The Solvent Concept

The fact that water undergoes some autoionization suggests that perhaps other solvents behave in a similar way. For predicting the products of reactions, it may not be important 

Just as the cation produced by dissociation of water (H30+) is the acidic species in aqueous solutions, the NH4+ ion is the acidic species in liquid ammonia. Similarly, the amide ion, NH2, is the base in liquid ammonia just as OH- is the basic species in water. Generalization to other nonaqueous solvents leads to the solvent concept of acid-base behavior. It can be stated simply as follows A substance that increases the concentration of the cation characteristic of the solvent is an acid, and a substance that increases the concentration of the anion characteristic of the solvent is a base. Consequently, NH4C1 is an acid in liquid ammonia, and NaNH2 is a base in that solvent. Neutralization becomes the reaction of the cation and anion characteristic of the particular solvent to produce unionized solvent. For example, in liquid ammonia the following is a neutralization  

The solvent concept for nonaqueous solvents works exactly like the Arrhenius theory does for aqueous solutions. Autoionization and typical neutralization reactions can be shown as follows for several solvents. For liquid S02, 

Amphoterism, the ability to react as both an acid and a base, is also exhibited by substances in nonaqueous solvents. For example, in aqueous solutions Zn2+ behaves as shown in the following equations  

In liquid ammonia, Zn2+ also produces a precipitate with the NH2 anion  

It has long been known that some autoionization occurs in water, and it was presumed that nonaqueous solvents behaved in a similar way. Although the reaction of sodium hydride with water, 

One of the primary pieces of evidence that water undergoes some autoionization comes from the conductivity of pure water. The equilibrium can be written as 

The conductivity of liquid ammonia is sufficiently high to indicate a very slight degree of autoionization. In order for ions to be produced, something must be transferred from one molecule to another, and in solvents such as water or ammonia it is proton transfer that occurs. Accordingly, the ionization of liquid ammonia can be shown as 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent  

Note that there is no requirement that the solvent actually undergo autoionization. 

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This reaction represents a neutralization reaction in liquid sulfur dioxide. It makes no difference that the solvent does not ionize or that SOCl2 is a covalent molecule. The utility of the solvent concept is not that it correctly predicts that solvents undergo some autoionization. The value of the solvent concept is that it allows us to correctly predict how reactions would take place if the solvent ionized. Note that in this case SOCl2 does not ionize, but if it did it would produce S02+ (the acidic species characteristic of the solvent) and Cl-. 

The coordination model provides a way to explain many reactions that occur in nonaqueous solvents without having to assume that autoionization takes place. As shown in Eq. (10.17), the fact that FeCl4 is produced can be explained by substitution rather than autoionization. However, as has been shown earlier in this chapter, it is sometimes useful to assume that the solvent concept is valid, and many reactions take place just as if the solvent has ionized to a slight degree into an acidic and a basic species. 

Although it is not necessary for autoionization to occur, the solvent concept shows that the cation characteristic of the solvent is the acidic species and the anion is the basic species. Therefore, when... 

Even though it is now known that the solvent concept does not represent the actual reacting species in some nonaqueous solvents, it is still a useful tool. 

Because the addition of FeCl3 to liquid OPCl3 increases the concentration of the OPCl2+ cation, ferric chloride is an acid in liquid OPCl3, which can be shown according to the solvent concept as... 

These equations show that FeCLT can form by coordination of the solvent rather than by postulating solvent autoionization according to the solvent concept. In the series of reactions represented by Eq. (5.69), nucleophilic substitution occurs in which a solvent molecule replaces a chloride ion that subsequently interacts with FeCl3. Undoubtedly, a similar situation exists for other reactions in which autoionization appears to occur. Autoionization probably occurs only in solvents in which a proton that is strongly solvated is transferred (H20, HF, NH3, etc.). Although the solvent concept is useful in a formal way, it is unlikely that autoionization occurs for a solvent such as liquid SO2. However, many reactions take place to give the products that would be predicted if autoionization had occurred. We will now describe the chemistry of three of the most extensively studied nonaqueous solvents. 

Acid-base reactions. According to the solvent concept, the acidic species characteristic of liquid ammonia is NH4+ and the basic species is NH2. Neutralization reactions in liquid ammonia thus become equivalent to the reaction of these ions ... 

Although it is unlikely that S02 undergoes ionization as shown in Eq. (5.98), some reactions take place as though these ions were present. For example, because according to the solvent concept S02+ would be the acidic species in liquid S02, a compound such as SOCl2 would be expected to react as an acid. Similarly, K2S03 would be a base because it would provide the basic S032- ions. Therefore, the reaction between the acid and base in liquid S02 would be... 

It should be noted that the products of Eq. (5.99) are exactly the same as if ionization had occurred. The solvent concept has some utility even in cases where it does not exactly represent the way in which the solvent behaves with regard to autoionization. 

Presuming the solvent concept to apply in the case of liquid sulfur dioxide, SOCI2 would be an acid that produces S02+. It may be that SOCI2 undergoes some slight autoionization that can be represented as... 

Meek, D. W., Drago, R. S. (1961). Journal of the American Chemical Society, 83, 4322. The classic paper describing the coordination model as an alternative to the solvent concept. 

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