Elimination, unimolecular, conjugate base

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. 

Like El elimination. ElcB requires two steps. Unlike E1, though, the intermediate in El cB is a carbanion, not a carbocation. ElcB stands for Elimination, unimolecular, conjugate base. 

Ad 2 (Addition, Electrophilic, Bimolecular), A -i- An and Hetero Ad Z, p.t. + AdN El (Elimination, Unimolecular), Dn + Dg and Lone-Pair-Assisted El, Ep + p.t. Se2Ar Electrophilic Aromatic Substitution, Ag + Dg ElcB (Elimination, Unimolecular, Conjugate Base), p.t.-i-Ep AdN2 (Addition, Nucleophilic, Bimolecular), AdN + P f ... 

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

Mechanism 20.2 is classified as Elcb (elimination-unimolecular, conjugate base) because the conjugate base of the reactant undergoes unimolecular dissociation. 

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. 

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. 

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