All Topics  
Elimination reaction

 

 
   Email Print
   Bookmark   Link
 

 

Elimination reaction
 
 
  Topics Elimination reaction Topic Home

An elimination reaction is a type of organic reaction
Organic reaction

Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions and organic redox reaction....
 in which two substituent
Substituent

In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl is used when naming organic compounds that contain a substituent....
s are removed from a molecule in either a one or two-step mechanism . 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 group
Leaving group

A leaving group in chemistry is an ion or substituent with the ability to detach itself from a molecule. The remaining molecule or fragment remaining is known as the residual or main part....
s, reacting with a Lewis base to form an alkene
Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
 in the reverse of an addition reaction
Addition reaction

An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one....
.






Discussion
Ask a question about 'Elimination reaction'
Start a new discussion about 'Elimination reaction'
Answer questions from other users
Full Discussion Forum



Encyclopedia


An elimination reaction is a type of organic reaction
Organic reaction

Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions and organic redox reaction....
 in which two substituent
Substituent

In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl is used when naming organic compounds that contain a substituent....
s are removed from a molecule in either a one or two-step mechanism . 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 group
Leaving group

A leaving group in chemistry is an ion or substituent with the ability to detach itself from a molecule. The remaining molecule or fragment remaining is known as the residual or main part....
s, reacting with a Lewis base to form an alkene
Alkene

In organic chemistry, an alkene, olefin, or olefine is an Saturation chemical compound containing at least one carbon-to-carbon double bond....
 in the reverse of an addition reaction
Addition reaction

An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one....
. When the substrate is asymmetric, regioselectivity
Regioselectivity

In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions . It can often apply to which of many possible positions a reagent will affect, such as which proton a strong Base will abstract from an organic molecule, or where on a substituted benzene ring a further s...
 is determined by Zaitsev's rule
Zaitsev's rule

In chemistry, Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich Zaitsev is a rule that states that if more than one alkene can be formed by an elimination reaction, the more stable alkene is the major product....
. The one and two-step mechanisms are named and known as E2 reaction and E1 reaction, respectively.

E2 mechanism

In the 1920s, Sir Christopher Ingold
Christopher Kelk Ingold

Sir Christopher Kelk Ingold Fellow of the Royal Society was a United Kingdom chemist based in Leeds and London. His groundbreaking work in the 1920s and 1930s on reaction mechanisms and the electronic structure of organic compounds was responsible for the introduction into mainstream chemistry of concepts such as nucleophile, electrophile,...
 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 transition state
    Transition state

    The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate....
    .
  • Typical of secondary or tertiary substituted alkyl halides. It is also observable with primary alkyl halides if a hindered base is used.
  • The reaction rate
    Reaction rate

    The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
    , 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
    Halogen

    |}The halogens or halogen elements are a chemical series of nonmetal chemical element from Periodic table group International Union of Pure and Applied Chemistry of the periodic table, comprising fluorine, F; chlorine, Cl; bromine, Br; iodine, I; and astatine, At....
    ) need to be coplanar. An antiperiplanar transition state
    Transition state

    The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate....
     has staggered
    Staggered

    A staggered conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 orbital hybridisation atoms as a conformational energy minimum....
     conformation with lower energy and a synperiplanar transition state is in eclipsed
    Eclipsed

    Main article: Alkane stereochemistryIn chemistry an eclipsed conformation is a chemical conformation that exists in any open chain single chemical bond connecting two sp3 orbital hybridisation atoms as a conformational energy maximum....
     conformation with higher energy. The reaction mechanism involving staggered conformation is more favourable for E2 reactions.
  • Reaction often present with strong base
    Base (chemistry)

    In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
    .
  • 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 step and therefore the deuterium isotope effect is larger than 1.
  • This reaction type has similarities with the SN2 reaction
    SN2 reaction

    The SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophile center and covalent bond to it, expelling another group called a leaving group....
     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
    Molecular geometry

    Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its Reactivity , Chemical polarity, Phase , color, magnetism, and biological activity....
    .


An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with potassium
Potassium

Potassium is a chemical element. It has the symbol K , atomic number 19, and atomic mass 39.0983. Potassium was first isolated from potash, hence the name....
 ethoxide
Alkoxide

An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO–, where R is the organic substituent....
 in ethanol
Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatility , flammable, colorless liquid....
. The reaction products are isobutylene
Isobutylene

Isobutylene is a hydrocarbon of significant industrial importance. It is a four-carbon branched alkene , one of the four isomers of butylene. At standard temperature and pressure it is a colorless flammable gas....
, ethanol
Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatility , flammable, colorless liquid....
 and potassium bromide
Potassium bromide

Potassium bromide is a salt , widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries. Its action is due to the bromide ion ....
.

E1 mechanism

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
      Ionization

      Ionization is the physics process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions....
      , Carbon-halogen breaks to give a carbocation
      Carbocation

      A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer Electron shell#Valence shell instead of the eight valence electrons that ensures maximum stability ....
       intermediate.
    • Deprotonation
      Deprotonation

      Deprotonation is a chemistry term that refers to the removal of a proton from a molecule, forming the conjugate base. The relative ability for a molecule to give up a proton is measured by a pKa value....
       of the carbocation.
  • Typical of tertiary and some secondary substituted alkyl halides.
  • The reaction rate
    Reaction rate

    The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
     is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step
    Rate-determining step

    The rate-determining step is a chemistry term for the slowest reaction step in a chemical reaction. The rate-determining step is often compared to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by the speed at which water is poured in....
    . 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 reaction
    SN1 reaction

    The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is molecule....
    s because they share a common carbocationic intermediate.
  • A deuterium isotope effect is absent.
  • No antiperiplanar requirement. An example is the pyrolysis
    Pyrolysis

    Pyrolysis is the chemical decomposition of a condensed substance by heating. The word is coined from the Greek language-derived morphemes pyro "fire" and lysys "decomposition"....
     of a certain sulfonate ester of menthol
    Menthol

    Menthol is an organic compound #Production or obtained from peppermint or other Mentha oils. It is a waxy, crystalline substance, clear or white in color, which is solid at room temperature and melts slightly above....
    :


Only reaction product A results from antiperiplanar elimination, the presence of product B is an indicator for a E1 mechanism .
  • Accompanied by carbocationic rearrangement reaction
    Rearrangement reaction

    A rearrangement reaction is a broad class of organic reactions where the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule ....
    s


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

The reaction rate
Reaction rate

The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
 is influenced by halogen
Halogen

|}The halogens or halogen elements are a chemical series of nonmetal chemical element from Periodic table group International Union of Pure and Applied Chemistry of the periodic table, comprising fluorine, F; chlorine, Cl; bromine, Br; iodine, I; and astatine, At....
's reactivity; iodide
Iodide

An iodide ion is an iodine with a −1 electric charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide....
 and bromide
Bromide

A bromide ion is a bromine atom with electric charge of -1.Compounds with bromine in formal oxidation state -1 are called bromides, and each individual chemical in this class can be called a bromide, as well....
 being favored. Fluoride is not a good leaving group. There is a certain level of competition between elimination reaction and nucleophilic substitution
Nucleophilic substitution

In organic chemistry and inorganic chemistry, nucleophilic substitution is a fundamental class of substitution reaction in which an "electron rich" nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom attached to a group or atom called the leaving group; the positive or partially positive atom...
. More precisely, there are competitions between E2 and SN2
SN2 reaction

The SN2 reaction is a type of nucleophilic substitution, where a lone pair from a nucleophile attacks an electron deficient electrophile center and covalent bond to it, expelling another group called a leaving group....
 and also between E1 and SN1
SN1 reaction

The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is molecule....
. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when
  • steric hindrance increases
  • basicity increases
  • temperature
    Temperature

    In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
     increases
  • the steric bulk of the base increases for example Potassium tert-butoxide
    Potassium tert-butoxide

    Potassium tert-butoxide is the chemical compound with the chemical formula 3COK. This colourless solid is a strong base useful in organic synthesis....
  • the nucleophile
    Nucleophile

    In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
     is poor


In one study the kinetic isotope effect
Kinetic isotope effect

The kinetic isotope effect is a dependence of the reaction rate of a chemical reaction on the isotope of an atom in a reactant. It is also called "isotope fractionation," although this term is somewhat broader in meaning....
 (KIE) was determined for the gas phase reaction of several alkyl halides with the chlorate
Chlorate

The chlorate anion has the formula ClO3-. In this case, the chlorine atom is in the +5 oxidation state. "Chlorate" can also refer to chemical compounds containing this anion; Chlorate#Compounds are the salt s of chloric acid....
 ion. In accordance with a E2 elimination the reaction with t-butyl chloride results in a KIE of 2.3. The methyl chloride reaction (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

The E1cB elimination reaction
E1cB elimination reaction

The E1cB elimination reaction is a special type of elimination reaction in organic chemistry. This reaction mechanism explains the formation of alkenes from alkyl halides through a carbanion intermediate given specified reaction conditions and specified substrates....
 is a special type of elimination reaction involving carbanions. In an addition-elimination reaction elimination takes place after an initial addition reaction and in the Ei mechanism
Ei mechanism

Ei elimination in organic chemistry is a special type of elimination reaction in which two Vicinal substituents on an alkane framework leave simultaneously in a single step to form an alkene in a syn elimination ....
 both substituents leave simultaneously in a syn addition.

In each of these elimination reactions the reactants have specific leaving group
Leaving group

A leaving group in chemistry is an ion or substituent with the ability to detach itself from a molecule. The remaining molecule or fragment remaining is known as the residual or main part....
s:
  • the dehydration reaction
    Dehydration reaction

    In chemistry, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule. Dehydration reactions are a subset of elimination reactions....
     is one where the leaving group is water.
  • the Bamford-Stevens reaction
    Bamford-Stevens reaction

    The Bamford-Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens ....
     with a tosyl hydrazone leaving group assisted by alkoxide
  • the Cope reaction
    Cope reaction

    The Cope reaction or Cope elimination, developed by Arthur C. Cope, is an elimination reaction of an amine oxide to form an alkene and a hydroxyl amine....
     with an amine oxide leaving group
  • the Hofmann elimination
    Hofmann elimination

    Hofmann elimination is a process where an amine is reacted to create a tertiary amine and an alkene by treatment with excess methyl iodide followed by treatment with silver oxide, water, and heat ....
     with quaternary amine leaving group
  • the Chugaev reaction with a methyl xanthate leaving group
  • the Grieco elimination
    Grieco elimination

    The Grieco elimination is an organic reaction describing the elimination reaction of an aliphatic primary alcohol through a selenide to a terminal alkene ....
     with a selenoxide leaving group
  • the Shapiro reaction
    Shapiro reaction

    The Shapiro reaction or tosylhydrazone decomposition is an organic reaction in which a ketone or aldehyde is converted to an alkene through an intermediate hydrazone in the presence of 2 equivalents of strong base ....
     with a tosyl hydrazone leaving group assisted by alkyllithium
  • Hydrazone iodination
    Hydrazone iodination

    Hydrazone iodination is an organic reaction in which a hydrazone is converted in a vinyl halide by reaction of iodine and a non-nucleophilic base such as DBU ...
     with a hydrazone leaving group assisted by iodine
  • A Grob fragmentation
    Grob fragmentation

    A Grob fragmentation, named for the British chemist Cyril A. Grob, is an elimination reaction taking place when an electrofuge and nucleofuge are situated in positions 1 and 3 on an aliphatic chain....
     with degree of unsaturation increasing in one of the leaving groups.
  • the Kornblum–DeLaMare rearrangement
    Kornblum–DeLaMare rearrangement

    The Kornblum?DeLaMare rearrangement is a rearrangement reaction in organic chemistry in which a primary or secondary organic peroxide is converted to the corresponding ketone and alcohol under base catalysis....
     (elimination over a (H)C-O(OR) bond) with an alcohol leaving group forming a ketone
  • the Takai olefination
    Takai olefination

    Takai olefination in organic chemistry describes the organic reaction of an aldehyde with a diorganochromium to form an alkene. In the original 1986 publication the aldehyde is benzaldehyde and the organochromium species is generated from iodoform or bromoform and an excess of chromium chloride....
     with two bulky chromium groups.


See also

  • Important publications in organic chemistry
    List of publications in chemistry

    This is a list of important publications in chemistry, organized by field.Some reasons why a particular publication might be regarded as important:...