The Solvent-System Concept

GermannIS found that aluminium trichloride reacts with carbonyl chloride to give a solution which yields KAICI4 on addition of potassium chloride. He considered aluminium chloride as a solvo-acid and potassium chloride as a solvobase, according to the following self-ionization equilibrium of the pure solvent (which is unlikely to be correct)  

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  

In liquid ammoniai self-ionization solvo-acid solvo-base 

SbFs BrF3 BrF2 t-SbFg KBrF4 K+ + BrF4  

It has recently been pointed out that the solvent system concept based on the mode of the self-ionization of a solvent has been overemphasized , but it has definitely served a useful purpose in the exploration of reactions in a number of non-aqueous solutions. 

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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. 

The solvent system concept has been used extensively as a method of classifying ... 

One criticism of the solvent system concept is that it concentrates too heavily on ionic reactions in solution and on the chemical properties of the solvent to the neglect of the physical properties. For example, reactions in phosphorus oxychloride (= phosphoryl chloride) have been systematized in terms of the hypothetical autoionization ... 

The application of the solvent system concept to liquid sulfur dioxide chemistry stimulated the elucidation of reactions such as those of aluminum sulfite. However, there is no direct evidence at all for the formation of S02+ in solutions of thionyl halides. In fact, there is evidence to the contrary. When solutions of thionyl bromide or thionyl chloride are prepared in 3JS-labeled (S ) sulfur dioxide, almost no exchange takes place. The half-life for the exchange is about two years or more. If ionization took place ... 

When the ionic species formed in solution are known, the solvent system approach can be useful. In solvents that are not conducive to ion formation and for which little or nothing is known of the nature or even the existence of ions, one must be cautious. Our familiarity with aqueous solutions of high permittivity (ch-o 81.760) characterized by ionic reactions tends to prejudice us toward parallels in other solvents and thus tempts us to overextend the solvent system concept. 

There has been some controversy in the literature over the proper interpretation of reactions in solvents such as phosphorus oxychloride. Drago and coworkers18 have suggested the "coordination model" as an alternative to the solvent system approach. They have stressed the errors incurred when the solvent system concept has been pushed further than warranted by the facts. In addition, they have pointed out that iron(lll) chloride dissolves in triethyl phosphate with the formation of letrachloro-... 

One of the chief difficulties with the solvent system concept is that in the absence of data, one is tempted to push it further than can be justified. For example, the reaction of thionyl halides with sulfites in liquid sulfor dioxide might be supposed to occur as follows, assuming that autoionization occurs ... 

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. 

In accordance with the solvent-system concept fluoride donors like NaF act as bases in this medium (giving Na+ and BrF-4), and fluoride acceptors such as SbF5 act as acids (giving BrF+2 and SbF-4). 

The solvent system concept has been used extensively as a method of classifying solvolysis reactions. For example, one can compare the hydrolysis of nonmetal halides with their solvolysis by nonaqueous solvents ... 

Acid-base reactions in the solvent system concept are the reverse of autodissociation ... 

Ionic fluorides such as KP behave as bases in Brp3, whereas some covalent fluorides such as Sbp5 behave as acids. On the basis of the solvent system concept, write balanced chemical equations for these acid-base reactions of fluorides with Brp3. 

In 1905, Franklin introduced the solvent system concept of acids and bases. This theory recognizes the ionization of a solvent to give a cation and an anion for example, 2H2O + OH" or 2NH3 NH4 + NH2". An acid is defined as a... 

Jander has made use of the solvent-system conceptions, but preferred to use the term acid-analogon in the place of solvo-acid and base-analogon in the place of solvo-base to stress the analogy with the reactions in water. ... 

The solvent-system concept definitions are, however, restricted to solvents with characteristic self-ionization. 

By a further extension of the solvent-system concept, the definitions of acid and base may be extended to molten salts. Thus the tetra-... 

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