Concerted reaction bimolecular elimination

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). 

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. 

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. 

The first mechanism to be classified, the bimolecular elimination (E2), was suggested to account for the Hofmann degradation (1), p. 163, of quaternary ammonium hydroxides to olefins and tertiary amines. A single-step reaction between the substrate and a base is involved and the Q-H and C -X bonds break and the Ca-Q double bond forms in a concerted fashion, viz-... 

Such a mechanism requires that the major product, 1,2,4-trichlorobenzene should contain deuterium when the reaction is performed in a deuterated medium , but the amount of label found was no greater than predicted from the rate of exchange of the product under the reaction conditions . Presumably fairly extensive double-bond character is exhibited in the transition state for bimolecular elimination from the benzene hexachlorides and the abnormally high activation energy for the beta isomer is most plausibly attributed to the lack of coplanarity in the iy -c//nfl/-elimination or the high energy and low population of the boat conformer required for jy/i-elimination. Alternatively, a sparsely populated conformation for a concerted syn-clinal-... 

The concerted reaction depicted in Equation 10.2 is classified as an E2 process, where E stands for elimination and 2 refers to the molecularity of the ratedetermining step of the reaction. For E2 processes, the rate of the reaction depends upon the concentrations of the organic substrate and the base (Eq. 10.3), so both reactants are involved in the transition state of the rate-determining step. This bimolecularity is illustrated in 4 (Eq. 10.2). 

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 ... 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

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. 

The concerted bimolecular /3-elimination reaction of substituted alkanes (X-Ca H2-Cp H2-...) has been studied using the semilocalized quantum chemical approach.3... 

In the bimolecular concerted beta elimination reaction, E2, heterolytic cleavage of the C—X and C—H bonds takes place within the same reaction step, without formation of an intermediate (see Vol. 9). It appears that the energy barrier for the concerted process is lower than each of the barriers for the separate steps involving either a carbanion or a carbonium ion intermediate. 

The E2 mechanism is a concerted one-step process in which a nucleophile abstracts a hydrogen ion from one carbon while the halide is leaving from an adjacent one. The Ei mechanism is two-steps and involves a carbocation intermediate formed upon departure of the halide ion in the first step. E2 reactions are bimolecular and the reaction rate depends on the concentrations of both the alkyl halide and nucleophile. E1 reaction rates depend on the slowest step, formation of the carbocation, and are influenced only by the concentration of the alkyl halide the reaction is unimolecular. E2 reactions involve anti elimination and produce a specific alkene, either cis or trans. E1 reactions involve an intermediate carbocation and thus give products of both syn and anti elimination. 

E2 reaction (elimination, bimolecular) A concerted elimination involving a transition state where the base is abstracting a proton at the same time that the leaving group is leaving. The anti-coplanar transition state is generally preferred, (p. 258)... 

The base, as proton acceptor, deprotonates alkyl halide causing the removal of a halide anion. The reaction proceeds via the transition state in which both C-H and C-Br bonds break simultaneously. Such reactions are called concerted. The reaction is bimolecular because its rate depends on the concentrations of two molecular species, alkyl-bromide and the ethoxy anion. This mechanism is called the E2 elimination mechanism. 

Thermal organic reactions are often classified in terms of the molecular and electronic structure of their transition state or reactive intermediate (which is often taken as a model of the transition state). Thus, for instance, one has the Sn2 transition state for concerted bimolecular nucleophilic substitution reactions one has the E2 and Ei transition states in elimination reactions, etc. Given the transient nature of the transition states, the use of quantum chemical methodologies is essential for the determination of their detailed geometrical and electronic structure. Furthermore, the computation of the associated transition vectors provides information on the reactive mode... 

The E2 reaction (elimination, bimolecular) involves a base and an alkyl structure with a good nucleofuge, such as halide or tosylate. As shown in Scheme 10.14 for a simple alkyl bromide, the reaction involves three simultaneous bonding changes. A base removes a proton from the carbon adjacent to the C-Br bond a new C-C tt bond is formed and the C-Br ct bond is cleaved. No intermediates are formed, and the reaction is completely concerted. 

There are a number of bimolecular nucleophilic elimination reactions like that of equation (5.188) that take place in a single, concerted step. Since the transition state for such an 2 reaction is odd, being isoconjugate with the pentadienate anion (CH2 CH—CH CH—CH2) , while the reactants and products are even, -type groups at the central carbon atoms can exert mesomeric effects in the transition state that are different from those in the reactants or products. Such eliminations are therefore anti-BEP reactions and are classed as EO. ... 

E2 reaction A bimolecular concerted elimination reaction that usually occurs via a coplanar transition state. 

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