Chemistry
Elimination Reaction
An elimination reaction is a chemical reaction in which a molecule loses atoms or functional groups to form a double bond or a ring. It typically involves the removal of two substituents from a molecule, resulting in the formation of a new unsaturated compound. This process often occurs in the presence of a base or acid catalyst.
Written by Perlego with AI-assistance
Related key terms
1 of 5
10 Key excerpts on "Elimination Reaction"
eBook - ePub- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
3 Elimination ReactionsRemoval of two atoms or groups from a molecule is called elimination. In a substitution reaction, the leaving group departs from the molecule with the bonding electrons and the nucleophile attacks a carbon atom in the substrate. In Elimination Reactions, one of the groups leaves the molecule with the bonding electrons, while the nucleophile acts as a base and removes a proton from the adjacent carbon atom. A double bond is formed between two carbon atoms, from which the leaving group and proton are eliminated. We can divide Elimination Reactions into three subgroups depending on the location of the leaving groups.- α-Elimination: When two atoms or groups are eliminated from a single atom of the substrate, such type of Elimination Reaction is called α-elimination, 1,1-elimination, or geminal elimination.
Because both groups leaving the molecule are bonded to the same carbon atom, a new bond will not be formed as a result of α-elimination. An electron pair remains on the atom from which the groups are eliminated. The product is a carbene when the groups are eliminated from a single carbon atom. The carbon atom is a divalent carbon.- β-Elimination: When the atoms or groups are eliminated from the adjacent atoms, it is called β-elimination, 1,2-elimination, or vicinal elimination. A double bond is formed between the atoms from which the groups or atoms are eliminated.
- γ- and Higher eliminations: There is a third type of elimination, which is called γ-elimination or 1,3-elimination, in which a three-membered ring is formed. Of course, higher cyclic systems can also be created depending on the position of the leaving groups in the molecule.
In this section, we will only discuss 1,2-elimination (β-elimination) reactions because they are one of the most important methods applied to generate a double bond as alkenes are essential industrial compounds. The application area of these reactions is extensive and they are also mechanistically very important.
eBook - PDF- Metin Balcı(Author)
- 2021(Publication Date)
- Wiley-VCH(Publisher)
87 3 Elimination Reactions Removal of two atoms or groups from a molecule is called elimination. In a substitution reaction, the leaving group departs from the molecule with the bonding electrons and the nucleophile attacks a carbon atom in the substrate. In Elimination Reactions, one of the groups leaves the molecule with the bonding electrons, while the nucleophile acts as a base and removes a proton from the adjacent carbon atom. A double bond is formed between two carbon atoms, from which the leaving group and proton are eliminated. We can divide Elimination Reactions into three subgroups depending on the location of the leaving groups. -Elimination: When two atoms or groups are eliminated from a single atom of the substrate, such type of Elimination Reaction is called -elimination, 1,1-elimination, or geminal elimination. C C Br R R H R′ R′ NaOH + + H OH NaBr Carbene Because both groups leaving the molecule are bonded to the same carbon atom, a new bond will not be formed as a result of α-elimination. An electron pair remains on the atom from which the groups are eliminated. The product is a carbene when the groups are eliminated from a single carbon atom. The carbon atom is a divalent carbon. -Elimination: When the atoms or groups are eliminated from the adjacent atoms, it is called -elimination, 1,2-elimination, or vicinal elimination. A double bond is formed between the atoms from which the groups or atoms are eliminated. C C R Br R H R′ R′ NaOH C C R R R′ R′ + H OH + NaBr - and Higher eliminations: There is a third type of elimination, which is called -elimination or 1,3-elimination, in which a three-membered ring is formed. Of course, higher cyclic systems can also be created depending on the position of the leaving groups in the molecule.
eBook - PDFOrganic Chemistry
A Mechanistic Approach
- Penny Chaloner(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
381 10.1 INTRODUCTION Elimination Reactions are those in which we remove two atoms or groups from a molecule to generate a multiple bond. Most of the examples we will discuss involve making a carbon–carbon double bond, but we will also make some triple bonds and a few carbon–heteroatom bonds. We previously saw elimination processes as a “side reaction” of substitution—now, we turn the tables and see substitution as a side reaction of a desired elimination. Although we have good method- ologies for inducing reactions to go in one direction or the other, we should recognize that there may always be some competition. In this chapter, we will concentrate initially on eliminations to give alkenes, turning later to alkynes, and multiple bonds involving heteroatoms. 10.2 MECHANISMS As with substitution, we have two mechanisms that are relatively common and one that is much rarer. The two common mechanisms, E1 and E2, have strong similarities to the S N 1 and S N 2 pro- cesses described in the previous chapter and are generally favored by similar conditions to these analogues. The third mechanism, E1cB, is rather different, with no direct analogy in substitution chemistry. We will initially exemplify all the reactions by removal of hydrogen halides, HX, in the presence of base and then explore the scope of the reactions. 10.2.1 E1 ELIMINATION, UNIMOLECULAR This mechanism (Figure 10.1) has strong similarities to the S N 1 process—indeed, the first RDS is identical. A leaving group is lost, taking the electrons from the bond being broken with it, in a slow step, to give a carbocation. However, the next step is not capture of the carbocation by a nucleophile but a rapid loss of a proton to give an alkene. The observed kinetics of the reaction are first order, with the rate proportional only to the concentration of the alkyl halide, not to any added base. The RDS is unimolecular, hence the name E1. The analogy to S N 1 is clear.
eBook - PDF- John A. Olmsted, Gregory M. Williams, Robert C. Burk(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
However, the number of distinctly different types of organic reactions is surprisingly small. In this chapter, we will study three important types of reactions; namely, substitution, elimination, and addition reactions. In a substitution reaction, as the name suggests, one functional group is substituted for another. An example is shown below, where a nitro group substitutes for a hydrogen atom on a benzene ring: HNO 3 NO 2 H 2 O H 2 SO 4 + + In an Elimination Reaction, atoms or groups of atoms that are bound to adjacent carbon atoms are eliminated, generally as a small molecule. This results in the formation of a double bond between the carbon atoms. For example, ethanol can undergo a reaction to form ethene with the elimination of water: H OH H H H H H H H H H 2 O + And finally, in an addition reaction, a molecule is added across a double (or triple) bond, resulting in a single (or double) bond. An example is the chlorination of ethene to make 1,2-dichloroethane: Cl Cl H H H H H H H H Cl 2 + Chemical Space—How Many Possible Drug Compounds Are There? New drug molecules have been synthesized and tested by the thou- sands over the years. However, recent estimates suggest that only a tiny fraction of the potential medicines that could be made have been synthesized so far. Some estimates suggest that there are as many as 10 60 potentially interesting small molecules that we have yet to synthesize or test. This staggering number is not too different from estimates of the number of atoms in the universe—how can we possibly decide which compounds to spend time and effort on? Combinatorial chemistry involves the automated synthesis of huge libraries of different but related compounds. Pharmaceu- tical companies in particular have used robotic approaches to syn- thesize hundreds of thousands of new and unique compounds per year.
eBook - PDF- David R. Klein(Author)
- 2020(Publication Date)
- Wiley(Publisher)
Throughout the chapters of this text, we will add many more reactions to our list of “func- tional group interconversions,” and we will also learn many reactions that involve formation of new carbon-carbon bonds. As our “toolbox” of reactions gets larger, we will be able to build molecules of increasing complexity. FIGURE 8.32 Substitution and Elimination Reactions involving tertiary substrates. 8.14 Substitution and Elimination Reaction with Other Substrates 377 In order to propose a synthesis, it is certainly important to know what reactions are available, but more importantly, you must know how and when to use those reactions. When proposing a synthesis (even a one-step synthesis), it is generally helpful to think backwards. To illustrate what this means, consider the following S N 2 reaction, in which an alkyl halide is converted into an ether: An ether An alkyl halide Br O NaOEt This reaction can also be viewed backwards, like this: An ether An alkyl halide Br NaOEt can be made from + An alkoxide ion O This figure represents a retrosynthetic analysis, in which the product is shown first, followed by reagents that can be used to make that product. The wavy line, also called a disconnection, identifies the bond that can be made by the reaction. The special arrow indicates a retrosynthesis, which means that we are thinking about this reaction backwards. Planning a retrosynthesis requires that we identify a suitable nucleophile and electrophile that will react with each other to give the target molecule (the desired product). In the case above, the target molecule is an ether, which has two C O bonds. The retrosynthe- sis above shows disconnection of one C O bond, while the retrosynthesis below shows disconnec- tion of the other C O bond: An ether An alkyl halide can be made from An alkoxide ion O ONa Br + This example illustrates how a retrosynthetic analysis can reveal more than one way to make a target molecule.
eBook - PDFOrganic Reaction Mechanisms 2016
An annual survey covering the literature dated January to December 2016
- A. C. Knipe(Author)
- 2019(Publication Date)
- Wiley(Publisher)
CHAPTER 9 Elimination Reactions M. L. Birsa Faculty of Chemistry, ‘Al. I. Cuza’ University of Iasi, Iasi, Romania E1cB and E2 Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 Solvolytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 Pyrolytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 Cycloreversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 Acid Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Oxygen Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 Elimination Reactions in Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 452 Other Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 461 E1cB and E2 Mechanisms The synthesis of ketone-derived enamides by elimination of HCN from cyanoamides has been reported. 1 An E1cB mechanism consistent with the Z configuration of the de resulting enamide has been proposed. The E2 mechanism has been proposed for the dehydrochlorination of 2,2-diaryl-1,1,1-trichloroethanes with nitrite ion, leading to 2,2-diaryl-1,1-dichloroethenes, on the basis of experimental kinetic study and quantum chemical simulation. 2 The Elimination Reactions of (E)-2,4,6-trinitrobenzaldehyde O-benzoyloximes pro- moted by R 2 NH/R 2 NH 2 + in 70 mol% MeCN (aq) have been investigated. 3 The reaction proceeded via a cyclic transition state, which is insensitive to the reactant structure variations and favours the E1cB irr mechanism. A regioselective approach to trifluoromethylated diarylethanes and ethenes has been described.
eBook - PDF- David R. Klein(Author)
- 2021(Publication Date)
- Wiley(Publisher)
• Bimolecular Elimination Reactions are called E2 reactions. SECTION 7.6 • A cis alkene will generally be less stable than its stereoiso- meric trans alkene. This can be verified by comparing heats of combustion for isomeric alkenes. • A trans π bond cannot be incorporated into a small ring. When applied to bicyclic systems, this rule is called Bredt’s rule, which states that it is not possible for a bridgehead car- bon of a bicyclic system to possess a CC double bond if it involves a trans π bond being incorporated in a small ring. SECTION 7.7 • E2 reactions are regioselective and generally favor the more substituted alkene, called the Zaitsev product. • When both the substrate and the base are sterically hindered, an E2 reaction can favor the less substituted alkene, called the Hofmann product. • If the β position has two different protons, the resulting E2 reaction can be stereoselective, because the trans isomer will be favored over the cis isomer (when applicable). • If the β position has only one proton, an E2 reaction is said to be stereospecific, because the proton and the leaving group must be anti-periplanar to one another. SECTION 7.8 • When a tertiary alkyl halide is dissolved in a polar solvent that is both a weak base and a weak nucleophile (such as ethanol, EtOH), substitution and elimination products are both observed. • Unimolecular nucleophilic substitution reactions are called S N 1 reactions. An S N 1 mechanism is comprised of two core steps: 1) loss of a leaving group to give a carbocation intermediate; and 2) nucleophilic attack. • When a solvent molecule functions as the attacking nucleo- phile, the resulting S N 1 process is called solvolysis. • Unimolecular Elimination Reactions are called E1 reactions. • S N 1 processes are favored by polar protic solvents. • S N 1 and E1 processes are observed for tertiary alkyl halides, as well as allylic and benzylic halides.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
Elimination is highly favored, especially when the reaction is carried out at higher temperatures. Any substitution that occurs must take place through an S N 1 mechanism: Without Heating + Tertiary O Br EtOH, 25 °C (room temp.) EtONa S N 1 Minor (9%) E2 Major (91%) With Heating E2 E1 Only (100%) Tertiary Br EtOH, 55 °C EtONa Temperature Increasing the reaction temperature favors elimination (E1 and E2) over substitution. Elimination Reactions have greater free energies of activation than substitution reactions because more bonding changes occur during elimination. When higher tempera- ture is used, the proportion of molecules able to surmount the energy of activation barrier for elimination increases more than the proportion of molecules able to undergo substitution, although the rate of both substitution and elimination will be increased. Furthermore, elim- ination reactions are entropically favored over substitution because the products of an elimi- nation reaction are greater in number than the reactants. Additionally, because temperature is the coefficient of the entropy term in the Gibbs free-energy equation ∆G ° = ∆H ° − T ∆S °, an increase in temperature further enhances the entropy effect. 308 CHAPTER 7 Alkenes and Alkynes I Size of the Base/Nucleophile Increasing the reaction temperature is one way of favorably influencing an Elimination Reaction of an alkyl halide. Another way is to use a strong sterically hindered base such as the tert-butoxide ion. The bulky methyl groups of the tert-butoxide ion inhibit its reaction by substitution, allowing Elimination Reactions to take precedence. We can see an example of this effect in the following two reactions. The relatively unhindered methoxide ion reacts with octadecyl bromide primarily by substitution, whereas the bulky tert-butoxide ion gives mainly elimination.
eBook - PDFOrganic Reaction Mechanisms 1990
An annual survey covering the literature dated December 1989 to November 1990
- A. C. Knipe, W. E. Watts, A. C. Knipe, W. E. Watts(Authors)
- 2008(Publication Date)
- Wiley(Publisher)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Pyrolytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Alkyl Halides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 376 Esters, Lactones, and Related Substrates . . . . . . . . . . . . . . . . . . . . . . . 377 Other Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379 Ring-opening and Ring-closure Reactions . . . . . . . . . . . . . . . . . . . . . . . . 379 Fragmentations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382 Dehydration Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Micellar and Phase-transfer Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . 385 Enzyme-catalysed Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386 Elimination Reactions in Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 Retro-cycloaddition and Electrocyclic Ring-opening Reactions . . . . . . . . . . . . 387 Other Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 391 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Reviews Only two reviews dealing with Elimination Reactions have been published during the year. The title of the first is ‘Mechanisms of base-catalysed alkene-forming eliminations’ and it has 323 references.’ The other covers gas-phase ion-molecule reactions as studied by Fourier transform ion cyclotron resonance.’ E2 and ElcB Mechanisms Mechanistic studies of base-promoted Elimination Reactions of 2-arylethyl deriva- tives have now been extended by an investigation of elimination from 2-(2,4,6- trinitropheny1)ethyl halides (1) in water.3 The bases employed were HO-, Organic Reaction Mechanisms 1990. Edited by A. C. Knipe and W. E. Watts 0 1992 John Wiley & Sons Ltd 367
eBook - PDFOrganic Reaction Mechanisms 2014
An annual survey covering the literature dated January to December 2014
- A. C. Knipe(Author)
- 2017(Publication Date)
- Wiley(Publisher)
CHAPTER 9 Elimination Reactions M. L. Birsa Faculty of Chemistry, “Al. I. Cuza” University of Iasi, Iasi, Romania E1cB and E2 Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 Pyrolytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 Acid Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424 Halogen Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Oxygen Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426 Other Pyrolytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 Elimination Reactions in Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . 427 Other Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433 E1cB and E2 Mechanisms Density functional theory (DFT) and a mixed implicit/explicit solvation model have indicated that base-catalysed dehydration of benzene cis-1,2-dihydrodiols takes place by producing an aromatic product and by-products potentially stabilized by hyperaromaticity. 1 Experiments show unusual shifts in isotope effects, indicating an uncommon mechanistic balance on the E2–E1cB continuum. The computational data help unravel hidden by-products in the reaction coordinate and provide a novel conceptual framework for distinguishing between competing pathways in this and any other system with borderline reaction mechanisms. Ketene-forming elimination from 2-X-4-nitrophenyl furylacetates promoted by R 2 NH–R 2 NH + 2 in 70 mol% aqueous MeCN has been studied kinetically. 2 When X = Cl and NO 2 , the reactions exhibited second-order kinetics, the Brönsted decreased with a poor leaving group, and | lg| increased with a weak base.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
