Strengths of Weak Bronsted Acids

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

Fig. 2.4C) and S04 /Al203 (5 P -49 and -3 ppm Fig. 2.4D). Likewise, the additional weak peak appearing at 5 P 34 ppm (Fig. 2.4C) has been ascribed due to the presence of TMPO. On the basis of earlier mentioned results obtained firom the P-TMP NMR approach, it is indicative while metal oxides such as Z1O2, Ti02, and AI2O3 possess mosdy Lewis acidity, further sulphonation treatment of these metal oxide catalysts tends to not only increase the Lewis acidic strength (as indicated by the downfield shift of the P resonance) but also promote formation of new Bronsted acid sites.

Pyridine, a base of moderate base strength, is used to distinguish between not only Lewis and Bronsted acid sites on solid surfaces, but also strong and weak Bronsted acids (33). When it is protonated to pyridinium (C5H5NH ), the diagnostic N -H deformation band appears in the region 1535-1550 cm and... 

In the Bronsted-Lowry theory, the strength of an acid depends on the extent to which it donates protons to the solvent. We can therefore summarize the distinction between strong and weak acids as follows ... 

A major development in the study of EGB s is the recently reported measurements of rates of protonation by acids of known pK. The correlation of such rates with pK, the Bronsted relationship, also enables bases of determined pK to be used in the measurement of kinetic acidities of weak acids. This quantitative approach will eventually lead to the optimisation of reaction conditions for preparative reactions by providing data which can be used to match the acid/base pairs more exactly. In many organic reactions involving bases the base chosen is stronger than is strictly neccessary and consequently such reactions are often complicated by side reactions such as condensation reactions and isomerisations. The advantage of an EGB of moderate strength has been seen in the vitamin A preparation described in Scheme 18, where the facile cisftrans isomerisation is avoided. 

Before continuing on to the last definition of acids and bases, it will be helpful to consider the definitions for strong and weak acids within the context of the Bronsted-Lowry model of acids and bases. The definitions are really an extension of the Arrhenius ideas. In the Arrhenius definitions, strong acids and bases were those that ionize completely. Most Bronsted-Lowry acids and bases do not completely ionize in solution, so the strengths are determined based on the degree of ionization in solution. For example, acetic acid, found in vinegar, is a weak acid that is only about 1 percent ionized in solution. That means that when acetic acid, HC2H302, is placed in water, the reaction looks like ... 

Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. 

In summary, physisorbed nitrogen appears to offer several advantages as an infrared probe of acid sites in zeolites. It clearly distinguishes between Bronsted and Lewis acid sites without interference from gas phase species, it is small enough to probe sites in smaller pore zeolites, and its interaction with the zeolite is sufficiently weak and reversible to have negligible influence on the zeolite chemistry. It is not yet clear whether the method can probe variations in Bronsted acid strength. 

The requirement for zeolites of high acid strength for the alkylation of arrunonia by alcohols contrasts with the need for weakly acidic zeolites for the addition reactions between alkenes and ammonia [79]. In these reactions the alkene has to be activated by the Bronsted acid site of the zeolite and that is only possible when the acid sites are not fully blocked by... 

---