Using Green Solvents

The processing of the feedstock e.g. extraction using green solvents... 

Selective extraction of organic conqx>unds using green solvents is attractive from economic and environmental points of view. Carbon dioxide and water are two of the cheapest and most environmentally acceptable solvents on the earth. As explained in die introduction section, liquid and supercritical CO2 are able to dissolve non-polar or slighdy polar organic conqiounds. Water is an excellent solvent for ionic conqiounds because of its high dielectric constant (8 =78.5 at 25 °C) which decreases with increasing tenqierature due to the... 

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. 

This makes for useful green solvents because having the ability to tune the density, solubility properties, and diffusivity of the supercritical fluid allows for reaction or extraction conditions to be sensitively manipulated. 

The chemical industry is broad, and it would not be possible to adequately cover all the areas here. However, some representative examples illustrate the earlier concepts and provide incentive for future work. The solvents presently used in chemical reactions and processes were chosen based upon on performance criteria, including optimal yield, chemical reactivity and selectivity. Each industry or chemical sector must determine how the regulatory, economic and technological drivers affect their solvent supply in the near and far term, and motivate them to use green solvents. 

The development of modem and reliable procedures for microwave-assisted organic synthesis has also been used advantageously for 1,2,4-oxadiazole synthesis by avoiding time-consuming steps, using green solvents or solvent-free conditions, or by directly employing acids rather... 

Use of safer chemicals including solvents satisfies one of the criteria of green chemisfry principles. From green chemistry perspective, solvent-free chemical fransformafions are indisputably more desirable than reactions in any kind of solvents. However, feasibility of solvenf-free reactions is often complicated by problems with reaction (especially exothermic) control, mass transfer limifafion, viscosity and melting point of reacfanfs, etc. Water is the solvent of first choice provided by nature. This is the most commonly used green solvent and heterocyclic S5mthesis in aqueous medium is discussed in details in Chapters 7 and 8 of this book. Unfortunately, very often water is not suitable for organic reactions and choice of another more efficient solvent becomes critical. 

Francos J, Cadierno V (2010) Palladium-catalyzed cycloisomerization of (Z)-enynols into furans using green solvents glycerol vs. water. Green Chem 12 1552-1555... 

To a solution of 0.6 g naphthalene in 10 ml tetrahydrofurane, anhydrous, used as solvent, add 96 mg sodium under a nittrogen atmosphere. After a few minutes, an intensive dark green coloration develops, while the sodium dissolves. The reaction is completed after a period of time ranging between 30 and 60 minutes. 

In cases in which product solubility in the ionic liquid and the product s boiling point are high, the extraction of the product from the ionic liquid with an additional organic solvent is frequently proposed. This approach often suffers from some catalyst losses (due to some mutual solubility) and causes additional steps in the workup. Moreover, the use of an additional, volatile extraction solvent may nullify the green solvent motivation to use ionic liquids as nonvolatile solvents. 

Ionic liquids have several important features that make them attractive for use as solvents, particularly in green chemistry ... 

Recently, room temperature ionic liquids (RT-ILs) have attracted much attention for their excellent properties, e.g., wide temperature range of liquid phase, ultra-low vapor pressure, chemical stability, potential as green solvents, and high heat capacities [64,65]. These properties make them good candidates for the use in many fields, such as thermal storage [66], electrochemical applications, homogeneous catalysis [67], dye sensitized solar cells [68], and lubricants [69,70]. 

Pyrrolidones fit well into the bio-refinery concept since they may be produced in a scheme beginning with the fermentation of a portion of the bio-refineiy s sugar product into succinate. Pyrrolidones are a class of industrially important chemicals with a variety of uses including polymer intermediates, cleaners, and green solvents which can replace hazardous chlorinated compounds. 

Mixtures of low molecular weight products and DEK can find large uses as low volatility green solvents [18,26]. 

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