Kinetics and Experimental Observations for ElcB

Although at first glance the ElcB mechanism seems simple, the kinetics of the reaction can be complex. Depending upon the relative rate constants for the individual steps, the kinetics of the reaction can take three different forms. Let s see this by examining the rate expressions for the schematic mechanism given in Eq. 10.70. We treat the carbanion as a transient intermediate and use the steady state approximation (see Section 7.5). This gives Eq. 10.71. 

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. 

When the deprotonation occurs in a reversible step prior to leaving group departure, isotope scrambling from the solvent to the site of deprotonation will occur. With a carbonyl containing reactant, the a-hydrogens will become deuterated at a rate faster than elimination if the reaction is performed in a deuterated protic solvent. 4-Methoxy-2-butanone is an example of a reactant that shows all the elimination attributes discussed here (Eq. 10.73). Note, as we mentioned above, these eliminations can occur with poor leaving groups— in this case methoxide. Such reactions are called EIcBr, where the R indicates a reversible first step. 

If in Eq. 10.70 k2 k i, Eq. 10.71 reduces to Eq. 10.74. Now the reaction is first order in both reactant and base under all experimental conditions, and the kinetics is identical to E2 (Eq. 10.68 A). However, because k2 is not in the expression, changing the leaving group does not have as large an effect on the rate of the reaction as on the rate of an E2 reaction. 

As stated, the prediction is that one can distinguish this mechanism from that of a simple E2 reaction by changing the leaving group. 4-Benzoyl-2-butanone is a reactant that follows the experimental predictions discussed here, where substitutions on the benzoyl group do not affect the rate (Eq. 10.75). These reactions are called ElcBi ., where the irr indicates an irreversible first step. 

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