Acid base catalysis

To calculate the catalytic constants of the acids H3O+ and CH3.CO2H and the base CHg.COjfor the general acid-base catah sed mutarotation of glucose in water. 

The reaction is strictly first-order. Bronsted and Guggenheim (J. Amer. Chem. Soc. 1927, 49, 2564), using a dilatometric method to follow the reaction at 18 C, obtained the values for the first-order rate constant k given in tables 1 and 2. 

In these solutions primary salt effects and catalj sis by hydroxyl ions are neglected. An approximate value for the acidity constant of acetic acid is 1.8 x 10 mole 1.  

From fig. 1, which relates to the experiments with perchloric acid, we obtain k f n 10 = 0.00530 min- and 

A rough calculation shows that catalysis by the hydrogen ion can be neglected for the third solution of table 2, we have 

Recall that Eqs. 8.48 and 8.50 are called BrDnsted linear free energy relationships. If an acid or base is involved in the rate-determining step of a reaction, the rate of that reaction should depend upon the strength of the acid or base. Hence, a Bronsted correlation is often found. Eqs. 8.51 and 8.52 relate the rate constants for an acid- or base-catalyzed reaction, respectively, to the pfC, of the acid or conjugate acid of the base. The sensitivity of an acid-catalyzed reaction to the strength of the acid is a, whereas the sensitivity of a base-catalyzed reaction to the strength of the base is p. The a and p reaction constants indicate the extent of proton transfer in the transition state. In Chapter 9 we explore the use of these two equations in much more detail, and we apply them in Chapters 10 and 11. 

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 classical example of specific acid-base catalysis is the hydrolysis of esters. The hydrolysis is catalyzed by H30 and OH but not by other acids or bases. The rate of hydrolysis in the absence of acid or base is extremely slow. 

The mechanism of acid hydrolysis of an ester may be illustrated as follows  

The concentration of water does not appear in the rate law, since it is effectively constant during the course of the reaction in aqueous solution. Because of the relation [H ] [OH ] = the rate constant k = /ch+ [H ] -h /cqh-EOH ] has a minimum at a pH that depends on /ch+, and /cqh-- The dependence of log o k on pH is shown schematically in Fig. 32.15. 

1 Describe the application of the isolation method to determine the rate law, Eq. (32.37). 

In aqueous solution, the rates of many reactions depend on the hydrogen-ion (H+ or h3o+) concentration and/or on the hydroxyl-ion (OH-) concentration. Such reactions are examples of acid-base catalysis. An important example of this type of reaction is esterification and its reverse, the hydrolysis of an ester. 

If we use the Bronsted concept of an acid as a proton donor and a base as a proton acceptor, consideration of acid-base catalysis may be extended to solvents other than water (e g., NH CH3COOH, and SO,). An acid, on donating its proton, becomes its conjugate base, and a base, on accepting a proton, becomes its conjugate acid  

For proton transfer between a monoprotic acid HA and a base B, 

In this connection, water, an amphoteric solvent, can act as an acid (monoprotic, with, say, NH, as a base)  

Acid-base catalysis can be considered in two categories (1) specific acid-base catalysis, and (2) general acid-base catalysis. We illustrate each of these in turn in the next two sections, using aqueous systems as examples. 

There are certain reactions, e.g. inversions of sucrose and methane etc. in which the rate of reactions were found to be proportional to the concentration of H+ ions. Similarly, there are reactions which are catalyzed by OH ions, e.g. conversion of acetone into diacetone alcohol or decomposition of nitroso-triacetoneamine. These are known as specific hydrogen ion catalyzed or specific hydroxyl-ion catalyzed reactions. Also there are some reactions in which both H+ and OFF ions act as catalysts probably along with water. The undissociated acid or base have negligible effect on the rate of reaction. The hydrolysis of ester is an example in which both H+ and OH ions act as catalyst 

The first order rate constant is, therefore, given by 

Equation (6.9) can also be written in a more convenient form as 

In most of the cases, one of these terms containing concentration is small compared with other terms and can be neglected. 

Similarly, in 0.1N NaOH solution, the rate constant may be represented as 

Many reactions are accelerated by the presence of H+ or OH-, or both. If both are effective catalysts, then the apparent rate constant over a wide pH range may be expressed by 

This equation contains a further term, ko, representing the solvent (water) term. In any given system, any of these terms may of course be negligible. The values of the catalytic constants and kon can be determined at the pH extremes. This equation states that the rate will be a minimum at an intermediate pH. It is easy to show that this occurs at a pH independent of ko, as expressed by 

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)  

The pH profile for the hydrolysis of methyl aspirin, which shows specific acid-base catalysis. The solid line shows the fit according to Eq. (10-21), and the dashed one where ko = 0. Data are from Ref. 16. 

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. 

Both acetone and water are strongly polar molecules, the dipole moments being 2.84 and 1.84 Debyes, respectively. Since they mix as liquids in all proportions with minor heat evolution, it is reasonable to expect that there is fairly strong dipole-dipole interaction between their molecules. If pressed for structural details of the liquid, we might guess that in dilute acetone solutions, each acetone molecule may be associated with about 4 H2O molecules in some such manner as the following  

In view of these interactions it is probably not surprising to find that acetone will slowly exchange its 0 with H20 and its H atoms with D2O. The reaction with pure H2O is extraordinarily slow, with a half-life for both these reactions of the order of 1 to 10 weeks. With addition of M NaOH or IICl, however, the half-life of the D exchange becomes of the order of an hour or so, while the exchange is almost too fast to measure. It can be shown that the rate is directly proportional to the stoichiometric concentration of H+ or OH in each case and is essentially independent of the other ion (Cl or Na+). 

See compilation by C. P. Smyth, Dielectric Behavior and Structure,McGraw-Hill Book Company, New York, 1955. 

Such observations lead to the designation of reactions as acid- or base-catalyzed. When the catalysis is limited to the species H+ (or OH ), the reaction is spoken of as being subject to specific ion (or OH ion) catalysis. Many reactions of both organic and inorganic chemistry fit such a designation. However, very early work on such systems soon showed that the catalysis was not limited to or OH but did extend to other species which could be subsumed under the category of what are now called Bronsted acids and bases. 

Ill the Bronsted scheme an acid is any substance HA capable of transferring a proton to a second species called a base B. The stoichiometric equation can be written as 

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Edsall, J. T. George Scatchard, John G. Kirkwood, and the electrical interactions of amino acids and proteins. Trends Biochem. Sci. 7 (1982) 414-416. Eigen, M. Proton transfer, acid-base catalysis, and enzymatic hydrolysis. Angew. Chem. Int. Ed. Engl. 3 (1964) 1-19. 

Acid amide herbicides Acid anhydrides Acid azine dyes Acid-base catalysis Acid-base chemistry Acid Black [1064-48-8]... 

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

Volume 90 Acid-Base Catalysis II. Proceedings of the International Symposium on... 

Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori, M. Misono and Y. Ono Volume91 Preparation of Catalysts VI. Scientific Bases forthe Preparation of... 

Modeling of Chemioal Kinetios and Reaotor Design ACID-BASE CATALYSIS HOMOGENEOUS LIQUID PHASE... 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Examples of acid-base catalysis in aqueous solution... 

Much of the study of kinetics constitutes a study of catalysis. The first goal is the determination of the rate equation, and examples have been given in Chapters 2 and 3, particularly Section 3.3, Model Building. The subsection following this one describes the dependence of rates on pH, and most of this dependence can be ascribed to acid—base catalysis. Here we treat a very simple but widely applicable method for the detection and measurement of general acid-base or nucleophilic catalysis. We consider aqueous solutions where the pH and p/f concepts are well understood, but similar methods can be applied in nonaqueous media. 

Bell. R.P. Acid-Base Catalysis Oxford University Press London, 1941 p 3. 

FIGURE 16.11 Specific and general acid-base catalysis of simple reactions in solution may be distinguished by determining the dependence of observed reaction rate constants (/sobs) pH and buffer concentration, (a) In specific acid-base catalysis, or OH concentration affects the reaction rate, is pH-dependent, but buffers (which accept or donate H /OH ) have no effect, (b) In general acid-base catalysis, in which an ionizable buffer may donate or accept a proton in the transition state, is dependent on buffer concentration. 

The Lysozyme Mechanism Involves General Acid-Base Catalysis... 

Eigen, M., 1964. Proton transfer, acid-base catalysis, and enzymatic hydrolysis. Angewandte Chemie, Int. Ed. 3 1—72. 

For exchange of non-labile organic hydrogen atoms, acid-base catalysis (or some other catalytic hydrogen-transfer agent such as palladium or platinum) is required. The method routinely gives tritiated products having a specific activity almost 1000 times that obtained by the Wilzbach method shorter times are required (2-12h) and subsequent purification is easier. 

Bronsted acid/base catalysis (equation, a, p) 113, 210, 355ff.,360ff., 392 Buckminsterfullerene, reaction with ArN 188... 

Gattermann-Koch reaction 230 Gattermann reaction 230 General acid/base catalysis, see Bronsted catalysis... 

One can test for general acid-base catalysis by varying [BH+] and [B] at constant pH. An easy test is to dilute the buffer progressively at a constant ratio of [BH+]/[B], making up any ionic strength change so as not to introduce a salt effect. If the rate is invariant with this procedure, then general acid-base catalysis is absent under the circumstances chosen. 

Weak acids and bases are, generally speaking, less effective catalysts than H+ and OH at the same concentrations. Proton transfer occurs in all acid-base catalysis, regardless of the detailed mechanism (this aspect is considered in the next section). It is only... 

Acid-base catalysis. Interpret the finding that a particular rate constant remains constant when the different values of [OAc"] and [HO Ac] are used such that [OAc" J/[HOAc] remains constant, whereas the rate constant increases with [HOAc] in solutions to which OAc" was not added. The ionic strength was held constant. 

Acid-base catalysis, 232-238 Brqnsted equation for, 233-236 general, 233, 237 mechanisms for, 237 specific, 232-233, 237 Activated complex (see Transition state) Activation enthalpy, 10, 156-160 for composite rate constants, 161-164 negative, 161 Activation parameters, 10 chemical interpretation of, 168-169 energy of activation, Ea, 10 enthalpy of activation (A// ), 10, 156-160... 

Smoluchowski see von Smoluchowski) Solvent cage, 198, 202 Solvent effects. 197-199, 204—206 Specific acid-base catalysis,... 

The HIV-1 protease, like other retroviral proteases, is a homodimeric aspartyl protease (see Fig. 1). The active site is formed at the dimer interface, with the two aspartic acids located at the base of the active site. The enzymatic mechanism is thought to be a classic acid-base catalysis involving a water molecule and what is called a push-pull mechanism. The water molecule is thought to transfer a proton to the dyad of the carboxyl groups of the aspartic acids, and then a proton from the dyad is transferred to the peptide bond that is being cleaved. In this mechanism, a tetrahedral intermediate transiently exists, which is nonconvalent and which is mimicked in most of the currently used FDA approved inhibitors. 

Zechel DL, Withers SG (2001) Dissection of nucleophilic and acid-base catalysis in glycosidases. Curr Opin Chem Biol 5 643-649... 

The ionizable functional groups of aminoacyl side chains and (where present) of prosthetic groups contribute to catalysis by acting as acids or bases. Acid-base catalysis can be eithet specific ot general. By specific we mean only protons (HjO ) or OH ions. In specific acid or specific base catalysis, the rate of reaction is sensitive to changes in the concenttation of protons but... 

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