Elimination, E2, and

However, there are a number of complicating factors to consider. First, the basic conditions needed to form the enolate ions often lead to side reactions such as aldol addition and E2 elimination of RX compounds. Aldol addition is minimized if the carbonyl compound is a ketone with a structure unfavorable for aldol addition or if all of the carbonyl compound is converted to its enolate. To convert all of a simple carbonyl compound to its enolate usually requires a very strong base, such as NH2 in an aprotic solvent or liquid ammonia. Because the enolate anion itself is a strong base, best results are obtained when the halide, RX, does not undergo E2 reactions readily. 

In practice, the same problems of polyalkylation and E2 elimination exist with the amine anion as with the neutral amine—and as far as E2 goes, much more so. 

The term stereoelectronic refers to the effect of orbital overlap requirements on the steric course of a reaction. Thus, because of stereoelectronic effects, the Sw2 substitution gives inversion (see Section 4.2) and E2 elimination proceeds most readily when the angle between the leaving groups is 0° or 180° (see Chapter 7, p. 369). Stereoelectronic effects also play an important role in pericyclic reactions, which are the subject of Chapters 11 and 12. 

DePue, J. S. Collum, D. B. Structure and reactivity of lithium diphenylamide. Role of aggregates, mixed aggregates, monomers, and free ions on the rates and selectivities of N-alkyla-tion and E2 elimination. /. Am. Chem. Soc. 1988, 220, 5524-5533. 

The addition-elimination pathway can become very significant if the SN2 substitution and E2 elimination are largely suppressed, such as in the reaction of 18Ov with neo-C5HuONO. In that case the product ions N160180" and N1603 have been found to be formed in the ratio 20 1... 

Instead, the displaced F- ion re-attacks the newly formed molecule within the complex, leading eventually to the products shown in (42). Although the lifetime of the F- ion/molecule complexes is not known, they must live sufficiently long to allow secondary reactions to occur. Depending upon the nature of the original nucleophile, the re-attack by the displaced F- ion can involve proton transfer, SN2 substitution and E2 elimination. Proton transfer to the displaced F" ions (43) is the dominant reaction if the neutral in the complex is more acidic than HF. This is the case when the primary... 

Until now, discussions have focused only on how carbanions and carbocations behave under conditions favorable for nucleophilic substitutions. However, these species may undergo other types of reactions in which unsaturation is introduced into the molecule. Such reactions are called elimination reactions and should be considered whenever charged species are of importance to the mechanistic progression of a molecular transformation. In previous chapters, SN1 and SN2 reactions were discussed. In this chapter, the corresponding El and E2 elimination mechanisms are presented. 

As already discussed, El and E2 eliminations differ, in part, by the electronic nature of the mechanism. Specifically, El eliminations depend on cationic intermediates, whereas E2 eliminations depend on anionic intermediates. This difference, however, does not eliminate the mechanistic similarities of these reactions as related to the necessary alignment of adjacent chemical bonds. While, as shown in Figure 6.4, El eliminations require alignment of a carbon-hydrogen bond with an adjacent empty p orbital, E2 eliminations, as shown in... 

In this chapter, elimination reactions were presented both independently and in association with their related nucleophilic substitution mechanisms. Furthermore, the processes by which molecules undergo both El and E2 eliminations were presented and explained using bonding and nonbonding orbitals and their required relationships to one another. While much emphasis was placed on the planar relationships of orbitals required for both elimination reaction mechanisms, the special case of frans-periplanar geometries were described as necessary for efficient E2 eliminations to occur. 

The first product has a trisubstituted double bond, with three substituents (circled) on the doubly bonded carbons. It has the general formula R2C=CHR. The second product has a disubstituted double bond, with general formula R2C=CH2 (or R—CH=CH—R). In most El and E2 eliminations where there are two or more possible elimination products, the product with the most substituted double bond will predominate. This general principle is called Zaitsev s rule, and reactions that give the most substituted alkene are said to follow Zaitsev orientation. 

Secondary alkyl halides 8 2 substitution and E2 elimination occur in competition, often leading to a mixture of products. If a... 

In cyclic compounds, there are much greater restrictions on conformational flexibility. In six-membered rings, the antiperiplanar requirement for E2 elimination is satisfied when both the leaving group and the adjacent H atom are axial. Compounds in which such a conformation is readily achievable undergo E2 elimination much more readily than those in which it is not. For example, in menthyl chloride, the C-Cl bond is not antiperiplanar to any adjacent C-H bond in the lowest energy conformation, and E2 elimination is therefore much slower than it is in the diastereomer neomenthyl chloride, in which the C-Cl bond is antiperiplanar to two C-H bonds in the lowest energy conformation. Moreover, two C-H bonds are antiperiplanar to the C-Cl bond in the reactive conformation of neomenthyl chloride, so two products are obtained upon E2 elimination, whereas only one C-H bond is antiperiplanar to the C-Cl bond in the reactive conformation of menthyl chloride, so only one product is obtained. 

The El, ElcB and E2 elimination pathways are all 1,2-elimination reactions. These account for the vast majority of elimination reactions that occur in solution. We will now briefly examine some other pathways that occur under particular conditions. 

J.P. Guthrie, Concertedness and E2 Elimination Reactions Prediction of Transition State Position using Two-Dimensional Reaction Surfaces based on Quadratic and Quartic Approximations, Can. J. Chem., 1990, 68, 1643. 

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