Specific acid or base catalysis

The acid-base catalysis illustrated in these equations is termed general to distinguish it from specific acid or base catalysis in which the catalyst is the proton or hydroxide ion. 

In acid-base catalysis, both an acid (or base) and its conjugate base (or acid) take part in different reaction steps and are eventually restored. Such reactions are first order in acid (or base) if the link-up with that species controls the rate, or first order in H+ (or OH") if a subsequent step involving the conjugate base (or acid) does so. Traditionally, the first alternative is called "general" acid or base catalysis the second, "specific" acid or base catalysis. However, this distinction is not always applicable as there may be no clear-cut rate-controlling step, and reversibility of later steps may produce a more complex behavior. 

There are many chemical reactions that are catalyzed by acids or bases, or by both. The most common acid catalyst in water solution is the hydronium ion and the most common base is hydroxyl ion. However, some reactions are catalyzed by any acid or by any base. If any acid catalyzes the reaction, the reaction is said to be subject to general acid catalysis. Similarly, general base catalysis refers to catalysis by any base. If only hydronium or hydroxyl ions are effective, the phenomenon is called specific acid or base catalysis. 

We should not expect that a given reaction will exhibit only general acid (or base) catalysis or specific acid (or base) catalysis. In principle, reactions may be subject to more than one kind of catalysis. For example, the catalytic rate expression for the reaction of iodine with acetone in buffer solutions was determined to be... 

Water is an amphoteric compound, and thus it can lend all types of assistance, whichever is required in a given process, or even a combination of several. It may itself be a general acid or base catalyst, or, in the presence of other acids and bases, serve as an environment deploying proton or hydroxide ion for specific acid or base catalysis. 

The mechanism of each of the four possible combinations may involve an Arrhenius or a van t Hoff intermediate. This leads to eight possible mechanisms, schematically presented in Table 13.2. Each of these mechanisms can be developed using either the preequilibrium or the steady-state approximation to arrive at the corresponding rate law. The lessons that can be learnt from the treatment of these mechanisms are also indicated in Table 13.2 some mechanisms lead exclusively to specific acid or base catalysis, while others lead to general acid or base catalysis. Furthermore, the specific catalysis is associated with the existence of a limiting rate, that is the rate that will not increase indefinitely with the H+, or OH , concentration, but attain a limiting valne eqnal to 2[S]o-... 

The rate of a reaction that shows specific acid (or base, or acid-base) catalysis does not depend on the buffer chosen to adjust the pH. Of course, an inert salt must be used to maintain constant ionic strength so that kinetic salt effects do not distort the pH profile. 

In the remainder of this chapter, well look at specific examples nucleophilic addition reactions. In so doing, well be concerned both wl the reversibility of a given reaction and with the acid or base catalysis of that reaction. Some nucleophilic addition reactions take place reversibl) and some do not. Some occur without catalysis, but many others require acid or base to proceed. 

In this equation, is the experimentally determined hydrolytic rate constant, /Cq h the uncatalysed or solvent catalysed rate constant, and /CgH- te the specific acid- and base-catalysis rate constants respectively, ttd ky - are the general acid- and base-catalysis rate constants respectively, and [HX] and [X ] denote the concentrations of protonated and unprotonated forms of the buffer. 

As we can see from Fig. 4.9, this dmg is very stable in unbuffered solution over a wide pH range but degrades relatively rapidly in the presence of strong acids or bases. Since the influence of buffer components has been removed, this plot allows us to calculate the rate constants for specific acid and base catalysis. Removing the terms for the effect of buffer from equation (4.44), we have... 

Observed pH rate profiles typically consist of several regions including segments with linear dependence on [H+] or [ OH], pH-independent, and curved transitions between linear areas. The occurrence of [H+] (or [ OH]) in the rate expression indicates either that a protonated (or deprotonated) form of the reactant is involved (preequilibrium) or that H+ (or OH) is involved in the rate-determining step. Figure 3.28 shows some pH dependencies that may be components of a specific profile. Curves (a) and (b) show linear dependence on [H+] and [ OH] that is due to specific acid and base catalysis, respectively. The horizontal portion of the profile corresponds to a reaction that does not involve acid or base catalysis. Usually the slope of the linear part of the curve is -1(H+) or - -l( OH) because there is only one protonation (or deprotonation) step. 

We have indicated how to determine the various kinetic constants appearing in the expression for specific acid and base catalysis. Let us now consider how to evaluate the various contributions to the rate constant in the case of general acid-base catalysis. For reactions of this type in a solution of a weak acid or base and its corresponding salt, the possible catalysts indicated by equation (7.3.3) are the hydro-nium ion, the hydroxide ion, the undissociated weak acid (or base), and the conjugate base (or acid), In the case of acetic acid the general acid would be the neutral CHjCOOH species and the conjugate base would be the acetate ion (CH3COO"). In this case the apparent rate constant can be written as... 

The kinetics for ligand exchange of the water-soluble (11 R = CH2CH2C02 ) as a function of pH and thiol concentration are in accord with four reversible mer-captan-lyate species exchange reactions followed by product formation through specific acid and base catalysis. It has also been found that (11) (R = Me, Bu , or Bu ) represent the basic forms of weak acids, with pKa, comparable to those of carboxylic acids. 

---