Basicity Br0nsted-Lowry

Zeolites may behave as Lewis acids at Al,+ sites, or as Br0nsted-Lowry acids by means of absorbed H+ ions. Because they have relatively open structures, a variety of small molecules may be accommodated within the —O—Al—O—Si— framework. These molecules may then be catalyzed to react by the acidic centers. Coordinatively, unsaturated oxide ions can act as basic sites, and in some catalytic reactions both types of centers are believed to be important. Catalysis by zeolites is discussed further in Chapter 15. 

The Br0nsted-Lowry theory expands the definition of acids and bases to allow us to explain n ch more ol solution chemistry. For example, the Brpnsted-Lowry theory allows us to explain why a solution af ammonium nitrate tests acidic and a solution of potassium acetate tests basic. Most of the substances that we cofcider acids in the Arrhenius theory are also acids in the Brpnsted-Lowry theory, and the same is true of bases. Injboth theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the weak acid, the ions, and the w ater. A similar statement can be made about weak bases ... 

Still another important concept related to electronegativity and polarity is that of acidity and basicity. We ll soon see that the acid-base behavior of organic molecules helps explain much of their chemistry. You may recall from a course in general chemistry that there are two frequently used definitions of acidity, the Br0nsted-Lowry definition and the Lewis definition. 

Carbon-based nucleophiles are generally regarded as soft bases, and Li is a hard acid. From this perspective alone, it would be difficult to rationalize that this reaction heavily favors the products, since the softness of the bases n-butyl and phenyl carbanions is similar. The significantly higher Br0nsted-Lowry basicity of the n-butyl carbanion (pK of n-butane is 50) relative to the phenyl carbanion (pKa of benzene is 43) plays a vital role in rationalizing the formation of phenyllithium. 

Basicity constants are not necessary in the Br0nsted-Lowry approach. Basicity is measured according to the p a of the conjugate acid. The weaker the conjugate acid, the stronger the base. 

Returning to the problem at hand, both 2 and 3 react quickly and easily with HBr, so the ti-bonds in each molecule are considered to be good Br0nsted-Lowry bases in their reactions with mineral acids. Benzene does not react with HBr, even with heating. Benzene has six 7i-electrons 2 has only two, so benzene is more electron rich relative to 2. The fact that benzene does not react suggests that it is too weak a base, which is a good indication that the six p-electrons are held tightly by the molecule and are not available for donation. The poor basicity of benzene in a reaction with HBr is presented as evidence of the special stability of benzene, and the explanation for that stability is the resonance delocalization shown for 1C. 

Br0nsted-Lowry bases include NHj and amines and the anions of weak acids. All produce basic solutions by accepting H+ from water, which yields OH, thus making [H3O+] < [OH ]. 

Certain reactions have the characteristics of acid—base reactions but do not fit the Br0nsted—Lowry concept. An example is the reaction of the basic oxide Na20 with the acidic oxide SO3 to give the salt Na2S04. ... 

The Arrhenius concept was the first successful theory of acids and bases. Then in 1923, Br0nsted and Lowry characterized acid-base reactions as proton-transfer reactions. According to the Br0nsted—Lowry concept, an acid is a proton donor and a base is a proton acceptor. The Lewis concept is even more general than the Br0nsted-Lowry concept. A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Reactions of acidic and basic oxides and the formation of complex ions, as well as proton-transfer reactions, can be described in terms of the Lewis concept. 

A salt may be regarded as an ionic compound obtained by a neutralization reaction in aqueous solution. The resulting salt solution may be neutral, but often it is acidic or basic (Figure 17.8). One of the successes of the Br0nsted-Lowry concept of acids and bases was in pointing out that some ions can act as acids or bases. The acidity or basicity of a salt solution is explained in terms of the acidity or basicity of individual ions in the solution. 

From the Br0nsted-Lowry point of view, the CN ion acts as a base, because it accepts a proton from H2O. You can also see, however, that OH ion is a product, so you expect the solution to have a basic pH. This explains why solutions of NaCN are basic. 

However, not all amphoteric substances are amphiprotic, because only Br0nsted—Lowry acids and bases accept and donate protons. If we also consider Lewis acids and bases, these either accept or donate electron lone pairs, which means that a substance that has more than one mechanism of action could show both types of acidity or basicity. For example, a metal oxide such as Mg(OH)2, when placed in water can dissociate to release hydroxide ions ... 

The Lewis definition implies the presence of high electron density centres in Lewis bases, and low electron density centres in Lewis acids. In a reaction between a Lewis acid and a Lewis base the electron pair donated by the base is used to form a new sigma bond to the electron-deficient centre in the acid. The identification of Lewis bases follows basically the same guidelines as the identification of Br0nsted-Lowry bases. They frequently contain atoms that have non-bonding electrons, or lone pairs. In contrast Lewis acids frequently contain atoms with an incomplete octet, a full positive charge, or a partial positive charge. 

The shortcomings of the Arrhenius theory led chemists to seek other explanations for the nature of acids and bases. The Br0nsted-Lowry theory was introduced independently in 1923 by the Danish chemist Johannes Nicolaus Brqnsted and the English chemist Thomas Martin Lowry, stating that any compound that can transfer a proton to any other compound is an acid, and the compound that accepts the proton is a base. Their theory explained the behaviour of all of the acids and bases covered by the Arrhenius theory, but also was able to resolve some of the problems with that theory. That is, they were able to explain why some salts are acidic and basic (due to salt hydrolysis) and why no free protons are found in the solutions of some acids. 

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