Acid base catalysis specific

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. 

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. 

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

Combining equations 7.3.4 and 7.3.2 indicates that in aqueous media for specific acid-base catalysis,... 

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. 

In specific acid-base catalysis in aqueous systems, the observed rate constant, kobs, depends on cH+ and/or on c0H-, but not on the concentrations of other acids or bases present ... 

The systematic variation of cH+, cOH-, etc. allows the experimental determination of each rate constant. If the terms in the first summation on the right of equation 8.2-9 predominate, we have general acid catalysis if those in the second summation do so, we have general base catalysis otherwise, the terminology for specific acid-base catalysis applies, as in the previous section. 

By way of illustration we will look at the case of the hydrolysis of mecillinam (XII), which is an antimicrobially active amidopenicillamic acid. This amphoteric dmg can exist as a cation, which we can write as MHJ, as a zwitterion MH or as an anion M. Figure 4.10 shows the pH-rate profile at zero buffer concentration. The reason this plot is so much more complex than that of codeine sulfate is that each of the species present in solution can undergo specific acid-base catalysis to var 4ng extents and so each contributes to the overall profile shown in Fig. 4.10. 

The hydrolysis rate of dmgs in liquid dosage forms is strongly influenced by the pH of the solution and can be catalysed not only by H+ and OH ions (specific acid-base catalysis) but also by the components of the buffer used (general acid-base catalysis). We have looked at the ways in which the effect of the buffer components can be removed so that the pH of maximum stability of the solution can be determined from the pH-rate profile and the rate constants for specific acid-base catalysis can be calculated. 

In the actual experiments, since buffers are used to control pH, the points are extrapolated to zero buffer concentration by making measurements at several buffer concentrations (but at the same pH). Such plots are linear if the reaction is subject to specific acid 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. 

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... 

There are several types of pH-dependent kinetic behavior that can be interpreted in terms of one or more of the various forms of the specific acid-base catalysis relation [equation (7.3.2)]. Skrabal (33) classified the various possibilities that may arise in reactions of this type, and Figure 7.3 is based on this classification. The various forms of the plots of log k versus pH reflect the relative importance of each of the various terms in equation (73.2) as the pH shifts. Curve a represents the most general type of behavior. This curve consists of a region where add catalysis is superimposed on the noncatalytic reaction, a region where neither acid nor base catalysis is significant. 

Sometimes when reactions are catalyzed by acids and bases there is little sign of any effect other than catalysis by hydrated hydrogen ions (usually written as H30" ) or by hydroxide ions. One then speaks of specific acid-base catalysis, and the rate of reaction might be of the form... 

Of greater relevance to our discussion is specific acid-base catalysis, which refers to the case where the rate of a chemical reaction is proportional only to the concentration of hydrogen and hydroxyl ions present. For these type of reactions, / ha and kp,- are negligible, and therefore... 

As the pH increases, the participation of the hydroxide ion in the catalysis becomes increasingly important, and the reaction rate for specific acid-base catalysis becomes... 

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