Elimination El reaction

Thus in the above case the elimination product is found to contain 82 % of (7). Unexpected alkenes may arise, however, from rearrangement of the initial carbocationic intermediate before loss of proton. El elimination reactions have been shown as involving a dissociated carbocation they may in fact often involve ion pairs, of varying degrees of intimacy depending on the nature of the solvent (cf. SN1, p. 90). 

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

Mechanism of the SnI substitution and El elimination reactions of tert-butyl bromide (2-bromo-2-methylpropane). 

Unlike the E2 reaction, the relative stereochemistry of the leaving group and the hydrogen is not important in the El elimination reaction. In the first step of the reaction, the leaving group departs, producing a planar carbocation. Only at this point must the C—H bond be aligned parallel to the empty p orbital of the carbocation so that pi over-... 

El elimination reactions, several potential products can be identified. These are illustrated below using arrow pushing. 

This is a solvolysis reaction where the alcohol is protonated and water leaves, generating a carbocation. Because protons adjacent to carbocations are acidic and, therefore, participate in El elimination reactions, several potential products can be identified. These are illustrated below using arrow pushing. 

Like Problems 3(b) and 3(c), this is a solvolysis reaction. However, due to the increased complexity of the starting compound, the potential product mixture is more complex. Specifically, if we consider the initial solvolysis step and elimination of water, we notice that an allyl carbocation is formed that is adjacent to a migratable hydrogen atom. While this carbocation can undergo an El elimination reaction, a... 

Selective formation of the more substituted a-aminoalkyl radical was, however, observed for amines of the type Me2NCH2R, where R is a radical stabilizing group with minimal steric requirements (R=CH2=CH-, Ph-, HCsC-, and C2H5CO2) [125]. It has been proposed that the base strength can also affect the selectivity of deprotonation, as observed in the formation of the less substituted alkene as the base strength is increased in El elimination reactions [122, 126, 127]. 

A THC and other active constituents are believed to be metabolically converted to substances oxidized at C(7), and this oxidation has been carried out by osmium tetroxide hydroxylation of A THC (333) and subsequent removal of the tertiary hydroxy-group (Scheme 20). The isomerization of the double bond to A THC is most conveniently carried out by photo-isomerization in propan-2-ol and xylene for 14 days, " although a second method is available that depends on an El elimination reaction of the hydrogen chloride adduct of A THC, using a sterically hindered base (after blocking the phenolic hydroxy-group). ... 

ThomsonNOW Click Organic Process to view an animation showing the mechanism of an El elimination reaction. 

In the dehydration the ketone is first converted into an enol by protonation and deprotonation. Protonation of the alcohol O is then followed by an El elimination reaction (loss of H2O and then loss of H+) to give the product. As in the formation of the enol ether discussed earlier, the principle of microscopic reversibility suggests that the ketone must be converted into an enol before El elimination occurs. 

This is the mechanism for the El elimination reaction. The similarities with the SN1 reaction are plain. Like the E2 and SN2 reactions that often... 

We have seen that an El elimination reaction takes place in two steps. The leaving group leaves in the first step, and a proton is lost from an adjacent carbon in the second step, following Zaitsev s rule in order to form the more stable alkene. The carbocation formed in the first step is planar, so the electrons from a departing proton can move toward the positively charged carbon from either side. Therefore, both syn and anti elimination can occur. 

If the two reactions described in this section were El elimination reactions, what value would you expect to obtain for the deuterium kinetic isotope effect ... 

In Chapter 17 you saw that El elimination reactions usually give mainly E alkenes (there s an example earlier in this chapter) because the transition state leading to an E double bond is lower in energy than that leading to a Z double bond. In other words, El reactions are stereoselective, and their stereoselectivity is kinetically controlled. E2 reactions are similar if there is a choice of protons that can be removed the E alkene is preferred, but a mixture is still formed. Again, this is kinetic control. 

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