Sustainability green solvents

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. 

As indicated, IL is a broad class of solvents and it is not correct to derive unique conclusions. However, it is also not possible to present ILs as green solvents [109], or to claim as sustainable a chemical process because it uses ILs as solvents. They can be corrosive, flammable or toxic. Their impact on aquatic ecosystems, due to their medium to high solubility in water, is another critical element. Their nonvolatile nature could be a factor for a lower impact on the environment and human health. 

Ionic liquids are indicated as green solvents because of their negligible vapor pressure. However, major concerns exist regarding their impact and persistence in the environment, raising several doubts about whether they could really be presented as an advance towards sustainable chemical production. 

The unique properties of highly fluorinated and perfluorinated ( fluorous ) solvents and reagents open several routes to a solution of these problems and to a sustainable green chemistry [1-5]. These properties include their very temperature-dependent miscibility with typical hydrocarbons, their non-toxicity, and their extreme chemical inertness. 

Reusability is a characteristic of the sensitizers prepared by stirring Ceo-fullerene with aminomethylated poly(styrene/vinylbenzene). They have been used to promote the standard O2 oxidation processes such as ene and Diels-Alder reactions (Scheme 7), and catalysts suitable for photoxidations in water have been prepared from them by reaction with poly(allylamine). The same reactions have been carried out using a novel solvent-free procedure which involves loading a porphyrin into solvent-swollen polystyrene beads and carrying out the photo-oxidation in neat liquid substrate. The formation of the allylic hydroperoxide (89) from p-pinene, with complete conversion and in 84% yield, is particularly noteworthy, as the standard liquid-phase reaction can be problematic. It is suggested that the possibility of using this approach under solar conditions is further evidence of the sustainable, green chemistry potential of synthetic photochemistry. 

Microwave energy is a key enabling technology in achieving the objective of sustainable (green) chemistry. It has been shown that solvent-free conditions are especially suited to microwave-assisted organic synthesis, because reactions can be run... 

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. 

The notion of sustainability in highly specialized markets where specifications and performance are key requirements is discussed in Chapter 10 (green solvent alternatives for fine chemicals, for metal treatment, for coatings and for crop protection formulations) and in Chapter 11 (sustainable solutions for adhesives and sealants). 

Sustainable Catalysis Without Metals or Other Endangered Elements, Parts 1 and 2 deal with catalysts that do not possess a metal centre as part of their structure. After an introductory chapter. Chapters 2-4 cover non-asymmetric acid and base catalysis. The subsequent chapters (5-24) deal with asymmetric organocatalysis as this area has exploded in importance over the last 20 years. Again, catalysts that contain endangered elements e.g. phosphorus) have been excluded and authors were asked to highlight any examples that have other sustainable features (use of green solvent, high atom economy, etc.). 

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