The Electronic Theory of Acids and Bases

The experimental criteria of acid-base phenomena as listed in Chapter 1 are (1) neutralization, (2) titration with indicators, (3) displacement, and (4) catalysis. When the chemical reactions between substances thus classified as acids or bases are examined in detail, the theoretical explanation of their fundamental nature becomes apparent. An acid is capable of accepting a share in a lone electron pair from a base to form a coordinate covalent bond. A base donates a share in a lone electron pair to the acid. The formation of the coordinate bond is the first step in neutralization reactions  

Sometimes the product is a covalent compound. At other times, formation of the coordinate bond may be followed—or accompanied—by ionization, so that the product is a salt. For example, the compound CsHsNiAlBra seems to be a typical salt. Both aluminum bromide and pyridine are covalent compounds. When 

The existence of the hypothetical intermediate addition compound in which the hydrogen bridge between the hydrogen chloride and water molecules involves 2-covalent hydrogen is now regarded as unlikely. Probably it would be better to represent the formation of the coordinate bond as taking place simultaneously with ionization, as follows  

Thus the electronic theory pictures the reaction in exactly the same manner as the proton-donor theory. 

This example is only one of many which could be given to show that the electronic theory of acids and bases includes the proton theory as a special case. The simultaneous coordination and ionization pictured by the electronic theory is equivalent to the pro-ton-transfer mechanism of the Br0nsted theory. The bases of the 

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Luder, W. F. (1940). The electronic theory of acids and bases. Chemical Reviews, 27, 547-83. 

Up to this point, we have dealt with the subject of acid-base chemistry in terms of proton transfer. If we seek to learn what it is that makes NH3 a base that can accept a proton, we find that it is because there is an unshared pair of electrons on the nitrogen atom where the proton can attach. Conversely, it is the fact that the hydrogen ion seeks a center of negative charge that makes it leave an acid such as HC1 and attach to the ammonia molecule. In other words, it is the presence of an unshared pair of electrons on the base that results in proton transfer. Sometimes known as the electronic theory of acids and bases, this shows that the essential characteristics of acids and bases do not always depend on the transfer of a proton. This approach to acid-base chemistry was first developed by G. N. Lewis in the 1920s. 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

Luder, W. F., and Zuffanti, S. (1946). The Electronic Theory of Acids and Bases. Wiley, New York. A small book that is a classic in Lewis acid-base chemistry. Also available as a reprint volume from Dover. 

W. F.Luder S.Zuff anti, "The Electronic Theory of Acids and Bases,"Wiley,NY... 

Luder, W.F. Zuffanti, S. The Electronic Theory of Acid and Bases. Dover Publications New York, 1946 2nd Edition, 1961. 

As a result of there being so many kinds of interactions that involve the donation and acceptance of electrons, the electronic theory of acids and bases pervades the whole of chemistry. In the 1950s, Ahrland, Chatt, and Davies had classified metals as class A metals... 

Lewis [13] proposed the electronic theory of acids and bases and gave a more general definition of acids and bases in its framework. Acids are determined as acceptors of an electron pair, and its donors are classified as bases. The principal scheme of the Lewis acid-base interaction is described by the following equation ... 

Also in 1923, G. N. Lewis introduced the electronic theory of acids and bases. In the Lewis theory, an acid is a substance that can accept an electron pair and a base is a substance that can donate an electron pair. The latter frequently contains an oxygen or a nitrogen as the electron donor. Thus, nbnhydrogen-containing substances are included as acids. Examples of acid-base reactions in the Lewis theory are as follows ... 

Lui)BR and Zufpanti, Catalysis from the Viewpoint of the Electronic Theory of Acids and Bases, Chem. Revs., 34, 346 (1644). 

Luder, W. F., and S. Zufpanti The Electronic Theory of Acids and Bases, Wiley, New York 1946. 

One of the most powerful theoretical tools available to the chemist is the electronic theory of the covalent bond, which we owe primarily to G. N. Lewis. Lewis was also the first to suggest an electronic explanation of acid-base phenomena. The theory of the covalent bond is now almost universally accepted among chemists, but the electronic theory of acids and bases until quite recently has been ignored where it has not been actively opposed. ... 

Such reactions, as well as those of electrolysis and of amphoteric behavior, have been observed in other solvents. Reactions that occur in ammonia, sulfur dioxide, acetic acid, hydrogen sulfide, hydrogen fluoride, phosgene, selenium oxychloride, alcohols, and sulfuric acid are analogous to those that take place in water. Some of them have been interpreted according to the solvent-systems theory others, according to the proton theory. AU of them may be understood more clearly on the basis of the electronic theory of acids and bases. Only a few examples will be discussed here. 

We have seen that Walden s fears that Lewis would deliberately eliminate the important part played by the solvent in acid-base properties were groundless. Lewises acids and bases, dissolved in suitable amphoteric solvents, have the typicaF properties of acids and bases. These typicaF properties are the properties with which we are familiar from our study of water chemistry. Now that we are beginning to branch out into other fields, we may expect to find increasingly that the electronic theory of acids and bases is the only one so far proposed that is at all adequate. 

Although consistent application of the electronic theory of acids and bases simplifies and systematizes a great deal of chemistry, the amount of correlation can be increased still further by developing the relationship between acids and bases on the one hand and oxidizing and reducing agents on the other. This will be done in the following chapter. 

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