Bimolecular elimination reaction mechanism

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. 

This is an example of the first step of an E2 (bimolecular elimination) reaction mechanism. Note the base-mediated deprotonation of the diester converting the ferf-butoxide anion to ferf-butanol. For clarity, the anion was repositioned and the bond was lengthened. Arrow pushing is illustrated below ... 

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

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. 

J. F. Bunnett, The Mechanism of Bimolecular -Elimination Reactions, Angew. Chem. Int. Ed. Engl. 1962,1, 225-235. 

E2 reaction or bimolecular elimination reaction (Section 9.2) An elimination reaction that follows a concerted mechanism in which a base removes a proton simultaneously with the departure of the leaving group. 

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). 

Pincock [Pi 64] characterized solvents on the basis of the solvent dependence of the ionic decomposition of t-butyl peroxyformate. However, not only the rate, but also the mechanism of decomposition of this compound is solvent-dependent. For example, in chlorobenzene it decomposes in a slow unimolecular reaction in which the peroxide bond is split in n-butyl ether the decomposition proceeds via radical attack on the peroxide oxygen atoms and in the presence of pyridine a bimolecular elimination reaction occurs, with the formation of t-butanol and carbon dioxide. Pincock used the solvent dependence of the rate of this latter reaction to characterize the solvent. 

Much like the Sn2 reaction, the rate is linearly dependent on the concentrations of two different compounds (the substrate and the base). This observation suggests that the mechanism must exhibit a step in which the substrate and the nucleophile collide with each other. This is consistent with a concerted mechanism in which there is only one mechanistic step involving both the substrate and the base. Because that step involves two chemical entities, it is said to be bimolecular. Bimolecular elimination reactions are called E2 reactions ... 

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). 

The interpretation of product data for competitive solvolysis and elimination reactions requires that the mechanism for these reactions be known. Two experiments are sufficient to show that the formation of solvolysis and elimination products occurs by partitioning of a common carbocation intermediate (Scheme 3 a) rather than by competing bimolecular reactions of the substrate (Scheme 3b).3... 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

Pathway b is the specific base catalyzed (HO -catalyzed) hydrolysis. This bimolecular 5n2 reaction leads to the alcohol and nitrate. A peculiar pathway is carbonyl elimination (Fig. 9.1,c). This bimolecular reaction is catalyzed by strong bases and produces a dismutation of the two moieties, the organic group being oxidized to a carbonyl compound and nitrate being reduced to nitrite. Note that proton-catalyzed hydrolysis does not appear in Fig. 9.1 since this mechanism either does not occur or is negligible. 

Similar qualitative relationships between reaction mechanism and the stability of the putative reactive intermediates have been observed for a variety of organic reactions, including alkene-forming elimination reactions, and nucleophilic substitution at vinylic" and at carbonyl carbon. The nomenclature for reaction mechanisms has evolved through the years and we will adopt the International Union of Pure and Applied Chemistry (lUPAC) nomenclature and refer to stepwise substitution (SnI) as Dn + An (Scheme 2.1 A) and concerted bimolecular substitution (Sn2) as AnDn (Scheme 2.IB), except when we want to emphasize that the distinction in reaction mechanism is based solely upon the experimentally determined kinetic order of the reaction with respect to the nucleophile. 

Banthorpe, D. V., E. D. Hughes, and C. K. Ingold Mechanism of elimination reactions. Part XX. The Inesscntiality of. steric strain in bimolecular olefin elimination. J. chem. Soc. [London] 1960, 4054. 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

A fairly general procedure, which has also been used on the industrial scale, involves heating the alkali metal sulphonate with either sodium or potassium hydroxide in the presence of a small amount of water to aid the fusion process. The reaction mechanism may be formulated as a bimolecular nucleophilic addition-elimination sequence. 

The mechanism presented here is somewhat at variance with that proposed by the authors (Yamamoto et al. 1995) who suggested that the /BuOI I-derived radical adds to the primarily formed electron-adduct radical. Since this has been shown above to have only a very short lifetime, it will not be capable of undergoing bimolecular recombination reactions. An isomerization of C(8)-H -adduct [reaction (183)] followed by an addition of the tert-butanol-derived radical and water elimination [reactions (184) and (185)] is not in conflict with the above pulse radiolysis results [note that the tautomerization reaction (183) cannot be excluded on the basis of the pulse radiolysis data]. 

The oxidant may aid the elimination in a concerted or E2 type of mechanism, as illustrated in Eq. (7) for such examples, the oxidant is not bonded to the substrate, except possibly in the transition state. Other oxidants, for example chromic acid, have been shown to form intermediate esters such as 1 (although other mechanisms have been proposed7), which subsequently decompose by a related, bimolecular elimination [Eq. (2)] here the leaving group is the reduced form of the oxidant, and the C-H bond must necessarily break with the liberation of a proton. As in Eq. (7), the capture of electrons by the oxidant is the driving force of the reaction, so that the breaking of the C-H bond occurs simultaneously in the rate-determining step (Scheme 1). 

Starting materials that are likely to undergo an bimolecular SN2 reaction undergo elimination reactions by a bimolecular E2 mechanism. This is a one-step reaction in which the nucleophile attacks a C—H bond on the carbon atom adjacent to the site of SN2 reaction. 

An ab initio study of the. S N2 and E2 mechanisms has been performed for the reaction between the cyanide ion and ethyl chloride in dimethyl sulfoxide solution.5 Theoretical calculations have predicted a free energy barrier for nitrile formation of 24.1 kcal mol-1, close to the experimental value of 22.6 kcal mol-1 (Scheme 3). It has also been predicted that the isonitrile formation is less favorable by 4.7 kcal mol-1, while the elimination mechanism is less favorable by more than 10 kcal mol-1. These results indicated that isonitrile formation and bimolecular elimination are not significant side-reactions for primary alkyl chloride reactions. 

The rate law is consistent with a concerted or one-step mechanism. Because two species, ethoxide ion and tort-butyl bromide, react in this step, the reaction is described as a bimolecular elimination or an E2 reaction. This reaction is quite common when alkyl halides are treated with strong bases. Because many nucleophiles are also quite basic, the E2 reaction often competes with the SN2 reaction. 

Some interesting results have recently been obtained in studies on elimination reactions of esters of hydroxy acids. The mechanism is not fully established, but probably is of the bimolecular type. An especially interesting observation is that sodium iodide promotes the removal of two vicinal sul-fonyloxy groups by a process of cfs-elimination a series of elimination reactions of this type is known in carbohydrate chemistry, but apparently does not yet include an example from which the stereochemistry of the reaction could be deduced. 

Instead of performing the one step bimolecular SN2 reaction, alkenes react via two closely related bimolecular pathways. The first of these is called the tetrahedral mechanism and proceeds via a negatively charged intermediate. This mechanism is sometimes called the addition/elimination reaction, which is given the label Adn/E. This alternative name is unfortunate, because the other pathway is called the addition/elimination mechanism and proceeds via a readily detectable neutral intermediate. This latter mechanism will be considered in the chapter on sequential addition/elimination reactions. In this book, in an attempt to reduce the confusion, we will call the mechanism that proceeds via an anionic intermediate the tetrahedral mechanism, and reserve the name addition/elimination mechanism for the mechanism that proceeds via a neutral species. 

Many of the early studies of rate effects of aqueous micelles were on reactions in which OH acted as a nucleophile, e.g., in ester saponification, but cationic micelles also speeded bimolecular eliminations in which OH is a base [53-55], and ester hydrolyses by the ElcB mechanism in which the first step of reaction is an equilibrium deprotonation [56]. 

The leaving groups in the bimolecular elimination mechanism (E2) are essentially the same as those in substitution reactions (that is, halides and tosylate). Alcohols do not usually react by the E2 mechanism, but quaternary ammonium salts, such as RN(CH3)3X and sulfonium salts, such as RS(CH3)2X- do. 

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