E1 Elimination
E1 elimination is a chemical reaction in which a molecule loses a leaving group and a proton from an adjacent carbon atom, resulting in the formation of a double bond. This process occurs in a single step, with the leaving group departing before the proton is removed. E1 elimination reactions are typically favored in the presence of weak bases and occur more readily with tertiary substrates.
4 Key excerpts on "E1 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 - ePubBiochemistry
An Organic Chemistry Approach
- Michael B. Smith(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...There are two reactions, ionization to a carbocation and subsequent removal of the β-hydrogen atom by a base. In this reaction, loss of bromide to yield the carbocation via ionization has a much higher activation energy and is a significantly slower step whereas removal of the β hydrogen by the base has a low activation energy and is a much faster step. Therefore, ionization is the rate-determining step and this reaction is termed a unimolecular elimination, or E1. FIGURE 2.13 Ionization of an alkyl halide and an E1 reaction. In the carbocation intermediate the β hydrogen of the carbocation is more polarized and the β hydrogen of the carbocation is more acidic when compared to the β hydrogen atom (H a) in 2-bromo-2-methylbutane. It is analogous to S N 1 (see Section 3.3) and the product will be 2-methylbutan-2-ol. Water may also react with the carbocation, and in the basic medium the product is also the alcohol. In fact, the attraction of hydroxide or water to C + is largely a function of the solvent, and in a protic solvent (e.g., water), substitution is usually faster. If a base is rather nucleophilic and the reaction is done in an aprotic solvent, the S N 1 reaction also competes with E1 if there is a carbocation intermediate. If the reaction is done in a protic solvent, elimination is usually faster than the S N 1 reaction, but this obviously depends on the nucleophile. Since reaction conditions used for an E1 reaction often favor S N 1 rather than E1, it is difficult to find a “pure” E1 reaction. In general, this is true, but there are exceptions when the base used in the reaction is a poor nucleophile, or if the S N 1 product is unstable and leads to a reversible reaction. If cyclohexanol is treated with concentrated sulfuric acid, for example, the observed product is cyclohexene in a very fast reaction...
eBook - ePub- Michael North(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...However, if either conformation of the carbenium ion has a significantly lower energy than the other conformation, then elimination will occur preferentially through the lower energy conformation and the reaction will be stereoselective but not stereospecific. Since an E1 Elimination proceeds via a carbenium ion, if a system cannot form a planar carbenium ion it will not undergo an E1 Elimination. Bridgehead bicyclic systems (such as 9.9 and 9.17) again provide a good example of this. Scheme 9.11 9.3.3 Syn-eliminations If an anti-elimination is impossible, then syn-elimination may occur provided the two groups being eliminated are synperiplanar. An example of a synelimination is the elimination of HCl from cyclopropane 9.23 as shown in Scheme 9.12. As the Newman projection of compound 9.23 shows, the chlorine and hydrogen atoms are exactly synperiplanar but the deuterium atom is not antiperiplanar to the chlorine. Hence, an anti-elimination of DCl cannot occur but syn-elimination of HCl can. Another class of elimination reactions that occur by a syn-elimination are pyrolytic eliminations, which occur when appropriate substrates are heated in the absence of base. The best known examples are the eliminations of esters (and their sulphur analogues), sulphoxides (or selenoxides) and amine oxides as shown in Scheme 9.13. The elimination of selenoxides is of particular synthetic utility, since the reaction occurs spontaneously at room temperature. In each case, the elimination occurs in a single step and in the transition state three pairs of electrons are moving around a five or six-membered ring...
eBook - ePub- J Fisher, J.R.P. Arnold, Julie Fisher, John Arnold(Authors)
- 2020(Publication Date)
- Taylor & Francis(Publisher)
...This is an important point as if the original alkyl halide had been chiral (Section E), then a racemic mixture (Section E) would result (Figure 2). If on the other hand formation of a carbocation was not favorable, then the nucleophile would interact with the carbon before the halogen departed. The rate determining step therefore involves the alkyl halide and the nucleophile and is bimolecular. If the alkyl halide was chiral, then the reaction would proceed with inversion of configuration (Section E), as the nucleophile enters from the face opposite the leaving group (Figure 2). Figure 1 Examples of (a) S N 1 and (b) S N 2 reactions. Figure 2 Mechanism of S N 1 and S N 2 reactions. Aromatic compounds undergo electrophilic substitution reactions; in the case of benzene a catalyst is generally required. These reactions are more favorable than addition reactions as the product retains aromaticity; Elimination reactions Elimination reactions may occur with alkyl halides and with alcohols, resulting in the formation of an alkene and HX (where X is a halogen) or H 2 O, respectively. Elimination reactions often compete with nucleophilic substitution reactions, and like those reactions may be subdivided depending on the molecularity of the rate determining step. The label E2 is given to bimolecular processes, E1 to unimolecular processes. For an elimination reaction to occur, a good leaving group is required and a strong base. A ‘good’ leaving group is any group that is stable in its leaving form (e.g. a leaving halide ion is stable due to the electronegativity of the halogens), and hence promotes the forward reaction. If a good leaving group is present, and the carbon to which it is attached can stabilize positive charge then carbonium ion formation will be the first step, and the process will be unimolecular in the rate determining step (E1). Clearly the S N 1 mechanism can compete here...
