Specific acid and base catalysis

Taking into consideration specific acid and base catalysis (the concentration of a catalyst is reflected in the rate law but is not reflected in the equilibrium constant), the hydrolysis rate term for hydrolyzable chemicals can be described by  

Because k yj is a pseudo first-order rate constant at a fixed pH (i.e., the hydrolysis is independent of RX concentration), the half-life for hydrolysis can be calculated from Equation 2.6. 

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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. 

For a complete evaluation of the stability of the dmg, we need to evaluate the catalytic coefficients for specific acid and base catalysis and also to determine the catalytic coefficients of possible buffers which we might wish to use in the formulation. 

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. 

I, pp. 162-8 jencks PP- uses the selectivity—reactivity relationship between Br nsted slopes and nucleophilic reactivity to distinguish between general acid catalysis and specific acid—general base catalysis. 

A reaction catalyzed by undissociated acid will have the dependence of log k on pH shown in Figure 3g. Specific acid and specific base catalysis are presumed to be absent. If specific and general acid and base catalysis are both operative, one is able to obtain a variety of interesting log k versus pH curves, depending on the relative contributions of the different terms in various pH ranges. Curves i and j of Figure 7.3 are simple examples of these types. 

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. 

A distinction is often made between specific catalysis by hydrogen and hydroxyl ions specific hydrogen and hydroxide-ion catalysis) and catalysis by acids and bases in general general acid and base catalysis). Actually such a distinction is somewhat artificial, as we shall see. As an example of general base catalysis, consider the simple mechanism... 

This is typical acid-base catalysis, known as specific acid-base catalysis because the specific acid and base involved are (or H30 ) and OH". The form of the pH dependence of the rate tells... 

For the earlier discussion of acid and base catalysis, and an outline of what the terms specific and general acid/base catalysis mean, see Chapter 12, pp. 262-264. 

Acid (and base) catalysis, however, is not just controlled by the pH of the solution, which quantifies variation in [H ] (or [HO ]), but can also be influenced by the concentration of other weaker acids in solution in addition to the conjugate acid of solvent. To kinetically distinguish the two options, the terms specific and general acid catalysis have been defined according to eqn (3.6) and (3.7) where is the pseudo first-order rate constant (s ) for the acid-catalysed reaction of substrate. The former refers to a reaction showing a kinetic dependence only on the concentration of the conjugate acid of solvent (eqn (3.6)), whereas the latter (eqn (3.7)) also involves a... 

It has already been mentioned that the K, in NCW is several orders of magnitude greater than in water at room temperature. Thus, as shown previously, acid and base catalysis can be facilitated without the use of additional acid. Certainly CO2 reacts with water to form carbonic acid and, as a consequence, the concentration of hydronium ion in NCW can be increased by enriching the medium with CO2. From an environmental point of view this procedure wiU not only facilitate specific acid-catalyzed reactions but will not require neutralization of the acid after the reaction is complete. A simple cooling and depressurization will eliminate the CO2 and phase separates the product(s) of reaction. Thus, Aleman et al. have reported that the conversion of mesitoic acid to mesitylene over a period of 120 min at 250°C increased from 50 to 80% in the presence of 10 bar (rt) of CO2. Hunter and Savage reported the dehydration of cyclohexanol in water at 250 and 275°C and the reaction of p-cresol with tert-butanol in water at 275°C in the absence and presence of CO2. Their results indicated that in the presence of CO2 the rate of dehydration of the cyclohexanol increased by more than a factor of 2 and the rate of formation of 2-tert-butyl-4-methylphenol increased 40-120%. Modest increases in rate were reported for the hydration of cyclohexene to cyclohexanol. 

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. 

Pohl and Osterholtz [50] used C and Si NMR to investigate the condensation of alkylsilanetriol to bis-alkyltetrahydroxydisiloxane in buffered aqueous solutions as a function of pD (-loglDjO" ]). The second-order rate constants for the disappearance of the triol are plotted in Fig. 18a as a pD rate profile. The slopes of the plot above and below the rate minimum at pD 4.5 are -t-1 and -1, respectively, indicating that the condensation is specific acid- and base-catalyzed. Assink [30] investigated the acid-catalyzed condensation of TMOS over a narrow range of pH. He observed that the condensation rate was proportional to [H30 ], which is also consistent with specific acid catalysis. 

The role that acid and base catalysts play can be quantitatively studied by kinetic techniques. It is possible to recognize several distinct types of catalysis by acids and bases. The term specie acid catalysis is used when the reaction rate is dependent on the equilibrium for protonation of the reactant. This type of catalysis is independent of the concentration and specific structure of the various proton donors present in solution. Specific acid catalysis is governed by the hydrogen-ion concentration (pH) of the solution. For example, for a series of reactions in an aqueous buffer system, flie rate of flie reaction would be a fimetion of the pH, but not of the concentration or identity of the acidic and basic components of the buffer. The kinetic expression for any such reaction will include a term for hydrogen-ion concentration, [H+]. The term general acid catalysis is used when the nature and concentration of proton donors present in solution affect the reaction rate. The kinetic expression for such a reaction will include a term for each of the potential proton donors that acts as a catalyst. The terms specific base catalysis and general base catalysis apply in the same way to base-catalyzed reactions. 

A reaction with a rate constant that conforms to Eq. (10-21)—particularly to the feature that the catalysts are H+ and OH-, and not weak acids and bases—is said to show specific acid-base catalysis. This phenomenon is illustrated by the kinetic data for the hydrolysis of methyl o-carboxyphenyl acetate16 (the methyl ester of aspirin— compare with Section 6.6) ... 

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