Ei mechanism

The reaction proceeds by an Ei-mechanism. The /3-hydrogen and the carboxy-late are cleaved synchronously from the substrate molecule, while forming a new bond. This elimination reaction belongs to the class of -eliminations in the case of the ester pyrolysis, the substrate molecule passes through a six-membered cyclic transition state 4 ... 

As in the E2 mechanism, it is not necessary that the C—H and C—X bond be equally broken in the transition state. In fact, there is also a spectrum of mechanisms here, ranging from a mechanism in which C—X bond breaking is a good deal more advanced than C—H bond breaking to one in which the extent of bond breaking is virtually identical for the two bonds. Evidence for the existence of the Ei mechanism is... 

First, we consider reactions in which a C=C or a C=C bond is formed. From a synthetic point of view, the most important reactions for the formation of double bonds are 17-1 (usually by an El mechanism), 17-6,17-12, and 17-22 (usually by an E2 mechanism), and 17-3,17-4, and 17-8 (usually by an Ei mechanism). The only synthetically important method for the formation of triple bonds is 17-12. In the second section, we treat reactions in which C=N bonds and C=N bonds are formed, and then eliminations that give C=0 bonds and diazoalkanes. Finally, we discuss extrusion reactions. 

Magnesium alkoxides (formed by ROH- -Me2Mg —>ROMgMe) have been decomposed thermally, by heating at 195-340°C to give the alkene, CEU, and MgO. Syn elimination is found and an Ei mechanism is likely. Similar decomposition of aluminum and zinc alkoxides has also been accomplished. ... 

The mechanism is usually E2. Hofmann s rule is generally obeyed by acyclic and Zaitsev s rule by cyclohexyl substrates (p. 1315). In certain cases, where the molecule is highly hindered, a five-membered Ei mechanism, similar to that in 17-7, has been shown to operate. That is, the OH in these cases does not attract the P... 

The obvious way to distinguish between this mechanism and the ordinary E2 mechanism is by the use of deuterium labeling. For example, if the reaction is carried out on a quaternary hydroxide deuterated on the P carbon (R2CDCH2NMe OH ), the fate of the deuterium indicates the mechanism. If the E2 mechanism is in operation, the trimethylamine produced would contain no deuterium (which would be found only in the water). But if the mechanism is Ei, the amine would contain deuterium. In the case of the highly hindered compound (Me3C)2CDCH2NMe OH , the deuterium did appear in the amine, demonstrating an Ei mechanism for this ca.se. With simpler compounds, the mechanism is E2, since here the amine was deuterium-free. 

As mentioned before, thermal or photolytic treatment of (9-acyl esters (2) generates the corresponding alkyl 2-pyridyl sulfides. Oxidation of the sulfides with raCPBA provides the corresponding sulfoxides, which can be further derived to olefins under heating conditions via Ei mechanism. One example is shown below (eq. 8.8) [26]. 

In water, ionization of the C-Br bond occurs first (Ei mechanism) to give the intermediate resonance-stabilized benzylic zwitterion C. After fast rotation about the C-C bond, carbon dioxide leaves conformer D perpendicularly to the plane of the car-benium ion, to give mainly the most stable ( )-isomer of / -bromostyrene. In butanone, after fast rotation about the C-C bond, elimination of CO2 and Br occurs in a concerted single-step (E2 mechanism) for stereoelectronic reasons (Br and C02 must be anti to one another) to give conformer B, which decomposes exclusively to the thermodynamically less stable (Z)-isomer. In more polar solvents, the partly zwitterionic activated complex, leading to zwitterion C in the rate-determining step, will clearly be more stabilized by solvation than the less dipolar activated complex leading directly to the (Z)-isomer of / -bromostyrene from conformer B [851]. 

Methyl aryl ethers, such as anisole, are cleaved to iodomethane and a phen-oxide ion by treatment with Lil in hot DMR Propose ei mechanism for this reaction. 

The pyrolytic elimination is a model reaction, which probably dominates many pyrolytic processes. The (3 elimination with two groups lost from adjacent atoms is common in pyrolysis. A model pyrolytic elimination takes place with no other reagent present and often requires gas phase. For this reason, the typical E2 mechanism where a proton and another group from a molecule depart simultaneously, the proton being pulled by a base, is not common in pyrolysis in gas phase. The same is true for the Ei mechanism. More common for the gas phase pyrolysis is an E, mechanism. However, for polymers where the pyrolysis takes place in condensed phase, E2 and Ei mechanisms are not excluded. There are also several other mechanisms that have been found to operate in pyrolytic eliminations. 

In some cases, an Ei mechanism appears to be followed and the more stable olefin is formed. Instead of Hofmann s rule, Zaitsev s rule is followed (the double bond goes mainly toward the most highly substituted carbon). Also, in some reactions the direction of elimination is determined by the need to minimize steric interactions, sometimes even when the steric hindrance appears only during the transition state. 

When occurring for large molecules, it is not always possible to assign to the elimination an Ei mechanism. An example is the elimination of water or ethanol during the pyrolysis of cellulose or ethyl cellulose, respectively ... 

This reaction may have either an Ei mechanism or an E mechanism because it takes place in condensed phase. It should be remembered that an E2 reaction occurs as follows ... 

The -elimination with two groups lost from adjacent atoms is another common reaction in pyrolysis, usually taking place with an Ei mechanism and not involving free radicals. An a-atom is the atom bound to a specific group or bond, and any atom adjacent to it is indicated as a p-atom. p-Eliminations or 1,2-eliminations involve, for example, the elimination of a group from a-atom and a hydrogen from the p-atom. For polymers where the pyrolysis takes place in condensed phase, E2 and Ei mechanisms are not excluded. The Ej mechanism involves a cyclic transition state, which may be four-, five- or six-membered [4]. No discrete intermediate is known in this mechanism (concerted mechanism). Two examples of reactions with E mechanism involving different sizes of cyclic transition state are shown below [3] ... 

Ethers with a tertiary, benzylic, or allylic group cleave by an S>jl or Ei mechanism because these substrates can produce stable intermediate carbocations. T hese reactions are often fast and take place at moderate temperatures. ferf-Butyl ethers, for example, react by an El mechanism on treatment with trifluoroacetic acid at 0 °C. WeTl see in Section 26.7 that the reaction is often used in the laboratory synthesis of peptides. 

The trans isomer of the alkene stilbene is formed, as indicated by the presence of the deuterium atom in the product. In this case, elimination is via a cyclic transition state, and so the opposite stereochemistry is obtained than was achieved in the E2 mechanism. This cyclic pyrolytic elimination is labelled the Ei mechanism, which stands for intramolecular, or internal, elimination. 

In those versions of the Ei mechanism that involve a four-or five-membered cyclic transition state, there is a requirement that all the atoms are co-planar or... 

This is an example of a five-membered ring version of the Ei mechanism. Moreover, it is an example of a synthetic reaction that proceeds via two different pathways depending upon the exact conditions. These pyrolytic elimination reactions are valuable synthetically, because there is normally no opportunity for the starting material to rearrange, which is always a possibility when the elimination occurs via the El route, because in that pathway the reaction proceeds via a carbonium ion intermediate. 

The Hofmann exhaustive methylation usually follows the anti-E2 bimolecular elimination pathway. Normally, the hydroxide anion removes the P-hydrogen and so initiates the E2 mechanism. If, however, the quaternary ammonium species is so highly hindered that this is not possible, then the hydroxide anion removes a proton on one of the methyl groups that forms part of the ammonium cation and reacts via a five-membered ring version of the Ei mechanism. 

Classify each step or group of steps as a proton transfer, substitution, elimination, addition, or rearrangement. Then check for other alternative forms of the same process that may fit the reaction conditions better. For example, your first impulse in a particular problem might be to write some sort of elimination because it seems to get you closer to the answer. If you recognize that what you need is a type of elimination, you would know that eliminations could go by El, E2, ElcB, or Ei mechanisms. You can then pick the most appropriate one, rather than remaining with the first one that occurred to you. 

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. 

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