An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism ^[1]. Organic reactions are Chemical reactions involving Organic compounds The basic Organic chemistry reaction types are Addition reactions Elimination 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 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. In Organic chemistry, an unsaturated compound is a Chemical compound that contains carbon - carbon Pi bonds such as an Alkene or an Alkyne Oxidative addition and reductive elimination are two important classes of reactions in Organometallic chemistry. An important class of elimination reactions are those involving alkyl halides, or alkanes in general, with good leaving groups, reacting with a Lewis base to form an alkene in the reverse of an addition reaction. The haloalkanes (also known as halogenoalkanes or alkyl halides) are a group of Chemical compounds consisting of Alkanes such as Methane A leaving group is an Atom or group of atoms that detaches from a chemical substance In Organic chemistry, an alkene, olefin, or olefine is an unsaturated Chemical compound containing at least one Carbon An addition reaction, in 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 is determined by Zaitsev's rule. In Chemistry, regioselectivity is the preference of one direction of Chemical bond making or breaking over all other possible directions. In Chemistry, Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich Zaitsev (number of different spellings due The one and two-step mechanisms are named and known as E2 reaction and E1 reaction, respectively.

Contents 1 E2 mechanism 2 E1 mechanism 3 E2 and E1 elimination final notes 4 Specific elimination reactions 5 See also 6 References

E2 mechanism

In the 1920s, Sir Christopher Ingold proposed a model to explain a peculiar type of chemical reaction: the E2 mechanism. Sir Christopher Kelk Ingold ( October 28 1893 - December 8 1970, Edgware) was a British Chemist based in Leeds E2 stands for bimolecular elimination and has the following specificities.

• It is a one-step process of elimination with a single transition state. The transition state of a Chemical reaction is a particular configuration along the 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, influenced by both the alkyl halide and the base, is second order. The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes
• Because E2 mechanism results in formation of a Pi bond, the two leaving groups (often a hydrogen and a halogen) need to be coplanar. Abundance Owing to their high Reactivity, the halogens are found in the environment only in compounds or as Ions Halide ions and oxoanions In Geometry, a set of points in space is coplanar if the points all lie in the same geometric plane. An antiperiplanar transition state has staggered conformation with lower energy and a synperiplanar transition state is in eclipsed conformation with higher energy. Alkane stereochemistry concerns the Stereochemistry of linear Alkanes and the linear alkane Conformers The existence of more than one conformation is due The transition state of a Chemical reaction is a particular configuration along the Reaction coordinate. A staggered conformation is a Chemical conformation that exists in any open chain single Chemical bond connecting two sp3 hybridised atoms as Alkane stereochemistry concerns the Stereochemistry of linear Alkanes and the linear alkane Conformers The existence of more than one conformation is due For other possible meanings of the terms "eclipse" and "eclipsed" see eclipse. The reaction mechanism involving staggered conformation is more favourable for E2 reactions.
• Reaction often present with strong base. In Chemistry, a base is most commonly thought of as an aqueous substance that can accept Protons This refers to the Brønsted-Lowry theory of acids and
• In order for the pi bond to be created, the hybridization of carbons need to be lowered from sp³ to sp². -->In Chemistry
• The C-H bond is weakened in the rate determining step and therefore the deuterium isotope effect is larger than 1. The rate-determining step (RDS is a Chemistry term for the slowest step in a Chemical reaction. The kinetic isotope effect ( KIE) is a variation in the Reaction rate of a Chemical reaction when an Atom in one of the reactants is replaced
• This reaction type has similarities with the S_N2 reaction mechanism. The SN2 reaction (also known as bimolecular nucleophilic substitution) is a type of Nucleophilic substitution, where a lone pair from a Nucleophile

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 or molecular structure is the three- Dimensional arrangement of the Atoms that constitute a Molecule.

An example of this type of reaction in scheme 1 is the reaction of isobutylbromide with potassium ethoxide in ethanol. In Organic chemistry, butyl is a four- Carbon Alkyl substituent with Chemical formula -C4H9. Potassium (pəˈtæsiəm is a Chemical element. It has the symbol K (kalium from qalīy Atomic number 19 and Atomic mass 39 An alkoxide is the Conjugate base of an Alcohol and therefore consists of an organic group bonded to a negatively charged Oxygen atom The reaction products are isobutylene, ethanol and potassium bromide. Isobutylene (or 2-methylpropene is a Hydrocarbon of significant industrial importance Potassium bromide ( K[[Bromine Br]] is a salt, widely used as an Anticonvulsant and a Sedative in the late 19th and early 20th centuries

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, Carbon-halogen breaks to give a carbocation intermediate. Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons A carbocation (ˌkɑrboʊˈkætaɪɒn is an Ion with a positively-charged Carbon Atom.
  • Deprotonation of the carbocation. Deprotonation is a Chemistry term that refers to the removal of a Proton ( Hydrogen Cation H+ from a Molecule, forming
• Typical of tertiary and some secondary substituted alkyl halides. A carbon-carbon bond is a Covalent bond between two Carbon Atoms.
• The reaction rate is influenced only by the concentration of the alkyl halide because carbocation formation is the slowest, rate-determining step. The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes The rate-determining step (RDS is a Chemistry term for the slowest step in a Chemical reaction. Therefore first order kinetics apply. The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes
• Reaction mostly occurs in complete absence of base or presence of only a weak base.
• E1 reactions are in competition with S_N1 reactions because they share a common carbocationic intermediate. The SN1 reaction is a Substitution reaction in Organic chemistry.
• A deuterium isotope effect is absent. The kinetic isotope effect ( KIE) is a variation in the Reaction rate of a Chemical reaction when an Atom in one of the reactants is replaced
• No antiperiplanar requirement. An example is the pyrolysis of a certain sulfonate ester of menthol:

Only reaction product A results from antiperiplanar elimination, the presence of product B is an indicator for a E1 mechanism ^[2]. Pyrolysis is the Chemical decomposition of organic materials by heating in the absence of Oxygen or any other reagents except possibly Steam A sulfonate ion is an ion that contains the -S(=O2-O&minus Functional group. Menthol is an Organic compound made synthetically or obtained from Peppermint or other mint oils

• Accompanied by carbocationic rearrangement reactions

An example in scheme 2 is the reaction of tert-butylbromide with potassium ethoxide in ethanol. 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

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 S_N1 and E1 pathways are competing, the E1 pathway can be favored by increasing the heat.

E2 and E1 elimination final notes

The reaction rate is influenced by halogen's reactivity; iodide and bromide being favored. The reaction rate or rate of reaction for a Reactant or product in a particular reaction is intuitively defined as how fast a reaction takes Abundance Owing to their high Reactivity, the halogens are found in the environment only in compounds or as Ions Halide ions and oxoanions An iodide Ion is an iodine atom with a &minus1 charge. Compounds with iodine in formal Oxidation state &minus1 are called iodides A bromide Ion is a Bromine atom with charge of −1 Compounds with bromine in formal Oxidation state −1 are called bromides Fluoride is not a good leaving group. There is a certain level of competition between elimination reaction and nucleophilic substitution. In organic and Inorganic chemistry, nucleophilic substitution is a fundamental class of Substitution reaction in which an "electron rich" More precisely, there are competitions between E2 and S_N2 and also between E1 and S_N1. The SN2 reaction (also known as bimolecular nucleophilic substitution) is a type of Nucleophilic substitution, where a lone pair from a Nucleophile The SN1 reaction is a Substitution reaction in Organic chemistry. Substitution generally predominates and elimination occurs only during precise circumstances. Generally, elimination is favored over substitution when

• steric hindrance increases
• basicity increases
• temperature increases
• the steric bulk of the base increases for example Potassium tert-butoxide
• the nucleophile is poor

In one study ^[3] the kinetic isotope effect (KIE) was determined for the gas phase reaction of several alkyl halides with the chlorate ion. See also Intramolecular forces ' Steric effects arise from the fact that each Atom within a Molecule occupies a certain Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature An alkoxide is the Conjugate base of an Alcohol and therefore consists of an organic group bonded to a negatively charged Oxygen atom In Chemistry, a nucleophile (literally nucleus lover as in nucleus and phile) is a Reagent that forms a Chemical bond to The kinetic isotope effect ( KIE) is a variation in the Reaction rate of a Chemical reaction when an Atom in one of the reactants is replaced The chlorate anion has the formula ClO3− In this case the Chlorine atom is in the +5 Oxidation state. In accordance with a E2 elimination the reaction with t-butyl chloride results in a KIE of 2. tert-Butyl chloride is a colorless liquid Organic compound at Room temperature. 3. The methyl chloride reaction (only S_N2 possible) on the other hand has a KIE of 0. Chloromethane, also called Methyl chloride, R-40 or HCC 40 is a Chemical compound of the group of Organic compounds called Haloalkanes. 85 consistent with a S_N2 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 is a special type of elimination reaction involving carbanions. E1cB elimination reaction is a special type of Elimination reaction in Organic chemistry. In an addition-elimination reaction elimination takes place after an initial addition reaction and in the Ei mechanism both substituents leave simultaneously in a syn addition. An addition reaction, in Chemistry, is in its simplest terms an Organic reaction where two or more molecules combine to form a larger one Ei elimination (Elimination Internal in Organic chemistry is a special type of Elimination reaction in which two vicinal Substituents on an

In each of these elimination reactions the reactants have specific leaving groups:

• the dehydration reaction is one where the leaving group is water. A leaving group is an Atom or group of atoms that detaches from a chemical substance In Chemistry, a dehydration reaction is usually defined as a chemical reaction that involves the loss of water from the reacting molecule
• the Bamford-Stevens reaction with a tosyl hydrazone leaving group assisted by alkoxide
• the Cope reaction with an amine oxide leaving group
• the Hofmann elimination with quaternary amine leaving group
• the Chugaev reaction with a methyl xanthate leaving group
• the Grieco elimination with a selenoxide leaving group
• the Shapiro reaction with a tosyl hydrazone leaving group assisted by alkyllithium
• Hydrazone iodination with a hydrazone leaving group assisted by iodine
• A Grob fragmentation with degree of unsaturation increasing in one of the leaving groups. The Bamford-Stevens reaction is a Chemical reaction whereby treatment of Tosylhydrazones with strong base gives Alkenes It is named for the British chemist The Cope reaction or Cope elimination, developed by Arthur C Cope, is an Elimination reaction of an Amine oxide to form an Alkene Hofmann elimination (also known as exhaustive methylation) is a process where an Amine is reacted to create a tertiary amine and an Alkene by treatment The Chugaev elimination is a chemical reaction that involves the elimination of water from primary Alcohols to produce terminal Alkenes The intermediate is a Xanthate The Grieco elimination is an Organic reaction describing the Elimination reaction of an aliphatic primary Alcohol through a Selenide to a terminal The Shapiro reaction or tosylhydrazone decomposition is an Organic reaction in which a Ketone or Aldehyde is converted to an Alkene Hydrazone iodination is an Organic reaction in which a Hydrazone is converted in a vinyl iodide by reaction of Iodine and a non-nucleophilic A Grob fragmentation, named for the British chemist Cyril A Grob, is an Elimination reaction taking place when an Electrofuge and Nucleofuge
• the Kornblum–DeLaMare rearrangement (elimination over a (H)C-O(OR) bond) with an alcohol leaving group forming a ketone
• the Takai olefination with two bulky chromium groups. The Kornblum–DeLaMare rearrangement is a Rearrangement reaction in Organic chemistry in which a primary or secondary Organic peroxide is converted to Takai olefination in Organic chemistry describes the Organic reaction of an Aldehyde with a diorganochromium compound to form an Alkene

See also

• Important publications in organic chemistry

References

1. ^ March, Jerry (1985). Some criteria for editing this page have been debated and are displayed Advanced Organic Chemistry, Reactions, Mechanisms and Structure, third Edition, John Wiley & Sons. ISBN 0-471-85472-7.  
2. ^ 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. The Journal of Chemical Education ( JCE) is a monthly subscription-only periodical available in both print and electronic versions 85 No. 4 April 2008 552
3. ^ Stephanie M. Villano, Shuji Kato, and Veronica M. Bierbaum (2006). "Deuterium Kinetic Isotope Effects in Gas-Phase SN2 and E2 Reactions: Comparison of Experiment and Theory". J. Am. Chem. Soc. 128 (3): 736 - 737. The Journal of the American Chemical Society (usually abbreviated as J doi:10.1021/ja057491d. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  

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