Ei reaction

The mechanism is Ei (see p. 1322). Lactones can be pyrolyzed to give unsaturated acids, provided that the six-membered transition state required for Ei reactions is available (it is not available for five- and six-membered lactones, but it is for larger rings ). Amides give a similar reaction but require higher temperatures. 

Analogously, solvent effects on alkene formation in Sn 1 and Ei reactions can be predicted [16, 44]. Owing to the fact that the first step in both reactions, the heterolysis of the C—X bond, is exactly the same, we have to consider the activated complexes which lead to either the alkene or substitution product. 

Ab initio and density functional calculations of the thermal syn elimination transition states for Ei reaction of organic amine oxide, sulfoxide, and phosphoxide have confirmed the expected planar geometry and known order of reactivity. ... 

Example 3.11. An Ei reaction Pyrolytic elimination from a sulfoxide. 

From what we already know of carbonium reactions we see that Ei reactions should indeed lead to the more highly branched olefin. 

Scheme 1.22. The naming convention for epoxy groups of TGDDM before (Eq) and after (Ei) reaction with amine has occurred. Reproduced with permission from St John (1993).

The E2 reaction proceeds exclusively via anti elimination whereas the Ei reaction is capable of both. Rotate the carbon-carbon bond to postion for syn and anti elimination. Eliminate the H and Br as highlighted and then look down the axis from the front to back carbon and imagine a double bond has formed. Translate this into the products shown. 

For esters, deprotonation is effected with triethyl amine (which favors E2), while El ionization is disfavored because it requires moving the bulky terr-butyl group into planarity. For thioesters, Ei reaction is facilitated by the thiophenyl group, while E2 reaction is slowed by use of the bulky diisopropyl ethyl amine. ... 

Alkyl halides have the same order of reactivity in S l reactions as they do in El reactions because both reactions have the same rate-determining step—dissociation of the alkyl halide (Table 11.5). This means that all alkyl halides that react under Sn1/E1 conditions will give both substitution and elimination products. Remember that primary alkyl halides do not undergo SnI/EI reactions because primary carbocations are too unstable to be formed. 

Alcohols and ethers undergo Sfj2/E2 reactions unless they would have to form a primary carbocatlon. In which case they undergo S I/EI reactions. 

Cis and trans isomers of 2-methylcyclohexanol were used by Kuhlmann et al. [103] to probe whether the eliminations occurred via an Ei or E2 mechanism. That is, higher reactivity of the cis isomer could indicate the bimolecular E2 pathway because only in this isomer could the leaving group assume the required antiperiplanar conformation. Equal conversion rates would be expected for Ei reactions. Cis- and /rans-2-methylcyclohexanol were eliminated to 1-methylcyclohexanol in low yield but high selectivity (100%) in pure superheated water at 300 °C [103]. However, the treatment of the trans isomer at 270 °C yielded a mixture of methylcyclohexenes at a conversion of about 70%. Similar results were obtained for c -2-raethylcyclohexanol however, 1-methycyclohexene was more predominant than double-bond migration products. These results are still not sufficient to elucidate the mechanism or the function of water in the dehydration. Dehydration of neopentyl alcohol or pentaerythritol, concomitant with the carbon-bond migration, did not occur within 60 min at 250-300 °C. None of the alcohols examined were dehydrated to ethers. 

The abbreviation El staixls for Elimination, unimolecular. The mechanism is called unimolecular because the rate-limiting transition state involves a single molecule rather than a collision between two molecules. The slow step of an EI reaction is the same as in the SnI reaction unimolecular ionization to form a carbocation. In a fast second step, a base abstracts a proton fr n the carbon atom adjacent to the C . The electrons that once fwmed the carbon-hydrogen bond now form a pi bond between two carbon attxns. The general mechanism for the E1 reaction is shown in the following Key Mechanism box. 

The EI reaction almost always occurs together with the S l. Whenever a carbocation is formed, it can undergo either substitution or elimination, and mixtures of products often result. The following reaction shows the formation of both elimination and substitution products in the reaction of r-butyl bromide with boiling ethanol. 

Figure 7.19. A representation of the course of a generalized SnI/EI reaction. The ratedetermining transition state leads to a carbocation that subsequently partitions between products of substitution and elimination.The transition state for elimination/row the common intermediate is generally higher than that for substitution.

Mechanism of the Ei reaction. Two steps are involved, the first of which is rate-limiting, and a carbocation intermediate is present. 

To predict the products of the reaction of an alkyl halide with a nucleophile/base, you must first decide whether the reaction conditions favor Sn2/E2 or SnI/EI reactions. (Recall that the conditions that favor an Sn2 reaction also favor an E2 reaction, and the conditions that favor an SnI reaction also favor an El reaction.)... 

Now let s look at what happens when conditions favor SnI/EI reactions—that is, a poor nucleophile/weak base. Recall that in Sn1/E1 reactions, the alkyl halide dissociates to form a carbocation, which can then either combine with the nucleophile to form the substitution product or lose a proton to form the elimination product. 

When Sn1/E1 conditions are favored, tertiary alkyl halides, allylic halides, and benzylic halides can form both substitution and elimination products primary and secondary alkyl halides do not undergo SnI/EI reactions. 

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