The E2, El and Elcb Mechanisms

An elimination reaction—the expulsion of a small molecule from an organic substrate—can be classified according to the relative placement of the carbon atoms from which elimination occurs  

The jS-eliminations can be further subdivided by closer examination df the mechanisms involved. The three distinct mechanisms that can operate in such reactions are outlined below  

We will shortly discuss the most important structure-reactivity features of the E2, El, and Elcb mechanisms. The variable transition state theory allows discussion of reactions proceeding through transition states of intermediate character in terms of the extreme mechanistic types. The most important structural features to be considered in such a discussion are (1) the nature of the leaving group, (2) the nature of the base, (3) steric factors in the substrate, and (4) solvent effects. 

E2 reactions are distinguished from E1 reactions in that the base is present in the transition state for the rate-determining step. The reactions therefore exhibit overall second-order kinetics. The precise nature of the transition state is a function of 

The nature of the transition state is of great importance, since it controls the direction of -elimination in compounds in which the new unsaturation can be introduced in one of several possible positions. These orientation effects are discussed in the next section. 

Variable-transition-state theory of elimination reactions. J. F. Bunnett, Angew. Chem. Int. Ed. Engl 1, 225 (1962) J. F. Bunnett, Surv. Prog. Chem. 5, 53 (1969) W. H. Saunders, Jr., and A. F. Cockerill, Mechanisms of Elimination Reactions, Wiley, New York, 1973, pp. 48-55 D. J. McLennan, Tetrahedron 31, 2999 (1975) W. H. Saunders, Jr., Acc. Chem. Res. 

There is another useful way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbonium ion, whereas the Elcb proceeds via a carbanion intermediate. 

As depicted, the E2 mechanism involves a bimolecular transition state in which removal of a proton P to the leaving group is concerted with departure of the leaving group. In contrast, the rate-determining step in the El mechanism is the unimolecular ionization of 

The 6-eliminations can be subdivided on the basis of the mechanisms involved. Three distinct limiting mechanisms are outlined below. 

The presence of a substituent on the ethyl group that stabilizes the carbocation intermediate lowers the right-front corner of the diagram, which corresponds 

2 illustrates one of the distinguishing characteristics of E2 reactions. Both the a- and (3-carbons show isotope effects because rehybridization occurs at both carbons. Entry 

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The (3-hydride elimination reactions can be classified into three distinct mechanisms which are the E2, El, and Elcb mechanisms.3... 

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