Acidity solvent-system definition

Use the Lewis definition of acids and truces to explain the examples given tor the Br0nsted-Lowry, Lux-Flood, and solvent system definitions (Eqs 9.1-9.4, 9.S-9.8, 9 9-9.361. 

Aprotic nonaqueous solutions require a similar approach, but with a different definition of acid and base. The solvent system definition applies to any solvent that can dissociate into a cation and an anion (autodissociation), where the cation resulting from autodissociation of the solvent is the acid and the anion is the base. Solutes that increase the concentration of the cation of the solvent are considered acids and solutes that increase the concentration of the anion are considered bases. 

Lewis defined a base as an electron-pair donor and an acid as an electron-pair acceptor. This definition further expands the list to include metal ions and other electron pair acceptors as acids and provides a handy framework for nonaqueous reactions. Most of the acid-base descriptions in this book will use the Lewis definition, which encompasses the Brpnsted-Lowry and solvent system definitions. In addition to all the reactions discussed previously, the Lewis definition includes reactions such as... 

The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless they are always useful and have broadened their meaning and applications, covering concepts such as bond-formation, central atom-ligand interactions, electrophilic-nucleophilic reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory of the electron-pair bond, which he had first proposed in 1916. In this small volume which had since become a classic, Lewis independently proposed both the proton and generalized solvent-system definitions of acids and bases. He wrote ... 

Cady and Elsey gave the so-called solvent-system definitions, which depend on the mode of self-ionization of the particular solvent. Each solvent is considered as a parent of acids and bases. A solvo-acid is defined as a solute which increases the concentration of cations characteristic of the pure solvent and a solvo-base as a solute which increases that of the anions characteristic of the pure solvent ... 

Since acceptor molecules such as B(OH)3 and SbFj can enhance the acidity of water and HF respectively, although they are not simple Bronsted acids, there is considerable merit in defining an acid as any substance which increases the concentration of the characteristic cation of a solvent, and a base as any species which increases the concentration of the characteristic solvent anion. These are the solvent-system definitions of acid and base, and are very useful for amphiprotic and related solvents. 

These substances cannot be regarded as Bronsted bases in the normal sense of simple proton acceptors, since there is no evidence for the existence, under the experimental conditions in which the above ionizations are observed, of the conjugate acids (C6H5)3COH2 and H2N03. It might seem desirable to have a special name for such bases, but no reasonable name has been suggested. If, however, one accepts the solvent-system definition of a base as a species that increases the concentration of the characteristic anion of a solvent, no new name is needed. 

The solvent-system definition is also particularly useful for extending the concepts of acid and base to non-protonic non-aqueous systems. Many such systems are considered to undergo a self-ioniza-tion reaction analogous to autoprotolysis but involving the transfer of an anion such as F , Cl , O ", etc., rather than the transfer of a proton. In a few cases conductivity and other measurements provide substantial support for the existence of the postulated equilibrium, while in other cases there is no evidence at all for such a self-dissociation equilibrium. Nevertheless, the assumption that the equilibrium exists in a given solvent has proved useful for systematizing the chemistry in that solvent. 

R. J. GtLLESPtF. and E. A. Robinson. Sulfuric acid. Chap. 4 in T. C. WAUurNOTON (ed.), Nonaqueous Solvent Systems, pp. 117-210. Academic Press. London. 1965. A definitive review wiih some 250 references. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

An example of the different points of view and different tastes in the matter of acid-base definitions was provided to one of the authors in graduate school while attending lectures on acid-base chemistry from two professors. One Felt that the solvent system was very useful, but that the Lewis concept went too far because it included coordination chemistry. The second used Lewis concepts in all of his work, but felt uncomfortable with the Usanovich definition because it included redox chemistry To the latter s credit, however, he realized that the separation was an artificial one, and he suggested the pyridine oxide example given above. 

Almost all of the reactions that the practicing inotganic chemist observes in the laboratory take place in solution. Although water is the best-known solvent, it is not the only one of importance to the chemist. The organic chemist often uses nonpolar solvents sud) as carbon tetrachloride and benzene to dissolve nonpolar compounds. These are also of interest to Ihe inoiganic chemist and, in addition, polar solvents such as liquid ammonia, sulfuric acid, glacial acetic acid, sulfur dioxide, and various nonmctal halides have been studied extensively. The study of solution chemistry is intimately connected with acid-base theory, and the separation of this material into a separate chapter is merely a matter of convenience. For example, nonaqueous solvents are often interpreted in terms of the solvent system concept, the formation of solvates involve acid-base interactions, and even redox reactions may be included within the (Jsanovich definition of acid-base reactions. 

Such an equilibrium system is termed a conjugate (or corresponding) acid-base system. A and B are termed a conjugate acid-base pair. It is important to realize that the symbol H+ in this definition represents the bare proton (unsolvated hydrogen ion), and hence the new definition is in no way connected to any solvent. The equation expresses a hypothetical scheme for defining the acid and base - it can be regarded as a half reaction which takes place only if the proton, released by the acid, is taken up by another base. 

Use the generalized deHnilion of acids and buses to explum the examples given for the Br0nsicd-Lowry, Lux-Flood, solvent system, and Lewis definitions (Eqs. 9.I-9.4. 9 5-9 8. 9.9-9 36 9.37-9.40). 

For each of the following reactions identify the acid and the base. Also indicate which acid-base definition (Lewis, solvent system, Brpnsted) applies. In some cases, more than one definition may apply. 

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