Application of Green Solvents

The elimination of solvents in chemical processes, or the replacement of hazardous solvents with environmentally benign ones, is one of the Twelve Principles of Green Chemistry [13]. The main advantage of solventless chemistry is that it is conceptually the simplest solution for the problems with solvents. However, not many reactions can be carried out under such conditions, as exothermic reactions can be dangerous, heating and stirring can be inefficient, especially if solid reactants or products are present, and usually solvents are needed for working up the product from solventless reaction media. 

The most abundant and perhaps the most obvious solvent is water. It is cheap, readily available, nonflammable, nontoxic, and useful for certain types of reaction 

In the synthesis of phenylacetic acid, the Pd-TPPTS system was used as a catalyst by Kohlpaintner and Beller (Hoechst) [15] in a biphasic carbonylation of benzyl chloride as a greener alternative to the classical process the reaction of benzyl chloride with sodium cyanide (Equations 4.4 and 4.5). Although in the new process 1 equiv. of sodium chloride is formed, this is far less salt waste than in the original process. Moreover, sodium cyanide is about seven times more expensive per kilogram than carbon monoxide. 

The synthesis of adipic acid in the laboratory can be carried out by the oxidation of cyclohexene with potassium permanganate (Equation 4.6). The E-factor of this reaction is 2.61, which means that for 1kg of adipic acid 2.61kg waste (mainly Mn02 and KOH) is produced. The atom economy is 27.8%, indicating that only 27.8% of the atoms in the reactants will be incorporated into the product. 

A greener method has been developed using hydrogen peroxide as the oxidant, with catalytic amounts of sodium tungstate and a quaternary ammonium phase-transfer catalyst (Equation 4.7) [16]. Since the solvent and the by-product are water, the reaction is indeed much greener (E-factor = 0.49, atom economy 67%). 

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The same authors have also reported the application of green solvents in additions of terminal alkynes to aldehydes in the presence of Zn(OTf)2 and l,8-diazabicyclo[5,4,0]-7-undecene (DBU, Scheme 109).287 The reactions proceeded very slowly, but afforded desirable alcohols 195 in moderate to good yields. 

The examples of this section summarise the recent application of the solvent-drop grinding approach to solid-state cocrystal preparation. The approach has been shown in certain instances to provide for either acceleration of CO crystallisation kinetics or selection of a particular polymorph via solid-state grinding. The approach is attractive, as it appears to incorporate some of the beneficial aspects of solvent participation while maintaining an essentially green, eco-friendly process. 

To a great extent the influence of green solvents in biochemistry will come via changes made in organic and analytical chemistries. For example, a typical application... 

Table 7.5. Sampling of green solvents for some industrial applications.

The most common solvents in the organic laboratory are carbon-based such as toluene, chloroform, acetone, acetonitrile, ethyl acetate, and various ethers and alcohols. None of these chemicals should be poured down a sink because they are mostly water-insoluble, toxic, and flammable, and in addition this is unlawful. Many of the solutes that will be dissolved in these solvents will also have varying degrees of toxicity and flammability that make them inappropriate for easy disposal. All of these features make organic chemistry a prime target for the application of green chemistry. 

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