Elimination, unimolecular, conjugate base reactions

The ElcB (elimination, unimolecular, conjugate base) reaction... 

In Chapter 10 we looked at El reactions (two-step elimination reactions that form a car-bocation intermediate) and E2 reactions (concerted elimination reactions). The preceding base-catalyzed dehydration represents the third kind of elimination reaction— namely, an ElcB (elimination unimolecular conjugate base) reaction, a two-step elimination reaction that forms a carbanion intermediate. ElcB reactions occur only if the carbanion can be stabilized by electron delocalization. 

When this mechanism does occur, the second step often determines the rate. Because this step involves a unimolecular reaction of the conjugate base of the initial reactant, the mechanism is designated as elimination, unimolecular, conjugate base—or Elcb. 

This type of elimination is known as an ElcbR (elimination, unimolecular, conjugate base, reversible) reaction, and a generalized reaction coordinate diagram is shown in Figure 10.14. Such reactions exhibit C s-H/D exchange and a 1° hydrogen kinetic isotope effect (ku/ko) of 1.0. An example of an ElcbR is shown in Figure 10.15. ... 

Just as ion pair intermediates are important in substitution reactions, they may also play an important role in elimination reactions. Figure 10.16 shows an example of an Elcbjp (elimination, unimolecular, conjugate base, ion pair) mechanism. Here again there is a fast preequilibriiun formation of a carbanion. In this case, however, the carbanion and the cation are held together as an ion pair due to Coulombic forces that are not overcome by solvation. 

This reaction type is called the ElcB mechanism, which stands for unimolecular elimination conjugate base reaction, because the conjugate base of the starting material is being formed as the reactive intermediate. It is sometimes called the carbanion mechanism. As this mechanism results from the removal of a proton, it is not surprising that it is favoured by those substrates that possess an acidic hydrogen atom. Thus, would you expect the ElcB mechanism to be more prevalent in reactions that result in a carbon/carbon double bond or in reactions that result in a carbon/carbon triple bond ... 

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. ... 

Ingold examined the elimination 7 and found that it was first order in both hydroxide ion and quaternary ammonium ion . The reaction was described as having an E2 (elimination, bimolecular) mechanism, though Ingold noted that a second mechanism, namely Elat, (elimination, unimolecular, from the conjugate base (3) of the substrate), would also fit the kinetic data. Attempts have been made to distinguish between these two mechanistic paths. Since the iSlob mechanism involves an equilibration between the quaternary ion (1) and the zwitterion intermediate (3) it should be possible to observe an 14 ... 

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