Acid and base catalysis, general

Species World Avg. River World Avg. Seawater World Interstitial Water d 

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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. 

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 addition of buffers is required to maintain constant pH during the reaction when experiments are to be carried out in the range 3 < PH <11. However, keto-enol tautomerization reactions usually exhibit so-called general acid and base catalysis. 26 The observed rate acceleration with increasing buffer concentration implies that the components of the buffer participate in some rate-determining step of the reaction. In most cases, the rate of reaction increases linearly with increasing buffer concentration at constant buffer ratio, chb/cb3 = const (Fig. 4a). 

Here, A soivent includes the solvent-induced reaction (A ) plus terms for possible catalysis by H30+ (k ) and OH (icon). These pathways can be investigated in the usual manner by measurements in the absence of general acids and bases. Catalysis by general acids (HA) and/ or bases (B) is observed experimentally when the rate constant is plotted against the total concentration of general acid and general base at constant pH (Fig. 11.2A, where different possible outcomes are shown). 

A reaction exhibiting general acid catalysis, the ester hydrolysis (XXXVIII), has been discussed in Sect. 2.3. The present section deals with a classic reaction which is subject to both general acid and base catalysis in homogeneous media, the mutarotation of D-glucose. 

The mutarotation reaction exhibits general acid-and-base catalysis, the rate depending on the concentrations of all the acids and bases present. 

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. 

Scheme 11.1 General acid and base catalysis of deamidation. The tetrahedral oxyanion intermediate is inferred to be the transition state.

A dehydration of this type has actually been observed as a side reaction of a Lobry de Bruyn-Alberda van Ekenstein transformation in a very simple system. Thus, in experiments with the DL-glycerose-l,3-dihy-droxy-2-propanone isomerization in acetate, formate, and trimethylacetate buffers, pyruvaldehyde appeared in the reaction mixtures. (The formation of pyruvaldehyde from l,3-dihydroxy-2-propanone- and dl-glycerose-mineral acid mixtures had been observed much earlier.) Since these experiments in acidic buffers established that this reaction is subject to general acid and base catalysis, pyruvaldehyde must be formed in alkaline mixtures also. The results of Wohl s and Evans and Hass s experiments with DL-glycerose in alkaline solutions containing phenyl-hydrazine, in which pyruvaldehyde phenylosazone was isolated, support this view. 

The simplicity of the DL-glycerose-1,3-dihydroxy-2-propanone isomerization in acidic media, together with the application of the newer analytical techiuques, has made possible a kinetic examination of this reaction (see Section VI, Part 3 of this Chapter for further discussion of these experiments). In this study, the magnitudes of the catalytic effects caused by such species as acetic acid and acetate ion were determined. These data confirmed previous reports of catalysis produced by Bronsted acids and bases for this kind of reaction. They also indicated the operation of general acid and base catalysis in all Lobry de Bruyn-Alberda van Ekenstein transformations. 

Kinetic evidence from the DL-glycerose-1,3-dihydroxy-2-propanone isomerization has indicated that aldose-ketose isomerization and formation of a 3-deoxyosone proceed through a common intermediate. Ashmarin and coworkers and Petuely s observations, which indicate general acid and base catalysis of 2-furaldehyde formation, tend to support such a mechanism, since 2-furaldehyde and its derivatives may indeed be formed from 3-deoxyosones. 

Lowry and co workers141,222 studied the mutarotation of tetra-O-methyl-a-D-glucopyranose, and found that the rate of reaction is low in dry pyridine or in dry cresol, but high in a mixture of the two solvents or in either solvent when moist. Lowry and Smith57 concluded that the mutarotation requires an acid catalyst and a base catalyst, and that amphoteric solvents are complete catalysts for the process, whereas aprotic solvents are not. They also showed that molecules of undissociated acids, cations of weak bases, and anions of weak acids have catalytic properties. Much the same concept was developed independently by Bronsted and Guggenheim,189,223 and came to be known as generalized acid and base catalysis. It was found that the rate of mutarotation of a sugar in the presence of a mixture of several catalysts may be represented by an equation of the type ... 

The Aldol Reaction General Acid and Base Catalysis... 

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