Elimination reaction mechanisms

Four types of mechanisms are inherent to Organic Chemistry 1. These are substitution reaction mechanisms (S l and Sf 2) and elimination reaction mechanisms (El and E2). The principles of these four types apply to Organic Chemistry 11, and no review would be complete without a few reminders about these processes. 

With elimination reactions, you were told how to find the information you need to understand elimination reactions (mechanisms, factors, etc.), and you recorded the information as you went along (in the form of charts and other drawings). 

El an elimination reaction mechanism in which the slow step is a self-ionization of the molecule to form a carbocation. Thus, the ratecontrolling step is unimolecular. 

E2 an elimination reaction mechanism in which the rate-controlling step is the simultaneous removal of a proton from the molecule by a base, resulting in the creation of a double bond. The rate controlling step is bimolecular. 

Tertiary alkyl chlorides are easily dehydrochlorinated by base (via the E2, or bimolecular elimination reaction mechanism), but the environment of the degrading resin is not basic. Loss of hydrogen chloride to yield an olefin can occur principally by the El, or monomolecular elimination reaction. This is a slow reaction because, in the rate-determining step, the C—Cl bond is broken to form two separated oppositely charged particles. The reaction rate is not assisted by the acid present. 

In this chapter, elimination reactions were presented both independently and in association with their related nucleophilic substitution mechanisms. Furthermore, the processes by which molecules undergo both El and E2 eliminations were presented and explained using bonding and nonbonding orbitals and their required relationships to one another. While much emphasis was placed on the planar relationships of orbitals required for both elimination reaction mechanisms, the special case of frans-periplanar geometries were described as necessary for efficient E2 eliminations to occur. 

While frarcs-periplanar relationships are important to E2 elimination reactions, it is important to remember that, as illustrated in Schemes 6.6 and 6.7, E2 elimination reaction mechanisms do not have to occur in a concerted manner. After deprotonation, if the relevant orbitals do not line up, elimination will not occur until they do. Furthermore, recall that rotation around an acyclic single bond, as illustrated in Figures 6.4 and 6.5, occurs readily. Therefore, elimination reactions should not be removed from consideration if a molecule is drawn in a conformation that makes these reactions appear unfavorable. When looking at any type of nucleophilic reaction, initial identification of relevant trans-periplanar relationships will aid in the identification of potential side products and their respective mechanisms of formation. 

This is an example of the first step of an addition-elimination reaction mechanism converting an ester (methyl acetate) to an amide (A - mcth y I acctam ide). For clarity, the anion was repositioned in the scheme. Arrow pushing is illustrated below ... 

The reaction presented in this problem is known as a Friedel-Crafts acylation. Technically, this example belongs to a class of reactions referred to as electrophilic aromatic substitutions. Furthermore, the actual mechanism associated with this reaction, utilizing Lewis acid reagents as catalysts, proceeds through initial formation of an electrophilic acyl cation followed by reaction with an aromatic ring acting as a nucleophile. This mechanism, shown below, reflects distinct parallels to standard addition-elimination reaction mechanisms warranting introduction at this time. 

Please note that while the Friedel-Crafts acylation reaction is presented in discussions of addition-elimination reaction mechanisms, this reaction is actually an electrophilic aromatic substitution reaction. The correct mechanisms for a Freidel-Crafts acylation was presented in the solution for Problem 6 (h) from Chapter 7. 

Kinetic studies of the nucleophilic reactions of azolides have demonstrated that the aminolyses and alcoholyses proceed via a bimolecular addition-elimination reaction mechanism, as does the neutral hydrolysis of azolides of aromatic carboxylic acids. Aliphatic carboxylic acid azolides which are subject to steric hindrance can be hydrolyzed in aqueous medium by an 5n1 process. There have been many studies of these reactions, and evidence supporting both 5n1 and 5n2 processes leaves the impression that there are features of individual olysis reactions which favour either an initial ionization or a bimolecular process involving a tetrahedral intermediate (80AHC(27)241, B-76MI40701). 

The elimination reaction mechanisms are analogous to those of nucleophilic substitution. 

The proposed elimination (El cb) mechanisms (mechanisms A and B) were the result of studies of the behavior of substrate analogs and secondary deuterium isotope effects in the PAL reaction,solvent isotope exchange studies with HAL, ° and kinetic isotope effects studies of a ring perdeuterated phenylalanine in PAL. Later, a synthetic model putatively mimicking the PAL elimination reaction (mechanism B) was also described (Figure 15), taking into consideration the electrophilic behavior of a Michael acceptor within a sterically appropriate distance from a Phe (l)-like substrate. Upon treatment with a Lewis acid (AICI3) and... 

Several weakly basic anion-exchange resin catalysts were screened in laboratory-scale and the results showed that a very good product distribution with respect to aldol can be obtained. Extensive kinetic studies demonstrated that weakly basic anion-exchange resin catalysts promoted both the aldolization and elimination processes, but the product distribution can be steered by selection of the solvent and the ratio of the reactants. A kinetic model based on the aldolization and elimination reaction mechanisms was developed and sin5>lified. The model predictions were in good agreement with the... 

Are there uni- and bimolecular elimination reaction mechanisms, like there were for substitution reactions ... 

The products form by charge exchange, proton abstraction, or elimination. Reaction mechanisms were frequently discussed see for example [4, 8, 9]. Multiple H-D exchange, as in the formation of OH from D2O, was occasionally observed, but its minor importance indicates a short lifetime of the reactive intermediate complex [19]. The average dipole orientation (ADO) theory usually predicts the rate constants satisfactorily an exception is the value calculated for the reaction with N2O which is much too small [8]. Calculated structural parameters of the adduct of NHJ with NH3 are given on p. 269. 

NMR spectra of the product characteristic signals at 6.47 ppm were present which could be assigned to methine protons of vinyl ether. Their intensity increased with the reaction time. The authors discussed two possible elimination reaction mechanisms pyrolytic (Ei) and bimolecular (E2) eliminations (Scheme 21). 

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