Green Solvents for Organic Synthesis

Ahluwalia, V.K. Green Solvents for Organic Synthesis, Alpha Science International Oxford, U.K., 2009. 

The choice of reaction solvent is a critical factor for development of sustainable synthetic pathways to any chemical product in general, and for polymers in particular. To this end, it is important to try to avoid volatile organic solvents, chlorinated solvents, and solvents that can damage the environment (e.g., fluorinated hydrocarbons). The most widely used green solvents for polymer synthesis are water, ionic liquids, and supercritical CO2. In addition, polymerizations can often be performed solvent free. 

Ionic liquids (ILs) are not only viewed as potent designer solvents for organic synthesis, they are also often cited in the context of the recently developed field of green chemistry . The main rationale encountered for calling ILs green is their negligible vapor pressure, which, in comparison to conventional molecular organic solvents, reduces occupational health risks because of the limited exposure via air. Furthermore, the risk of explosions is limited because of the same property. 

Ionic liquids are organic salts with room temperature melting points. They are environmentally friendly, are able to dissolve most organic molecules, have high thermal stabilities and are nonflammable— properties that make these liquids attractive as solvents for organic synthesis. For a discussion of ionic liquids, go to the Focus On feature for Chapter 27, Green Chemistry and Ionic Liquids, on the MasteringChemistry site. 

Sheldon, R. (2005) Green Solvents for Sustainable Organic Synthesis-State of the Art. Green... 

Sheldon, R.A. 2005. Green solvents for sustainable organic synthesis State of the art. Green Chemistry, 7 267-78. 

Polyethylene glycol (PEG) has drawn considerable attention as a green reaction medium for organic synthesis [92]. Alkynes are successfiilly employed for the synthesis of Qxs using PdCyCuCl2 as the catalyst system in PEG-400 and water (8 2 Scheme 2.14) [93], The solvent system assists the oxidation of the alkyne to form the 1,2-dione as the intermediate species that undergo PEG-assisted cyclocondensation with the 1,2-diamine to form the Qx. 

From the environmental and economical viewpoints, water is the ideal green solvent for both the synthesis of coordination compounds and the catalytic transformations of organic molecules including the oxidative functionalization of alkanes [23]. However, the performance of catalytic reactions in aqueous medium typically requires the use of hydrosoluble catalysts that often mimic the functions of enzymes. Although various bioinspired multicopper complexes were synthesized as models of pMMO and related copper-based enzymes [3-5a], those catalysts were often not soluble in water, exhibited modest activities, or were almost not tested in oxidative transformations wherein alkanes are used as substrates. 

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