Acids and Bases in Kinetics

Excess Acidity in Aqueous Solutions of Sulfuric, Perchloric, and Hydrochloric Acids As Functions of the Mole Fraction of Acid, with the Standard Error of Estimate, S  

FIGURE 3.1 Excess acidity, X, in aqueous perchloric (light curve), sulfuric (black), and hydrochloric (dashed) acid solutions versus mole fraction of the acid, x. Source Cox (2000), with permission. 

Cox (2000) shows how the concept of excess acidity can be used in kinetic studies as an aid in assigning mechanisms. For example, data (Bell et ai, 1956 Cox, 2000) for the acid-catalyzed depolymerization of the cyclic trimer of formaldehyde (trioxane) 

S represents trioxane (1) and is the reciprocal of the (large) acid dissociation constant of the protonated trioxane (2). 

In contrast, in the A-2 mechanism, the species SH requires the assistance of a nucleophile, usually from the solvent, in the rate-determining step  

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Nelly R.N., Schulman S.G., Proton-Transfer Kinetics of Electronically Excited Acids and Bases, in Molecular Luminescence Spectroscopy Methods and Applications, part 2, Schulman S.G. (ed.), Wiley-Interscience, New York, 1988 pp 461-510. 

The concept that a catalyst provides an alternate mechanism for accomplishing a reaction, and that this alternate path is a more rapid one, has been developed in many individual cases. The basis of this idea is that the catalyst and one or more of the reactants form an intermediate complex, a loosely bound compound which is unstable, and that this complex then takes part in subsequent reactions which result in the final products and. the regenerated catalyst. Homogeneous catalysis can frequently be explained in terms of this concept. For example, consider catalysis by acids and bases.. In aqueous solutions acids and bases can increase the rate of hydrolysis of sugars, starches, and esters. The kinetics of the hydrolysis of ethyl acetate catalyzed by hydrochloric acid can be explained by the following mechanism ... 

Cirilli et al.11171 cloned the gene of diaminopimelate epimerase from Haemophilus influenzae, and purified and crystallized the enzyme. The enzyme is monomeric and has a unique protein fold, in which the amino terminal and carboxyl terminal halves of the molecule fold into structurally homologous and superimposable domains (Fig. 17-13). Cys 73 of the amino terminal domain is found in the disulfide linkage, at the domain interface, with Cys 217 of the carboxy terminal domain 117. Thus, it is most conceivable that these two cysteine residues stay in reduced form in the active enzyme and function as the acid and base in the mechanism. Koo and Blanchard 118 explored a number of kinetic and isotope approaches to clarify the mechanism of the enzyme. However, which of the two cysteine residues is responsible for proton abstraction from the two enantiomeric Ca-H bonds is not yet known. 

Despite the errors that can arise in both the Forster cycle and the steady-state fluorescence the pAT obtained by both methods usually agree to within 1 or 2 pK units (see tables in Ref. 28). For triplet states appUcation of either method is often difficult or even impossible. Instead it is necessary to use a kinetic approach. The amounts of conjugate acid and base in the triplet state are followed by time-resolved excited-state absorption spectroscopy. ... 

R. Kelly and S. Schulman, Proton transfer kinetics of electronically excited acids and bases, in Molecular Luminescence Spectroscopy—Methods and Applications Part 2 (ed. S. Schulman), 1st edn., Wiley-Interscience, New York, 1988, pp. 461-510. 

Kinetic Considerations. Extensive kinetic and mechanistic studies have been made on the esterification of carboxyHc acids since Berthelot and Saint-GiHes first studied the esterification of acetic acid (18). Although ester hydrolysis is catalyzed by both hydrogen and hydroxide ions (19,20), a base-catalyzed esterification is not known. A number of mechanisms for acid- and base-catalyzed esterification have been proposed (4). One possible mechanism for the bimolecular acid-catalyzed ester hydrolysis and esterification is shown in equation 2 (6). 

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. 

An inflection point in a pH-rate profile suggests a change in the nature of the reaction caused by a change in the pH of the medium. The usual reason for this behavior is an acid-base equilibrium of a reactant. Here we consider the simplest such system, in which the substrate is a monobasic acid (or monoacidic base). It is pertinent to consider the mathematical nature of the acid-base equilibrium. Let HS represent a weak acid. (The charge type is irrelevant.) The acid dissociation constant, = [H ][S ]/[HS], is taken to be appropriate to the conditions (temperature, ionic strength, solvent) of the kinetic experiments. The fractions of solute in the conjugate acid and base forms are given by... 

E. Rapid-Reaction Technique Because this technique and the apparatus involved are considered in detail in the following review, only a qualitative discussion is given here. This is the most valuable method for the confirmation of covalent hydration because it can usually give conclusive results even when the percentage of the hydrated species is as low as 2%. It makes use of the facts that aU known examples of the formation or disappearance of the hydrated species followed first-order kinetics and that the rates are both acid- and base-catalyzed. It also depends on the usual state of affairs that the ratio of the hydrated to the anhydrous species, although pH independent (see Section II, A), is different in the three species, i.e. in the cation, neutral species, and anion. In principle, a solution of one... 

Quantitative studies based on kinetic measurements using strongly electrophilic diazonium ions and, as coupling components, 1-naphthol, 2-naphthol-6-sulfonic acid, and resorcinol in aqueous acid were made by Sterba and coworkers (Kropacova et al., 1970 Kavalek et al., 1970 Sterba and Valter, 1972 Machackova et al., 1972a). In a typical case (2,6-dichloro-4-nitrobenzenediazonium ion and 1-naphthol) the dependence of the logarithm of the measured rate constant (ks) on pH was linear with a slope of 1. At pH < 1, however, a practically constant value of ks was obtained. The measured rate constants therefore correspond to Scheme 12-62, in which the first term relates to the reaction of the naphthoxide ion and the second to that of the undissociated naphthol Ka is the acidity constant of 1-naphthol. 

The terms rate, speed, and velocity are all synonymous in chemical kinetics, though this is not so in mechanics. It takes different periods of time to complete different reactions. The neutralization reaction between acids and bases, mentioned earlier as an example of homogeneous reactions, takes place almost instantaneously at room temperature and under atmospheric pressure. However, it takes many days for iron to rust under these conditions. Thus, the rates of reactions that may take place under the same conditions of temperature and pressure may differ very significantly. When carbon or sulfur or phosphorus bums in... 

Although the concepts of specific acid and specific base catalysis were useful in the analysis of some early kinetic data, it soon became apparent that any species that could effect a proton transfer with the substrate could exert a catalytic influence on the reaction rate. Consequently, it became desirable to employ the more general Br0nsted-Lowry definition of acids and bases and to write the reaction rate constant as... 

To summarize, when the kinetic data predict that only bromonium ions or only bromocarbocations are formed, the bromination products are obtained stereospecifically and regiospecifically, respectively, whatever the solvent. Olefin brominations involving open intermediates lead to more solvent-incorporated products in methanol or acetic acid than those involving bridged ions. This chemoselectivity can be interpreted in terms of the hard and soft acid and base theory (Dubois and Chretien, 1978). Methanol assistance to intermediate formation also plays a role in determining product-selectivity (Ruasse et al, 1991). 

Most pyrethroids undergo acid- and base-catalyzed hydrolysis to form the corresponding acid and alcohol (Fig. la), typically with U-shaped pH-rate profiles [8, 40]. The hydrolysis of pyrethroids in water basically obeys first-order kinetics with a half-life simply calculated from hydrolysis rate constant (A obs) as 0.693/kobs. Pyrethroids are generally stable under the acidic and neutral conditions at pH 4—7,... 

Priebe and coworkers [107,178] have attempted to rationalize the product distribution in terms of Pearson s theory of hard and soft acids and bases (HSAB) [179], concluding as a broad generalization that soft bases (S-, N- and C-nucleophiles) form bonds at the softer C-3 electrophilic center, whereas hard bases (O-based nucleophiles) react preferentially at the harder C-l center to give glycosides. They acknowledge that other factors may overrule this interpretation, such as when C-nucleophiles give kinetic C-l-alkylated products whose formation is not reversible. 

These descriptors have been widely used for the past 25 years to study chemical reactivity, i.e., the propensity of atoms, molecules, surfaces to interact with one or more reaction partners with formation or rupture of one or more covalent bonds. Kinetic and/or thermodynamic aspects, depending on the (not always obvious and even not univoque) choice of the descriptors were hereby considered. In these studies, the reactivity descriptors were used as such or within the context of some principles of which Sanderson s electronegativity equalization principle [16], Pearson s hard and soft acids and bases (HSAB) principle [17], and the maximum hardness principle [17,18] are the three best known and popular examples. 

The primary literature now contains a very large body of kinetic data for the catalysis of enolization and ketonization, not only of ketones and aldehydes but also of )3-diketones, )3-keto esters, and dienones, much of which could be treated by the Kurz approach. Also, data exist for third-order enolization, due to combined general acid and base catalysis, that could also be analysed. Such treatment is beyond the scope the present review. However, one study of metal ion catalysis of enolization is discussed later in this section. 

The appearance or disappearance of the U.V. absorption of the carbonyl group can in principle be used for kinetic measurements. Bell and Jensen (1961) applied this method to 1,3-dichloroacetone the reaction is too fast in pure water, but proceeded at a convenient rate in 5% water-I-dioxan mixtures, in which there is about 50% hydration at equilibrium. Catalysis by many acids and bases was observed. Much faster reactions can be studied by relaxation methods, and the pressure-jump technique has been applied to the reaction Me0(OH)2.CO2H MeC0.C02H-hH20 by Strehlow (1962). 

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