Bronsted acid-base interactions

In contrast to protic (Bronsted) acids, a common quantitative method to determine the strength of Lewis acids does not exist. Whereas the Bronsted acid-base interaction always involves a common denominator—the proton (H+) transfer, which allows direct comparison—no such common relationship exists in the Lewis acid-base interaction. The result is that the definition of strength has no real meaning with Lewis acids. 

A concept that has become increasingly popular is that of acid-base interactions at interfaces. In this definition, acid-base is not confined to the older Bronsted acid-base concepts, but includes the broader concept of donor-acceptor interactions, as would be included in the Lewis acid-base concept. Thus phenomena ranging from Bronsted acid-base interactions to hydrogen bonding and to tt-donor-tt-acceptor interactions are included in the concept of acid-base interactions. 

Donor-acceptor bonds Bronsted acid-base interactions Up to 1000... 

A subset of Lewis acids and Lewis bases is represented by Brpnsted acids and Bronsted bases. Whilst Lewis acids are electron accepters and Lewis bases are electron-donors, Bronsted acids are defined as hydrogen (or proton) donors and Bronsted acids are defined as hydrogen (or proton) acceptors. The occurrence of Bronsted acid-base interactions thus is less general than that of Lewis acid-base interactions. Because of its wider application and to avoid confusion, the Lewis terminology is the preferred one in this work. 

I. Definition of Basicity A. General Comments on the Acid-base Concept The current general definition of the acid-base concept is based on the definitions by Bronsted and Lewis. According to Bronsted s theory (1923) an acid-base interaction can be described by the general relation (1) ... 

In the case of H-SSZ-24, the values of the pre-exponential factor experimentally obtained (see Table 5.4) do not agree with the values theoretically predicted by the equation for a jump diffusion mechanism of transport in zeolites with linear channels, in the case of mobile adsorption [6,26], Furthermore, the values obtained for the activation energies are not representative of the jump diffusion mechanism. As a result, the jump diffusion mechanism is not established for H-SSZ-24. This affirmation is related to the fact that in the H-SSZ-24 zeolite Bronsted acid sites were not clearly found (see Figure 4.4.) consequently p- and o-xylene do not experience a strong acid-base interaction with acid sites during the diffusion process in the H-SSZ-24 channels, and, therefore, the hopping between sites is not produced. 

Bronsted base catalysis is expected to operate when acid-base interaction strongly enhances the substrate nucleophilicity or deeply changes the structure favoring the reaction. The silylation of phosphorous acid with the Me3SiCl-amine system may serve as a good example [Eq. (74)] (see references cited in Ref. 273). The first two molecules of silyl chloride enter the... 

This ability to interact strongly with positively and negatively charged ions is consistent with water s nnique amphoteric (acid-base) properties, and is why it is fundamental to the Bronsted acid-base theory (equations 4 and 5). 

In the context of non-covalent molecular imprinting, the most important ionic interaction is the Bronsted acid-base or proton transfer process where the removal of an acidic proton by a base results in electrostatic attraction between the resulting ions. The process is governed by the dissociation/association constants of the acidic and basic moieties, the relative acidity or basicity of the solvent and the differential solvating power of the solvent for the intermediate complexes of proton transfer Figure 6.13). 

In the context of molecular imprinting, ionic interactions occur as a result of a proton transfer in a Bronsted acid/base type process. Alternatively, this type of ionic interaction can be viewed as an extension of the hydrogen bonding process. Figure 6.22 illustrates that the proton transfer ion-pair is in equilibrium with the covalent H-bonded complex, the position of the equilibrium being dependent upon the pKas of the acidic and basic groups and the solvent environment [34]. 

Infrared spectroscopy is a powerful method that allows the direct determination of the Bronsted centers. When pyridine (py) is adsorbed on the catalyst simultaneous determination of both types of center is possible, since it is bound to Bronsted centers in the form of a pyridinium ion through a hydrogen bond (Eq. 5-63), whereas on Lewis acid centers, adsorption occurs by a coordinative acid-base interaction (Eq. 5-64). 

The importanee of aeid-base interactions in various fields of ehemistry led to extensive research in the 1960s to obtain acid-base scales. This resulted in the Hard and Soft Aeids and Bases (HSAB) seales of Pearson [30], Drago s E and C eonstants [31], and Gutmann s donor and aeeeptor numbers [32]. Bolger and Miehaels [33] have used Bronsted acid base ehemistry to predict the adhesion of organie and inorganic species. 

In the case of organic inorganic materials interaction (e.g., polymer-metal oxide), Bolger and Michaels [33] suggested a model based on Bronsted acid-base chemistry to account for the strength of the interaction. They defined a parameter A for organic adds and bases ... 

Finally, AFM appears as an extraordinary technique to study acid-base interactions at the molecular scale. It enables the determination of pXa for surface confined carboxylic and other Bronsted groups. With the systematic studies which have appeared over recent years using thiol-treated tips and surfaces, clearly AFM has become a very well established and powerful tool for fundamental and applied research studies on acid-base interactions in adhesion. 

The strength of Lewis acid-base interactions cannot be expressed in terms similar to the acidity and basicity constants. K. and of the Bronsted-Lowry theory. Consequently an equilibrium constant resembling the protolytic constant of Bronsted acid-base couples, Eq. (1), cannot be specified. Because of the broad variety of Lewis acid-base interactions there would be as many acid strength scales as there are interacting bases. 

Similar to homogeneous one-phase systems, the acidity of solid surfaces manifests itself only in relation to a base that must be present the strength of the acid-base interaction therefore depends on the particular base involved. This is true for both types of acid sites, Bronsted or Lewis. The reverse can be said, of course, for surface basic sites. 

It is important to emphasize that spectroscopic evidence shows that water transforms the Lewis acid sites of sulfated zirconia into Bronsted acid sites [80]. At the same time, water promotes isomerization reactions over sulfated zirconia for a moderate extent of catalyst dehydration. Similarities were reported between the effect of rehydration on the isomerization activity of sulfated zirconia [81] and on that of other oxide catalysts [49] that are consistent with the role of surface donor sites in hydrocarbon isomerization reactions. However, when spectroscopic methods using basic probes were used to compare sulfated zirconia and zeolites in terms of the strength of their acid sites, the results were inconsistent with all catalytic data. These findings illustrate the danger of comparing the acidity of catalyst systems that differ in structure and composition, such as zeolites and sulfated zirconia in these systems the "catalytic" and the "physicochemical" scales for the strength of acid-base interaction may contain significantly different parameters. 

A special type of interaction, the acid-base interaction, is a fairly recent discovery. It is based on the chemical concept of a Lewis acid and base, which is briefly described. The acid/base definition was proposed separately by J. N. Bronsted and G. N. Lewis. Restatement of these definitions by Lewis in 1938 led to their popularity and acceptance. The Lewis definitions are an acid is a substance which can accept an electron pair from a base a base is a substance which can donate an electron pair. By this definition, every cation is an acid in addition to chemical compounds such as BF3 and Si02. Conversely, anions and compounds like NHj, PH3, and CgHjCH2NH2 are bases. According to the acid-base theory, adhesion results from the polar attraction of Lewis acids and bases (i.e., electron-poor and electron-rich elements) at the interface. This theory is attributed to the work by Fowkes et Gutmann, and Bolger and Michaels . ... 

The Lewis theory, by virtue of its broader definition of acids, allows acid—base theory to include all of the Bronsted-Lowry reactions and, as we shall see, a great many others. Most of the reactions we shall study in organic chemistry involve Lewis acid—base interactions, and a sound understanding of Lewis acid—base chemistry will help greatly. 

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