ElcB reaction rate-limiting step

In Figure 10.7 the transition structure for the Elcb reaction indicates that the /8-proton has already been abstracted by a base, so the rate-limiting step is detachment of the leaving group. As discussed on page 642, there are many variations of the Elcb mechanism. 

In contrast to the El reaction, which involves a carbocation intermediate, the ElcB reaction takes place through a carbanion intermediate. Base-induced abstraction of a proton in a slow, rate-limiting step gives an anion, which expels... 

When the deprotonation occurs in a reversible step, leaving group departure is rate-limiting, k.i lc2, and Eq. 10.71 reduces to Eq. 10.72. Recall from Chapter 5 that the ratio of a base to its conjugate acid (here [B ] / [HB]) sets the pH of a buffer, and is equal to R.,/ [H, where is the acid dissociation constant of the conjugate acid of the base. We find that the rate of the elimination does not depend upon the concentration of the base, if the pH is kept constant. When the rate of a reaction that involves a base added to the solution depends only upon the pH, it is specific-base-catalyzed (see Section 9.3.2). Hence, many standard ElcB reactions are specific-base-catalyzed eliminations. 

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