Elcb Eliminations bimolecular

Fig. 4.39. A bimolecular Elcb elimination and its energy profile K refers to the constant of the acid/base equilibrium.

It just turns out that the termoiecular rate constant k3 is actually the product of an equilibrium constant K and a genuine bimolecular rate constant k2 such that k3 = Ki x k2. You saw a similar thing in the rate expressions for amide hydrolysis (Chapter 13) and ElcB elimination (Chapter 19, p. 497)... 

The ElcB Mechanism (Elimination, Monomolecular, Conjugate Base) 5.1,2.3 The E2 Mechanism (Elimination, Bimolecular)... 

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 ... 

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... 

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. 

The observed first-order kinetics and independence of the nature of the attacking base make the bimolecular E2 or ElcB mechanisms unlikely. The El mechanism or iraws-elimination would involve the formation of the amide anion NR. Even when this cannot be ruled out, this possibility seems less probable in aqueous solutions. A synchronous mechanism, possibly with ring formation, seems to be more probable. 

Many of the early studies of rate effects of aqueous micelles were on reactions in which OH acted as a nucleophile, e.g., in ester saponification, but cationic micelles also speeded bimolecular eliminations in which OH is a base [53-55], and ester hydrolyses by the ElcB mechanism in which the first step of reaction is an equilibrium deprotonation [56]. 

The sulfene formation may occur by either a stepwise ElcB or a bimolecular E2 elimination mechanism depending on the particular reaction. 

Among the different types of elimination processes only bimolecular pathways will be in agreement with Eq. 12.1 and hence we will center the discussion on E2 and ElcB mechanisms. Prior to flie discussion of flie experimental data, it may be necessary to talk about the main features of the E2 and ElcB reactions in detail. 

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