Elcb Eliminations unimolecular

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. 

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. 

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. ... 

Unimolecular Elcb Eliminations Energy Profile and Rate Law... 

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. 

Sfsjl, S)s]2, El, ElcB, E2—how can you keep it all straight and predict whal will happen in any given case Will substitution or elimination occur Wil the reaction be bimolecular or unimolecular There are no rigid answers tc... 

ElcB reaction (Section 11.10) A unimolecular elimination reaction in which a proton is first removed to give a car-banion intermediate, which then expels the leaving group in a separate step. 

This rate law contains only one concentration variable—the term [nitroalcohol]0 is, of course, a constant and not a variable—and thus refers to a unimolecular process. This elimination is therefore designated Elcb. 

Thus, this elimination takes place bimolecularly. Therefore, the designation Elcb for this mechanism is justified only if one thinks of the unimolecularity of the rate-determining step. 

Unimolecular Elimination, Conjugate Base (Elcb) Focus On Box, pages 333—334... 

The rate-determining elimination step in an ElcB reaction is unimolecular, so you might imagine it would have a first-order rate equation. But, in fact, the rate is also dependent on the concentration of base. This is because the unimolecular elimination involves a species—the anion—whose concentration is itself determined by the concentration of base by the equilibrium we have just been discussing. Using the following general ElcB reaction, the concentration of the anion can be expressed as shown. 

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