E2 Elimination
E2 elimination is a chemical reaction in which a leaving group is expelled from a molecule, resulting in the formation of a double bond. This process occurs in a single step and requires a strong base to abstract a proton from the molecule. E2 elimination is commonly observed in organic chemistry reactions, particularly in the synthesis of alkenes.
3 Key excerpts on "E2 Elimination"
eBook - ePub- Graham Patrick(Author)
- 2004(Publication Date)
- Taylor & Francis(Publisher)
...The rate-determining step is the first stage involving loss of the halide ion. As a result, the reaction is first order, depending on the concentration of the alkyl halide alone. The carbocation intermediate is stabilized by substituent alkyl groups. E2 versus E1 The E2 reaction is more useful than the E1 reaction in synthesizing alkenes. The use of a strong base in a protic solvent favors the E2 Elimination over the E1 elimination. The E1 reaction occurs when tertiary alkyl halides are dissolved in protic solvents. Related topics (H3) Electrophilic addition to symmetrical alkenes (M4) Reactions of alcohols Definition Alkyl halides which have a proton attached to a neighboring β-carbon atom can undergo an elimination reaction to produce an alkene plus a hydrogen halide (Figure 1). In essence, this reaction is the reverse of the electrophilic addition of a hydrogen halide to an alkene (Section H3). There are two mechanisms by which this elimination can take place — the E2 mechanism and the E1 mechanism. Figure 1. Elimination of an alkyl halide. The E2 reaction is the most effective for the synthesis of alkenes from alkyl halides and can be used on primary secondary and tertiary alkyl halides. The E1 reaction is not particularly useful from a synthetic point of view and occurs in competition with the S N 1 reaction of tertiary alkyl halides. Primary and secondary alkyl halides do not usually react by this mechanism. Susceptible β-protons An alkyl halide can undergo an elimination reaction if it has a susceptible proton situated on a β-carbon; that is, the carbon next to the C–X group. This proton is lost during the elimination reaction along with the halide ion. In some respects, there is a similarity here between alkyl halides and carbonyl compounds (Figure 2). Alkyl halides can have susceptible protons at the β-position whilst carbonyl compounds can have acidic protons at their α-position...
eBook - ePub- Michael North(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...The l -diastereomer is chiral and can exist as a pair of enantiomers; however, both enantiomers will give the same diastereomer of the product. Scheme 9.9 The reason why an E2 reaction requires the two groups being eliminated to be antiperiplanar is stereoelectronic in nature. During the elimination of X and Y, the two electrons occupying the C–Y σ -bond are used to form the π -bond of the alkene and for this to be possible there must be an interaction between this σ -bond and the σ *-orbital of the C–X bond. This is only possible if the two groups being eliminated are antiperiplanar as shown in Figure 9.1. Figure 9.1 The stereoelectronic requirement for an E2 Elimination. In cyclohexane systems, it is only possible for the two groups being eliminated to be antiperiplanar if the two groups are both in axial positions. In the isomer of hexachlorocyclohexane shown in structure 9.18, there are no hydrogen and chlorine atoms trans to one another on adjacent carbon atoms. The minimum energy conformation of compound 9.18 has all of the chlorine atoms in equatorial positions, and the other chair conformation would have all of the hydrogen atoms in equatorial positions. Thus there is no conformation of compound 9.18 in which an adjacent hydrogen and chlorine are both in axial positions and hence compound 9.18 cannot eliminate HCl via an E2 mechanism. Another example of the stereochemical consequences of an E2 Elimination in cyclohexane derivatives is shown in Scheme 9.10. The minimum energy conformation of compound 9.19 is as shown in Scheme 9.10, since this allows two of the three substituents to adopt equatorial positions. Compound 9.19 undergoes rapid elimination of HCl, as in the minimum energy conformation there are two hydrogen atoms axially located adjacent to the axial chlorine atom. Thus a mixture of the two regioisomeric alkenes 9.20 and 9.21 is produced...
eBook - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...If there is no intermediate, then the characteristics of the reaction are described by the transition state for the reaction that is shown in Figure 2.11. If there is no intermediate, all of the bond-making and -breaking occurs simultaneously (this is a synchronous reaction). The initial reaction of ethoxide and the β hydrogen is a collision process, and experimental kinetic data show it to be a second-order reaction. Since the overall transformation is an elimination reaction, with loss of H and Br, the symbol E for elimination is used. It is a bimolecular elimination reaction, so the symbol 2 is used. This an E2 reaction. Note that it is the electrons in the C β —βH bond that expels the bromine leaving group as the π-bond is formed. Therefore, the leaving group and the β H must be anti in the E2 transition state, as shown in Figure 2.11. Many alkenes have more than one β hydrogen atom, and removal of those β hydrogen atoms via an E2 reaction can lead to different isomeric alkenes. When 3-bromo-3-methylpentane is heated with KOH in ethanol, for example, the major product is 3-methylpent-2-ene. However, as shown in Figure 2.12, 3-bromo-3-methylpentane has two β hydrogen atoms (H a and H b). In principle, there should be two products, 3-methylpent-2-ene and 2-ethylbut-1-ene, formed by loss of each of the two β hydrogen atoms. Experimentally, the observed major product is 3-methylpent-2-ene formed by removal of H b. FIGURE 2.12 Competitive E2 reactions of 3-bromo-3-methylpentane. 3-Methylpent-2-ene is a trisubstituted alkene with three carbon substituents attached to the C=C unit, whereas 2-ethylbut-1-ene is a disubstituted alkene with two carbon substituents. A carbon substituent (an alkyl group) is electron releasing relative to the adjacent carbon atom, and a C=C unit with more alkyl groups has more electron density released to the π-bond, making the bond stronger and that alkene more stable...
