El mechanism, elimination

Hydroxyls can act as nucleophiles, although they are less nucleophilic than amines or thiols. Under acidic conditions, hydroxyls can be eliminated in a dehydration reaction (Fig. 79). Elimination reactions can occur as an El reaction (elimination unimolecular) or E2 reaction (elimination bimolecu-lar). The El elimination mechanism proceeds through formation of a carbo-cation intermediate as the rate-determining step with loss of water whereas the E2 mechanism is second order with the base abstraction of a proton and loss of the leaving group occurring simultaneously (120). 

Scheme 6.1 Dissociation of a proton through hyperconjugation completes the final stage of an El elimination mechanism.

For the compound below, draw an El elimination mechanism and show both major and minor products. 

The best examples of El eliminations are those carried out m the absence of added base In the example cited m Eigure 5 12 the base that abstracts the proton from the car bocation intermediate is a very weak one it is a molecule of the solvent ethyl alcohol At even modest concentrations of strong base elimination by the E2 mechanism is much faster than El elimination... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

An example of an (El)anion mechanism has been found with the substrate 14, which when treated with methoxide ion undergoes elimination to 16, which is... 

The prevailing negative slopes in Table II indicate a strong tendency toward the El-type elimination mechanism, which begins by the splitting of... 

When the reaction proceeds by this pathway, 24 and similar intermediates are not involved and the mechanism is exactly (by the principle of microscopic reversibility) the reverse of El elimination of alcohols (7-1).149 It is likely that the mechanism involves both pathways. 

We note that in Eq. 13-11 we have introduced the El (elimination, unimolecular) reaction, which commonly competes with the SN1 reaction provided that an adjacent carbon atom carries one or several hydrogen atoms that may dissociate. We also note that similar to what we have stated earlier for nucleophilic substitution reactions, elimination reactions may occur by mechanisms between the E2 and El extremes. 

The frequently observed preference for anti-elimination over syn-elimi-nation on alumina (for a summary of earlier results see ref. 7, later especially ref. 96) has been a cause of much controversy. However, as has been explained in Sect. 2.1.2, it is a natural reaction course for concerted elimination, provided that suitably spaced acidic and basic sites are available on the surface. Catalysts which operate by means of the El-like mechanism... 

These relations seem to be valid for the dehydration of primary alcohols, but secondary and tertiary alcohols may need other combinations of acidic and basic sites. It has been observed that the dehydration of tert-butanol was more sensitive to the presence of strongly acidic sites than the reaction of methanol, but both processes required basic sites [8]. All this is in accordance with the dynamic model of elimination mechanisms presented in Sect. 2.1, which allows transition from El to E2 or further to ElcB according to the structure of the reactant and the nature of the catalyst. 

The reaction mechanism of amine deamination and disproportionation has been put forward by analogy with other eliminations, namely dehydration and dehydrochlorination [149,155], its characteristic feature being the cooperation of acidic and basic sites. In the deamination, /3-hydrogen and the NR2 group (R is hydrogen or alkyl) are eliminated by an E2-like mechanism on alumina, but by an El-like mechanism on silica-alumina. The nature of the acidic sites is not clear, protons from surface hydroxyls or aluminium ions are possible candidates. However, it seems... 

The hypothesis of a continuous transition of the elimination mechanism from the extreme El through concerted E2 to the other extreme ElcB with the change of reactant structure and catalyst nature, described in Sect. 2.1, can be easily adopted for dehalogenation also. The data summarised in Sect. 2.4.3 show some inconsistencies but the over-all picture is clear. This can be demonstrated for some selected examples. 

In the dehydrogenation of isobutyric acid, the by-products in addition to CO and C02 are propylene and acetone. Two reaction mechanisms were proposed (340, 341) and the latter is shown in Scheme 9 (340). The formation of methacrylic acid and acetone involves a common intermediate The El elimination of a proton from I yields the methacrylic acid while a nucleophilic SN1 attack of oxide ion produces C02 and acetone (344). On the other hand. 

The mechanism of electrophilic addition of hydrogen chloride to 2-methylpropene as outlined in text Section 6.6 proceeds through a carbocation intermediate. This mechanism is the reverse of the El elimination. The E2 mechanism is concerted—it does not involve an intermediate. 

Fig. 4.4. Energy profile of the C=C-forming step of the four mechanisms according to which the /3-eliminations of Figure 4.3 can take place in principle as a function of the chemical nature of the substituent Het and the reaction conditions. The conceivable starting materials for this step are, depending on the mechanism, the four species depicted on the left, where k is for E2 elimination,2> is for /3-elimination via a cyclic transition state,3> is for El elimination, and4> is for Elcb elimination.

Let us now turn to /3-eliminations that take place via acyclic transition states. There three elimination mechanisms (Figure 4.16) depending on the order in which the C—H and the C—Het bonds of the substrate are broken. If both bonds are broken at the same time, it is a one-step E2 elimination. When first one and then the other bond is broken, we have two-step eliminations. These can take place via the El or the Elcb mechanism. In the El mechanism, the C— Het bond is broken first and the C—H bond is broken second. Conversely, in the Elcb mechanism the C—H bond is broken first, by deprotonation with a base. In this way the conjugate base (cb) of the substrate is produced. Subsequently, the C—Het bond breaks. 

The rate law of this El reaction in the form of Equation 4.19 reveals unimolecularity once more. However, it hides what the more detailed form of Equation 4.18 discloses namely, that the elimination rate increases with increasing CF3C02H concentration, which means that this is a bimolecular El elimination. From Equation 4.18 we can also see that the El rate increases at a given concentration of the acid when a more acidic acid is used. In the end Equation 4.18 implies that the rate of ether cleavage following the El mechanism increases with the basicity of the substrate. 

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