Elimination Reactions

8 Key excerpts on "Elimination Reactions"

• 

  eBook - ePub

  Reaction Mechanisms in Organic Chemistry

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

    (Publisher)

  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.

  • 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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Reaction Mechanisms in Organic Chemistry

  • 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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Organic 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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Chemistry

  • 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.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Organic Reaction Mechanisms

  A Step by Step Approach, Second Edition

  • Michael Edenborough(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  In contrast, in organic chemistry there seems to be an enormously large number of reactions that are possible. Furthermore, many of these, particularly at first sight, seem to be unique and seemingly unrelated to any other example. This gives the initial impression that organic chemistry is incapable of being readily rationalised into a small number of principal reaction types like inorganic chemistry. Yet, thankfully, this first impression is wrong, and it is possible to organise this multitude of reactions into a small number of fundamental reaction types, namely: substitution, addition, elimination, rearrangement and redox reactions. Of these reaction types, the first three are fairly straightforward, while the latter two are generally more complex.

  In a substitution reaction, one group is replaced by another. Usually a heteroatom or group, i.e. not a hydrogen atom or hydrocarbon moiety, is replaced by another heteroatom or group. In an addition reaction, the adduct, which is the molecule that is going to be added to the substrate, is broken into approximately equal parts, and these parts then add across a multiple bond, which is commonly a carbon/carbon or carbon/oxygen double bond. Elimination Reactions are broadly overall the reverse of addition reactions. Thus, in an elimination reaction usually a compound is formed that has more multiple bonds than did the starting material, i.e. there is a greater degree of unsaturation in the final product than in the starting materials.

  As well as those reactions that fall neatly into one or other of the three simple reaction types, there are some reactions where an addition reaction is followed by an elimination reaction, or vice versa, resulting in an overall substitution of a group. These reactions will be considered separately in this book under the heading of sequential addition/Elimination Reactions.

  More complicated still, in a rearrangement reaction, the backbone of the molecule is broken and reformed in a different configuration. Often, this involves the breakage of a carbon/carbon bond, but on some occasions it may involve the cleavage of a carbon/oxygen or carbon/nitrogen bond. Lastly, there are redox reactions. On many occasions redox reactions may be considered conveniently under one of the first four headings. However, for some other reactions, it is more convenient to consider them separately, because some of the detailed pathways of the mechanisms do not readily fall within any of the other categories.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Principles of Organic Chemistry

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

    (Publisher)

  7

  Nucleophilic Substitution and Elimination Reactions

  7.1 Reaction Mechanisms and Haloalkanes

  We introduced the concept of functional groups and their role in the organization of the structures of organic molecules in Section 1.9 . We described the importance of reaction mechanisms as an organizational device to classify chemical reactions in Section 2.9 . The details of the electrophilic addition reactions of alkenes (Section 4.9 ) and electrophilic substitution reactions of aromatic compounds (Section 5.5 ) are examples of two important reaction mechanisms. In this chapter we examine two more types of reactions mechanisms—nucleophilic substitution and Elimination Reactions. These mechanisms often occur in competition with one another and describe the reactions of several classes of compounds, such as haloalkanes (also called alkyl halides) and alcohols. In this chapter we focus on the substitution and Elimination Reactions of haloalkanes. These reactions illustrate the role of structure in determining the degree to which a given reaction mechanisms occurs.

  Reactivity of Haloalkanes

  Haloalkanes have a halogen atom bonded to an sp3 -hybridized carbon atom. As a result of the greater electronegativity of the halogens, the carbon atom of the carbon-halogen bond bears a partial positive charge and the halogen atom has a partial negative charge.

  where X = F, Cl, Br, I

  Since a carbon-halogen bond is polar, a haloalkane has two sites of reactivity. One is at the carbon atom bonded to the halogen atom. This carbon atom is electropositive and reacts with nucleophiles. The second site of reactivity in a haloalkane is the hydrogen atom bonded to the carbon atom adjacent to the carbon atom bonded to the halogen atom. This hydrogen atom is more acidic than the hydrogen atoms in alkanes because the halogen atom on the adjacent carbon atom withdraws electron density by an inductive effect.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  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.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Organic Chemistry

  • 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.

  Sign up to read

  Learn more about book