over a (H)C-O(OR) bond) with an alcohol leaving group forming a ketone
Takai olefinationwith two bulky chromium groups.
* Important publications in organic chemistry
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