Solvents for Chemical Reactions

Solvents for chemical reactions must be chemically inert under the reaction conditions. The following solvents have proved suitable [14.240]  

1) Hydrogemiiion alcohols, glacial acetic acid, hydrocarbons, dioxane 

2) Oxidation glacial acetic acid, pyridine, nitrobenzene 

4) Esterification benzene, toluene, xylene, dibutyl ether 

5) Nitration glacial acetic acid, dichlorobenzene, nitrobenzene 

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Today SCFs are used for natural product extractions, chromatographic separations, pollution prevention, material processing and as solvents for chemical reactions.[75-77] Chemical applications include catalysis, polymerization, enzymatic reactions and organic synthesis. 

Supercritical fluids show unique physicochemical properties, such as density, diffusivity, solubility, and viscosity all can be easily controlled by changing temperature and pressure. Thus, these fluids are attractive as a useful solvent for chemical reactions and the following purification. Particularly, supercritical C02(scC02) has the advantages of relatively low critical temperature and pressure (critical temperature (71.) = 304.2 K, critical pressure (Pc) = 7.28 MPa), non-flammability, and inexpensiveness. 

The benefits from tuning the solvent system can be tremendous. Again, remarkable opportunities exist for the fruitful exploitation of the special properties of supercritical and near-critical fluids as solvents for chemical reactions. Solution properties may be tuned, with thermodynamic conditions or cosolvents, to modify rates, yields, and selectivities, and supercritical fluids offer greatly enhanced mass transfer for heterogeneous reactions. Also, both supercritical fluids and near-critical water can often replace environmentally undesirable solvents or catalysts, or avoid undesirable byproducts. Furthermore, rational design of solvent systems can also modify reactions to facilitate process separations (Eckert and Chandler, 1998). 

There are several potential advantages which may be realized with the use of supercritical carbon dioxide as a solvent for chemical reactions (Tanko et al., 1994) ... 

The biggest problem in switching to water as a solvent for chemical reactions is that the reagents may be quite insoluble in water. A second problem is that they may... 

Organic Reactions in Micro-Organized Media—Why and How Water-Promoted Organic Reactions The design of green oxidants Water as a solvent for chemical reactions Water as a benign solvent for chemical syntheses... 

We see that the distribution coefficient for metal ion extraction depends on pH and ligand concentration. It is often possible to select a pH where D is large for one metal and small for another. For example. Figure 23-4 shows that Cu2+ could be separated from Pb2+ and Zn2+ by extraction with dithizone at pH 5. Demonstration 23-1 illustrates the pH dependence of an extraction with dithizone. Box 23-1 describes crown ethers that are used to extract polar reagents into nonpolar solvents for chemical reactions. 

In recent years, researchers have become increasingly interested in the use of carbon dioxide as a source of carbon because of its ideal properties as a Crunit in future chemistry 1-3], The three most important advantages are its abundancy, low cost and non-toxicity. Supercritical carbon dioxide (scC02) has also been considered as an ideal apolar solvent for chemical reactions due to its increased diffusion rates and reactant solubilities, and to its easy product separation compared to conventional solvents [4-7]. 

In the last couple of years, ionic liquids have attracted much interest in their potential applications as solvents for chemical reactions, in... 

The considerable chemical stabiHty of BTF suggests also its consideration as an alternative solvent for chemical reaction processes (Sects. 3 and 4). The relative stability of the BTFs in general (BTF, PCBTF and 3,4-DCBTF) as well as their desirable environmental and toxicological properties promote their consideration for a broad spectrum of apphcations as industrial solvents. 

The present contribution highlights recent developments in the application of compressed (liquid or supercritical) carbon dioxide as a solvent for chemical reactions. After a brief introduction to the basic physical properties of SCCO2, some practical aspects of the use of compressed gases are discussed. A survey of successful applications of compressed and particularly supercritical CO2 in organic synthesis is provided with an emphasis on metal-catalyzed reactions. 

Why use SCFs as solvents for chemical reactions There are numerous advantages associated with the use of SCFs in chemical synthesis, all of which are based on the unique combination of properties of either the materials themselves or the supercitical state. Different types of reactions may benefit particularly from a specific property, and these sometimes spectacular effects will be discussed in detail in the individual chapters of the book. Here, we try to summarize briefly the various potential improvements that can be expected if SCFs are employed as solvents for synthetically useful chemical reactions. The advantages fall into four general categories environmental benefits, health and safety benefits, process benefits, and chemical benefits (Table 1.1-3). 

The critical point on a phase diagram designates the pressure (pc) and temperature (Tq) at which the vapor and liquid phases of a substance become indistinguishable. By definition, a supercritical fluid (SCF) is above pc and Tc- Generally, the physical properties of an SCF (density, viscosity, and dielectric constant) are intermediate between those of a liquid and a gas, and these properties vary dramatically as a function of temperature and pressure [1,2]. Because of these unique features, there is enormous interest in the use of SCFs as solvents for chemical reactions [3-6]. 

There are several potential advantages which may be realized with the use of supercritical fluids as solvents for chemical reactions from the standpoint of reactivity and selectivity. As many of the examples discussed in this chapter illustrate, the unique features of SCFs can be exploited to control the behavior (i.e. kinetics and selectivity) of many chemical processes in a way not possible with conventional liquid solvents. 

In contrast to inorganic molten salts, the fluidity of ionic hquids at room temperature permits their use as solvents for chemical reactions. Electrostatic properties and charge mobility in ionic hquids can play a distinctive role in chemical reactivity, as compared with neutral solvents. In particular, hydrogen and proton transfer reactions are likely to be sensitive to an ionic environment due to the hydrogen-bond acceptor ability of the anions. Such type of reactions are fundamental in acid-based chemistry and proton transport in solution. 

Since the discovery of microemulsion phases in supercritical fluids in the mid-1980s [1] and their subsequent characterization [2-16], there has been much interest in exploiting the unusual properties of the supercritical fluid phase in applications of these systems. One such application is as a new type of solvent for chemical reactions. In the following sections, I discuss the properties of these systems for reactions, review the progress so far, and analyze the future potential. As a prelude to these discussions, I begin with a brief overview of what is known about the molecular structure of microemulsions in near-critical and supercritical fluids. The details of the primary and secondary molecular structures of various types of microemulsion phases can dramatically affect the reactivity in these systems. 

J. Dupont, C.S. Consorti, J. Spencer, Room temperature molten salts Neoteric Green solvents for chemical reactions and processes. J. Braz. Chem. Soc. 11 (2000) 337-344. 

Even though the first ionic liquids have been known since 1914, these ionic liquids have only been investigated intensively as solvents for chemical reactions in the past 15 years. In the middle of the 1980s, acidic chloroaluminate ionic liquids were successfully tested as catalysts in Friedel-Crafts reactions. The first application where an ionic liquid has been used as a catalytic solvent in biphasic catalysis was reported in 1990 by Chauvin et al. (see Sect. 20.3.2). However, the use of... 

DMF Dimethylformamide, common solvent for chemical reactions DMSO Dimethyl sulfoxide DoE Design-of-experiments... 

Breslow, R., Water as a solvent for chemical reactions. In Anastas, P.T. and Williamson, T.C. (Eds.), Green Chemistry, Oxford Press, New York, 1998, Chap. 13. 

Ethers are excellent solvents for a variety of substances due to their generally nonpolar nature combined with the ability to form hydrogen bonds with certain types of molecules. They are also relatively unreactive, so they make good solvents for chemical reactions to occur in. Unfortunately, they are also highly flammable and susceptible to slow oxidation by air to form peroxides which are highly explosive. 

Life requires a solvent for chemical reactions to occur. This solvent must have a number of characteristics which include (1) a liquid phase that is similar to the environmental conditions in which the biota is to exist, (2) a viscosity and density that allows molecules essential to biological function to be maintained at sufficient concentrations in the cells, (3) an environment that allows chemical reactions to occur, but in which the conditions also allow complex molecules to be synthesised that are not broken down by chemical reactions. 

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