E2 Elimination

11 Key excerpts on "E2 Elimination"

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  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  Therefore, the reaction is a second-order reaction. E2 reactions are also second-order reactions and the transition state is formed between the substrate and the base. Like the S N 2 reaction, the E2 reaction also proceeds in one step with one transition state and without the formation of any intermediates. The bond-breaking and bond forming that occur during E2 reactions are shown by the curved arrows. C C X H Base C C + H + X E2 Elimination mechanism β α Base In a bimolecular elimination reaction, the abstraction of a proton from a carbon atom next to the carbon bearing the leaving group, cleavage of the C—X bond, and the formation of a double bond occur in a concerted manner in a single step. The E2 Eliminations are observed in systems that cannot produce stable carbocations. Primary alkyl compounds are transformed into alkenes via E2 mechanism. There are remarkable similarities between E2 and S N 2 reactions, as between E1 and S N 1 reactions. Because the first attack comes from the base in E2 reactions, it is essential to have a strong base to abstract the proton. While the base abstracts the proton, the bonding electrons that it shared with carbon move toward the adjacent carbon atom bearing the leaving group. The electrons attack the carbon atom from the back as a nucleophile and depart the leaving group. When the reaction is completed, the electrons that were originally bonded to the hydrogen atom form the π bond. Like the S N 2 reaction, an E2 reaction requires a precise geometry. According to the mechanism given above, the four reacting atoms, i.e. the hydrogen to be eliminated, two carbon atoms to which the hydrogen and the leaving group are bonded, and the leaving group, must lie in a plane. There are two conformations in which the C—H bond and the C—X bond can be in the same plane. In an arrangement like that, the orbitals align appropriately for the formation of a π bond.

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  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  N 2 reaction, the E2 reaction also proceeds in one step with one transition state and without the formation of any intermediates. The bond-breaking and bond forming that occur during E2 reactions are shown by the curved arrows.

  In a bimolecular elimination reaction, the abstraction of a proton from a carbon atom next to the carbon bearing the leaving group, cleavage of the C—X bond, and the formation of a double bond occur in a concerted manner in a single step. The E2 Eliminations are observed in systems that cannot produce stable carbocations. Primary alkyl compounds are transformed into alkenes via E2 mechanism. There are remarkable similarities between E2 and SN 2 reactions, as between E1 and SN 1 reactions. Because the first attack comes from the base in E2 reactions, it is essential to have a strong base to abstract the proton. While the base abstracts the proton, the bonding electrons that it shared with carbon move toward the adjacent carbon atom bearing the leaving group. The electrons attack the carbon atom from the back as a nucleophile and depart the leaving group. When the reaction is completed, the electrons that were originally bonded to the hydrogen atom form the π bond.

  Like the SN 2 reaction, an E2 reaction requires a precise geometry. According to the mechanism given above, the four reacting atoms, i.e. the hydrogen to be eliminated, two carbon atoms to which the hydrogen and the leaving group are bonded, and the leaving group, must lie in a plane. There are two conformations in which the C—H bond and the C—X bond can be in the same plane. In an arrangement like that, the orbitals align appropriately for the formation of a π bond. The proton and the leaving group can be in either an anti-periplanar or syn-periplanar conformation. If the elimination proceeds from the anti-periplanar conformation, it is called anti- or trans-elimination. If it proceeds from the syn-periplanar conformation, it is called cis- or syn-elimination. Of these possible two conformations, the anti-periplanar conformation is the more commonly encountered in E2 reactions. In the transition state, the base is far away from the leaving group. The transition state for syn-periplanar elimination has higher energy resulting from the eclipsed interaction between the substituents. Furthermore, the base must approach close to the leaving group, and there will be strong repulsion between the leaving group and the base. However, some cyclic compounds have rigid geometry. Such compounds can undergo an E2 Elimination reaction by a concerted syn-periplanar mechanism. In acyclic systems, because the carbon–carbon bonds rotate freely, the molecule can always adapt the necessary conformation. However, it is not still possible to have an ideal trans

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  Organic Mechanisms

  Reactions, Methodology, and Biological Applications

  • Xiaoping Sun(Author)
  • 2020(Publication Date)
  • Wiley

    (Publisher)

  7 ELIMINATIONS

  Many useful alkenes can be effectively and efficiently prepared by elimination reactions of various functionalized organic compounds, typically haloalkanes (alkyl halides), as generalized in Equation 7.1 .

  (7.1)

  This general elimination (Eq. 7.1 ) occurs on two adjacent carbons. When an H–LG unit (X, Y = H, LG) is eliminated (β‐elimination), the reaction can follow an anti‐elimination mechanism (with a staggered conformation of the substrate and –H and –LG being anti‐coplanar) or a syn‐elimination mechanism (with an eclipsed conformation of the substrate and –H and –LG being syn‐coplanar). The anti‐elimination of H–LG is bimolecular (E2 reaction) and usually requires a strong base to initiate the reaction. Very often, the E2 reaction is best represented by the bimolecular dehydrohalogenation (elimination of hydrogen halide from two adjacent carbons) of haloalkanes RX (X = Cl, Br, or I). The elimination of H–LG can also follow a stepwise mechanism via a carbocation intermediate (E1 reaction). The E1 and E2 reactions are the most important types of elimination reactions. Their mechanisms, regiochemistry, and stereochemistry are discussed in this chapter. The analysis of E2 pathway by symmetry rules and molecular orbital theory are presented on the basis of recent research. Energetics of some E1 reactions, such as acid‐catalyzed dehydration of secondary and tertiary alcohols, is discussed by the aid of Bell–Evans–Polanyi principle.

  Some functionalized compounds (e.g., esters and organic selenides) undergo syn‐elimination of H–LG from two adjacent carbons. The reaction follows a unimolecular mechanism via a cyclic transition state. The study of the mechanism is aided by Huckel's (4n + 2) rule and will be discussed in the chapter.

  Some special bases and their effectiveness in bringing about the elimination reactions are presented. Since many bases are nucleophilic, the E1 and particularly E2 reactions are often accompanied by nucleophilic substitutions. The competitions between elimination and nucleophilic substitution reactions which occur to the same functionalized compounds will be addressed.

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  Principles of Organic Chemistry

  • Robert J. Ouellette, J. David Rawn(Authors)
  • 2015(Publication Date)
  • Elsevier

    (Publisher)

  N 2 reaction mechanism, the E2 mechanism is a concerted process. In an E2 dehydrohalogenation reaction, the base (nucleophile) removes a proton on the carbon atom adjacent to the carbon atom containing the leaving group. As the proton is removed, the leaving group departs and a double bond forms.

  Like the SN 2 reaction, an E2 reaction requires a precise molecular arrangement. The anticonformation of the hydrogen and halogen atoms is preferred for the reaction because it aligns the orbitals properly for the formation of the π bond. We can visualize the process as the removal of the proton to provide an electron pair that attacks the neighboring carbon atom from the back to displace the leaving group. An E2 reaction occurs at a rate that depends on the concentrations of both the substrate and the base. If the substrate concentration is doubled, the reaction rate also doubles, as in SN 2 processes. Thus, both E2 and SN 2 mechanisms are affected in the same way, and the two mechanisms compete with each other.

  The E1 Mechanism

  We recall that an SN 1 reaction proceeds in two steps, and that the rate-determining step is formation of a carbocation intermediate. Similarly, an E1 mechanism occurs in two steps, and the rate-determining step is the formation of a carbocation. Thus, just as E2 and SN 2 mechanisms compete with each other, an E1 mechanism competes with an SN 1 mechanism. Because the rate-determining step of an E1 reaction involves only the substrate, the formation of the carbocation is a unimolecular reaction. If the carbocation reacts with a nucleophile at the positively charged carbon atom, the result is substitution. But if the nucleophile acts as a base and removes a proton from the carbon atom adjacent to the cationic center, the net result in the loss of HL and the formation of a π bond—that is, an elimination reaction.

  7.7 Effect of Structure on Competing Reactions

  Let’s now examine the variety of product mixtures that result from competing substitution and elimination processes. We will divide our discussion according to the type of haloalkane. These results are summarized in Table 7.2

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  Organic Chemistry

  • 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 CC 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.

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  Addition, Elimination and Substitution: Markovnikov, Hofmann, Zaitsev and Walden

  Discovery and Development

  • David E. Lewis(Author)
  • 2022(Publication Date)
  • Elsevier

    (Publisher)

  Journal of the Chemical Society between 1933 and 1964. In the course of these studies, they identified a number of the critical factors affecting the reaction, including the leaving group, the structure of the alkyl group, and the effect of solvent. The eliminations studied by Hughes and Ingold are readily separated into two classes, based on the reaction kinetics and molecularity.

  E2 Elimination

  The first proposal of the bimolecular mechanism was for the Hofmann elimination by pyrolysis of quaternary ammonium hydroxides.

  108 –114

  The study of this reaction and the related eliminations of trialkylsulfonium ions,

  115 –123

  alkyl bromides,

  115 –119

  along with the “anomalous” hydrolysis reaction that frequently accompanied both provided much of the basis for the early mechanistic work of Hughes and Ingold. During these studies, the stereochemistry and mechanism of the E2 reaction were probed.

  124

  ,

  125

  One of these studies,

  125

  the E2 Eliminations from menthyl chloride (216) and neomenthyl chloride (219), has found its way into organic chemistry textbooks at both the graduate and undergraduate levels. The E2 Elimination of menthyl chloride, which can only eliminate with anti stereochemistry by attaining the high-energy conformation 217, where the leaving group and a single β-hydrogen are both axial, gave 2-menthene (218) as the only product of a slow reaction; the E2 Elimination of neomenthyl chloride, where there are two β-hydrogen atoms anti to the leaving group in the low-energy conformation, gave mainly the Zaitsev regioisomer, 3-menthene (220), and proceeds some 200 times faster (Scheme 7.6 ). These experiments provided clear evidence that E2 Elimination must occur with anti stereochemistry.

  The reason for the differing regiochemistry of Zaitsev and Hofmann eliminations is traced to the different leaving groups. In the Zaitsev elimination, the leaving group departs as the conjugate base of a strong Lowry–Brønsted acid: Cl– (HCl), Br– (HBr), I– (HI), OTs– (TsOH), OMs– (MsOH), H2 O (H3 O+ ), etc. These are usually described as good leaving groups. In contrast to this, the leaving group in Hofmann eliminations departs as a weak base itself or the conjugate base of a weak acid: R3 N (R3 NH+ ), R2 S (R2 SH+ ), F– (HF), RCO2 – (RCO2

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  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  7.9A How To Determine Whether Substitution or Elimination Is Favored S N 2 versus E2 S N 2 and E2 reactions are both favored by a high concentration of a strong nucleophile or base. When the nucleophile (base) attacks a β hydrogen atom, elimination occurs. When the nucleophile attacks the carbon atom bearing the leaving group, substitution results: C C Nu C H C (b) (a) − + X − Nu H + X + Nu C H C X − substitution S N 2 elimination E2 (b) (a) The following examples illustrate the effects of several parameters on substitution and elimination: relative steric hindrance in the substrate (class of alkyl halide), temperature, size of the base/nucleophile (EtONa versus t-BuOK), and the effects of basicity and polarizability. In these examples, we also illustrate a very common way of writing organic reactions, where reagents are written over the reaction arrow, solvents and temperatures are written under the 7.9 Elimination and Substitution Reactions Compete With Each Other 307 arrow, and only the substrate and major organic products are written to the left and right of the reaction arrow. We also employ typical shorthand notations of organic chemists, such as exclusive use of bond-line formulas and use of commonly accepted abbreviations for some reagents and solvents. Primary Substrate When the substrate is a primary halide and the base is strong and unhindered, like ethoxide ion, substitution is highly favored because the base can easily approach the carbon bearing the leaving group: + E2 Minor (10%) S N 2 Major (90%) Primary O Br EtOH, 55 °C EtONa Secondary Substrate With secondary halides, however, a strong base favors elimi- nation because steric hindrance in the substrate makes substitution more difficult: + S N 2 Minor (21%) E2 Major (79%) Secondary O Br EtOH, 55 °C EtONa Tertiary Substrate With tertiary halides, steric hindrance in the substrate is severe and an S N 2 reaction cannot take place.

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  Solomons' Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  This is because the reactive part of both nucleophiles and bases is an unshared electron pair. It should not be surprising, then, that nucleophilic substitution reactions and elimination reactions often compete with each other. Evaluating the relative potential of a reaction to lead to substitution or elimination can be a perplexing task for students of organic chemistry. To help you with mastering these concepts we shall now summarize factors that influence which type of reaction is favored, and provide some examples. 7.9A HOW TO Determine Whether Substitution or Elimination Is Favored S N 2 versus E2 S N 2 and E2 reactions are both favored by a high concentration of a strong nucleophile or base. When the nucleophile (base) attacks a β hydrogen atom, elimination occurs. When the nucleophile attacks the carbon atom bearing the leaving group, substitution results: C C Nu C H C (b) (a) − + X − Nu H + X + Nu C H C X − substitution S N 2 elimination E2 (b) (a) The following examples illustrate the effects of several parameters on substitution and elimination: relative steric hindrance in the substrate (class of alkyl halide), temperature, size of the base/nucleophile (EtONa versus t-BuOK), and the effects of basicity and polarizability. In these examples we also illustrate a very common way of writing organic reactions, where reagents are written over the reaction arrow, solvents and temperatures This section draws together the various factors that influence the competition between substitution and elimination. HINT 300 CHAPTER 7 ALKENES AND ALKYNES I: Properties and Synthesis. Elimination Reactions of Alkyl Halides are written under the arrow, and only the substrate and major organic products are writ- ten to the left and right of the reaction arrow. We also employ typical shorthand notations of organic chemists, such as exclusive use of bond-line formulas and use of commonly accepted abbreviations for some reagents and solvents.

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  Organic Chemistry

  • T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  This is because the reactive part of both nucleophiles and bases is an unshared electron pair. It should not be surprising, then, that nucleophilic substitution reactions and elimination reactions often compete with each other. Evaluating the relative potential of a reaction to lead to substitution or elimination can be a perplexing task for students of organic chemistry. To help you with mastering these concepts we shall now summarize factors that influence which type of reaction is favored, and provide some examples. • • 7.9A HOW TO Determine Whether Substitution or Elimination Is Favored S N 2 versus E2 S N 2 and E2 reactions are both favored by a high concentration of a strong nucleophile or base. When the nucleophile (base) attacks a β hydrogen atom, elimination occurs. When the nucleophile attacks the carbon atom bearing the leaving group, substitution results: C C Nu C H C (b) (a) - + X - Nu H + X + Nu C H C X - substitution S N 2 elimination E2 (b) (a) The following examples illustrate the effects of several parameters on substitution and elimination: relative steric hindrance in the substrate (class of alkyl halide), temperature, size of the base/nucleophile (EtONa versus t-BuOK), and the effects of basicity and polarizability. In these examples we also illustrate a very common way of writing organic reactions, where reagents are written over the reaction arrow, solvents and temperatures [ HELPFUL HINT ] This section draws together the various factors that influence the competition between substitution and elimination. 300 CHAPTER 7 ALKENES AND ALKYNES I: Properties and Synthesis. Elimination Reactions of Alkyl Halides are written under the arrow, and only the substrate and major organic products are writ- ten to the left and right of the reaction arrow. We also employ typical shorthand notations of organic chemists, such as exclusive use of bond-line formulas and use of commonly accepted abbreviations for some reagents and solvents.

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  Organic 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.

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  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Primary alcohols are also converted into alkenes, likely via an E2 process. SECTION 7.13 • A retrosynthetic analysis shows the product first, followed by reagents that can be used to make that product. A wavy line indicates a disconnection, which identifies the bond that can be made by the reaction. • 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). 334 CHAPTER 7 Alkyl Halides: Nucleophilic Substitution and Elimination Reactions SKILLBUILDER REVIEW 7.1 DRAWING THE PRODUCT OF AN S N 2 PROCESS Replace the LG with the Nuc, and draw inversion of configuration. OH OH Br Br + + ⊝ ⊝ Try Problems 7.3, 7.4, 7.54, 7.55 7.2 DRAWING THE TRANSITION STATE OF AN S N 2 PROCESS EXAMPLE Draw the transition state of the following process. STEP 1 Identify the nucleophile and the leaving group. STEP 2 Draw a carbon atom with the Nuc and LG on either side. Use δ– symbols to indicate partial charges. STEP 3 Draw the three groups attached to the carbon atom. Place brackets and the symbol indicating a transition state. Cl NaSH SH NaCl Cl SH Leaving group Nucleophile C Cl HS Bond Forming Bond Breaking C Cl HS CH 3 H H + ⊝ δ– δ– δ– δ– Try Problems 7.5, 7.6, 7.51 7.3 PREDICTING THE REGIOCHEMICAL OUTCOME OF AN E2 REACTION STEP 1 Identify all β positions bearing protons. STEP 2 Draw all possible regiochemical outcomes. STEP 3 Identify the Zaitsev and Hofmann products. STEP 4 Analyze the base to determine which product predominates. Cl Zaitsev Hofmann Zaitsev Hofmann Not sterically hindered Sterically hindered Major Minor Major Minor β β β Try Problems 7.18–7.20, 7.59, 7.60a,d, 7.63, 7.64b, 7.67 7.4 PREDICTING THE STEREOCHEMICAL OUTCOME OF AN E2 REACTION STEP 1 Identify all β positions bearing protons. STEP 2 If a β position has two protons, expect cis and trans isomers (with a preference for the trans isomer).

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