ElcB mechanisms, leaving-group

We will discuss shortly the most important structure-reactivity features of the E2, El, and Elcb mechanisms. The variable transition state theoiy allows discussion of reactions proceeding through transition states of intermediate character in terms of the limiting 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) electronic and steric effects of substituents in the reactant molecule, and (4) solvent effects. 

Elimination of hydrogen halides from polyfluoroalkanes by bases also usually involves earbanion intermediates (ElcB mechanism) [<8/l, and orientation is there tore governed by relative C H acidities and leaving group mobility Some examples are shown in equations 16-18 [145]... 

These reactions are often promoted by a strong base, which assists the departure of the proton. X is the leaving group. Both El and E2 mechanisms are known, as is a variant designated Elcb, for unimolecular elimination from the conjugate base of the substrate. ... 

In the ElcB reaction, C-H bond-breaking occurs first. A base abstracts a proton to give an anion, followed by loss of the leaving group from the adjacent carbon in a second step. The reaction is favored when the leaving group is two carbons removed from a carbonyl, which stabilizes the intermediate anion by resonance. Biological elimination reactions typically occur by this ElcB mechanism. 

In the El mechanism, X leaves first, and then H. In the E2 mechanism, the two groups leave at the same time. There is a third possibility the H leaves first, and then the X. This is a two-step process, called the ElcB mechanism, or the carbanion mechanism, since the intermediate is a carbanion ... 

Of the three cases of the ElcB mechanism, the one most difficult to distinguish from E2 is (ElcB)i. One way to make this distinction is to study the effect of a change in leaving group. This was done in the case of the three acenaphthylenes (12), where it was found that (1) the three rates were fairly similar, the largest being only about four times that of the smallest, and (2) in... 

Effect of Solvent on El versus E2 versus ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB... 

Saunders (1997) used quantum-chemical methods in the most recent attempt to establish the mechanistic importance of negative ion hyperconjugation. Earlier, Saunders (1976) had proposed that one could distinguish between an E2 and an irreversible Elcb(irr) mechanism (Lowry and Richardson, 1987) by measuring the leaving group KIE for the reaction. The... 

The ElcB mechanism has the same features as the E2 mechanism except that proton abstraction by the base proceeds essentially to completion prior to departure of the leaving group. A variant of this mechanism may intervene whenever the leaving group is a poor leaving group or an exceptionally stable carbanion may be formed (i.e., due to the presence of Z substituents in addition to the polar a bond and/or a hybridization effect). The factors which lead to stabilization of carbanions have been discussed in Chapter 7. 

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