Nonaqueous solvents liquid sulfur dioxide

Although no attempt will be made to describe the chemistry of all of the nonaqueous solvents listed in Table 5.4, the survey to this point has included ammonia as a basic solvent and liquid hydrogen fluoride as an acidic solvent. Another solvent that has been extensively utilized in both inorganic and organic chemistry is sulfur dioxide. Accordingly, we will give a brief survey of the chemistry of liquid sulfur dioxide for which the physical properties are presented in Table 5.8. 

Some proteins that are insoluble in a pure nonaqueous solvent may dissolve much more readily on the addition of a neutral salt (the analog of salting-in in water solutions). Thus, Katz (1955) found that whereas proteins are generally not directly soluble in pure liquid sulfur dioxide, they are readily soluble in sulfur dioxide containing 6 M NH4CNS. Zwitteri-onic salts (glycine, for example) may be useful in this connection in weakly protic solvents as they are in water (Cohn and Edsall, 1943, p. 617). 

The first of these reactions takes place because AgCl is insoluble in water. The second takes place because AgCl is insoluble in sulfur dioxide. The third reaction takes place because the highly ionic LiCl is not soluble in liquid ammonia. The last of these reactions takes place because AgCl is soluble in liquid ammonia but NaCl is not. It is clear that metathesis reactions may be different in some nonaqueous solvents. 

Indirect effects in nonaqueous media have received very little attention. It is likely that semiselective chemical reactions will occur in organic solvents containing active solutes. Liquid ammonia and sulfur dioxide also might offer some possibilities as solvents. The direct action of densely ionizing radiation on ammonia to produce hydrazine might yield reasonable quantities of this important reagent. 

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. 

Kamat et al. dlso support this hypothesis [18]. They compared lipase-catalyzed transesterification rates in supercritical carbon dioxide, fluoroform, ethylene, ethane, propane, and sulfur hexafluoride as well as in several conventional liquid solvents of different polarities. The reaction rates increased with increasing hydrophobicity of solvent within the SCFs and also within the liquid solvent group. Because the solvent s immiscibility with water and its apolarity, by themselves, are irrelevant to enzymatic activity [8], it appears that the activity loss is the result of the enzyme losing essential water. Although SCCO2 is generally considered to be a hydrophobic solvent, it is more hydrophilic than fluoroform or hexane and capable of stripping essential water from the enzyme in an essentially nonaqueous environment. 

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