Autoionizing solvents

Examples of acid-base reactions in the autoionizing solvents Brp3 and IF5 have been... 

This group of solvents ranges from the limiting case of a nonpolar solvent (Group I) to an autoionizing solvent (Croup III. see below). Within this range a wide variety of reactivity is obtained. Cutmann - has defined the donor number (DN) as a measure of the basicity or donor ability of a solvent. It is defined as the negative enthalpy of reaction of a base with the Lewis acid antimony penlachloride, SbClj ... 

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,... 

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. 

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 concepts just illustrated can be extended to other nonaqueous solvents. For example, in liquid N204 if autoionization occurred it would produce NO+ and N03 . In this solvent, a compound that furnishes N03- would be a base, and a compound that would (formally rather than actually) produce NO+ would be an acid. Therefore, the reaction of KN03 with NOC1 (actually linked as ONC1) is a neutralization in liquid N204. 

However, in liquid N204 reactions do not occur as if N02+ and N02 were present, and there is no evidence to indicate that they are. In most cases, autoionization of the solvent if it occurs is by transfer... 

For some nonaqueous solvents, the autoionization, if it occurs at all, must be to a degree so small that virtually no ions are present. If the ion product constant for a solvent is as low as 10-40, the concentra-... 

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... 

A solvent that resembles water in many ways is liquid hydrogen fluoride. The molecule is polar, there is some autoionization, and it is a fairly good solvent for numerous ionic solids. Although the boiling point of liquid HF is rather low (19.5 °C), it has a liquid range that is comparable to that of water, partially as a result of extensive hydrogen bonding. One of the problems associated with the use of liquid... 

The chemistry of the specific solvents discussed in this chapter illustrates the scope and utility of nonaqueous solvents. However, as a side note, several other nonaqueous solvents should at least be mentioned. For example, oxyhalides such as OSeCl2 and OPCl3 (described in the discussion of the coordination model earlier in this chapter) also have received a great deal of use as nonaqueous solvents. Another solvent that has been extensively investigated is sulfuric acid, which undergoes autoionization,... 

The nitronium ion, N02+, derived from N02, is the attacking species in nitration reactions (see Chapter 9). Also, liquid N204 has been rather extensively studied as a nonaqueous solvent, and autoionization, to the extent it occurs, appears to be as follows ... 

Sulfuric acid has been used as a nonaqueous solvent, and some proton transfer may take place as a result of autoionization in 100% H2S04. 

Photoelectron ejection from ArH in nonpolar solvents has been interpreted in terms of autoionizing excited states with x = 10 " sec, which can also be internally converted to the Do states [34,35,136,137]. It is assumed that t-Sf in the vibrational excited state ((t-St ) ) undergoes vibrational relaxation on the time scale of 10 " sec to yield t-St , which has a long enough lifetime v = k/y to be quenched by Bp via ET involving (t-St/Bp )soiv (Scheme 13). = 0.06 0.02 suggests that (t-St ) undergoes photoelectron ejection and... 

Amphiprotic solvents have both acidic and basic properties in terms of the Bransted acid-base concept. If we denote an amphiprotic solvent by SH, it donates a proton by SH S +H+ and accepts a proton by SH + H+ = SH2. Overall, the autoprotolysis (autoionization) occurs by 2SH = SH2 + ST The extent of autoprotolysis is expressed by the autoprotolysis constant, Kjh = aSH2 aS, the values of which are also included in Table 1.5 as pKSH values (for more details, see Table 6.6). 

Many solvents autoionize with the formation of a cationic and an anionic species as does water... 

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 physical properties of sulfuric acid are listed in Table 10.3. The dielectric constant is even higher than that of water, making it a good solvent for ionic substances and leading to extensive autoionization. The high viscosity, some 25 times that of water, introduces experimental difficulties Solutes are slow to dissolve and slow to crystallize. It is also difficult to remove adhering solvent from crystallized materials. Furthermore, solvent that has not drained from prepared crystals is not reudily removed by evaporation because of the very low vapor pressure of sulfuric acid... 

Meek and Drago8 showed that the reaction between tetramethylammonium chloride and iron(III) chloride can take place just as readily in triethyl phosphate, OP(OEt), as in phosphorus oxychloride, OPCl,. They suggested that the similarities in physical properties of the two solvents, principally the dielectric constant, were more impor tant in this reaction than the difference in chemical properties, namely, the presence or absence of autoionization to form chloride ions.9... 

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 ... 

The "equilibrium boxes" for the solvents (Fig. 10-1) indicate the range over which differentiation occurs outside the range of a particular solvent, all species are leveled. For example, water can differentiate species (i.e., they are weak adds and bases) with pKa s from about 0 to 14 (such as acetic acid). Ammonia, on the other hand, behaves the same toward acetic acid and sulfuric acid because both lie below the differentiating limit of —12. The extent of these ranges is determined by the autoionization constant of the solvent (e.g, —14 units for water). The acid-base behavior of several species discussed previously may be seen to correlate with Fig. 10.1.11... 

Thus for, tire solvents discussed have had one feature in common with water, namely, the presence of a transferable hydrogen and the formation of omum ions. In this section we shall look briefly at solvents which do not ionize in this way. These may be conveniently classified into three groups. The first group consists of solvents such as carbon tetrachloride and cyclohexane which are nonpolar, essentially nonsolvaiing, and do not undergo autoionization. These are useful when it is desired that the solvent play a minimum role in the chemistry being studied, for example, in the determination of and C parameters discussed in the previous chapter. 

A similar, although less reactive, aprotic solvent is phosphorus oxychloride (phosphuryl chloride). A tremendous amount of work on the properties of this solvent has been done by Gutmann and coworkers.16 They have interpreted their results m solvent system terms based on the supposed autoionization ... 

They argue that the similar coordinating ability of these phusphoryl (—P=0) solvents (and to a lesser extent their dielectric constants) is more important than their chemical differences (supposed autoionization and chloride ion transfer in phosphorus oxychloride). 

A point of this reaction is the release of H+ ions—the Bronsted-Lowry approach considers this the appearance of a proton from the acid. A Bronsted-Lowry acid is a proton donor. Note that the hydrogen ion is a proton, hydrogen s nucleus without the electron found in the atom. Then, a Bronsted-Lowry acid must contain a hydrogen. Of course, if the solvent were not to be water, this statement may not work because the cation released could be other than the hydrogen ion, but there might be other ions performing the same service (liquid ammonia autoionizes, Problem 17.3). 

Liquid NH3, like water, is an amphiprotic solvent. Write the equation for its autoionization. 

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... 

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,... 

---