Solvent-free cleaning

Clearly, the most effective way to avoid problems with solvent contamination of the environment is to eliminate the use of the solvent entirely. Even though this clearly is not possible for most applications, there are some innovative solvent-free cleaning processes that deserve mention. 

The solvent-free Cannizzaro reaction has some advantages. In addition to simplicity and cleanness of the procedure, the solvent-free reaction proceeds much faster than a solution reaction. For example, reaction of 51 in 60% aqueous NaOH takes 24 h to complete [10], although the solvent-free reaction is completed within 5 min [9]. 

Many of the adverse consequences of injecting dirty samples can be prevented or minimised by the use of guard columns, as discussed earlier, but often some form of sample clean-up is needed as well. The goal of sample preparation is to obtain, from the sample, the components of interest in solution in a suitable solvent, free from interfering constituents of the matrix, at a suitable concentration for detection and measurement. Naturally we want to do this with the minimum time and expense. 

Epoxides have been found to cleanly react with acetic anhydride to provide the diacetate under solvent-free conditions <06TL6865>. Treatment of epoxides with ammonium-12-molybdophosphate and a slight excess of acetic anhydride (1.2 equivalents) provides the corresponding diacetate in excellent yields. A number of epoxides were examined and all worked quite well. It was also found that /V-losyl aziridines participate in this reaction providing the corresponding acetoxysulfonamides. 

Microwave effects are most likely to be observed under solvent-free reactions [3]. In addition to the preparative interest of these methods in terms of use, separation, and economical, safe and clean procedures, absorption of microwave radiation... 

Significant improvements in yields or reaction conditions can be achieved, together with considerable simplification of operating procedures. The powerful synergistic combination of PTC and microwave techniques has certainly enabled an ever increasing number of reactions to be conducted under clean and mild conditions. The inherent simplicity of the method can, furthermore, be allied with all the advantages of solvent-free procedures in terms of reactivity, selectivity, economy, safety, and ease in manipulation. 

Property measurements of fullerenes are made either on powder samples, films or single crystals. Microcrystalline C6o powder containing small amounts of residual solvent is obtained by vacuum evaporation of the solvent from the solution used in the extraction and separation steps. Pristine Cgo films used for property measurements are typically deposited onto a variety of substrates (< . , a clean silicon (100) surface to achieve lattice matching between the crystalline C60 and the substrate) by sublimation of the Cr,o powder in an inert atmosphere (e.g., Ar) or in vacuum. Single crystals can be grown either from solution using solvents such as CS and toluene, or by vacuum sublimation [16, 17, 18], The sublimation method yields solvent-free crystals, and is the method of choice. 

As with cleaning, many researchers are looking toward solvent-free coatings as the answer to solvent replacement. 

A clean, solvent-free method has been developed for the bis-hydroxylation of alkenes by the use of Nafion-based acidic catalysts and 30% H202.655 Nafion NR50 and SAC-13 exhibited high activity in the oxidation of isomeric C alkenes, cyclohexene [Eq. (5.228)], 1,4-cyclohexadiene, and allylic alcohols in the temperature... 

Tetrasubstituted pyrroles could be obtained by skeletal rearrangement of 1,3-oxazolidines, a reaction that is substantially accelerated by microwave irradiation. Dielectric heating of a 1,3-oxazolidine 7, absorbed on silica gel (1 g silica gel/mmol) for 5 min in a household MW oven (900 W power) cleanly afforded the 1,2,3,4-tetrasubstituted pyrrole 8 in 78% yield, thus reducing the reaction time from hours to minutes (Scheme 5) [24], 1,3-Oxazolidines are accessible in one-pot, two-step, solvent-free domino processes (see also Sect. 2.6). The first domino process, a multi-component reaction (MCR) between 2 equivalents of alkyl propiolate and 1 equivalent of aldehyde furnished enol ethers 9 (Scheme 5). Subsequent microwave-accelerated solvent-free reactions of enol ethers 9 with primary amines on silica support afforded intermediate 1,3-oxazolidines, which in situ rearranged to the tetrasubstituted pyrroles (2nd domino process). Performed in a one-pot format, these... 

Solvent is easily removed, due to its zero surface tension, leaving the product in an easily processable, clean and solvent-free form. 

Analytical studies of the PVC profile surface indicated that the reasons for failure of the adhesive bond between the PVC film and PVC profile in decorative film laminated structures, was finely dispersed lubricant and wax particles on the PVC profile. These contaminants prevented the build-up of bond forces at the interface between the adhesive and PVC profile. A biodegradable alkaline cleaning agent was developed to remove the contaminants, which, when combined with flame treatment and a solvent free polyurethane hot melt... 

Fraga-Drubreuil, J. and Cherouvrier, J.R. 2000. Clean solvent-free dipolar cycloaddition reactions assisted by focused microwave irradiations for the synthesis of new ethyl 4-cyano-2-oxazoline-4-carboxylates. Green Chemistry, 2 226-29. 

For the development of a sustainable chemistry based on clean technologies, the best solvent would be no solvent at all. For this reason, considerable efforts have recently been made to design reactions that proceed under solvent-free conditions, using modern techniques such as reactions on solid mineral supports (alumina, silica, clays), solid-state reactions without any solvent, support, or catalyst between neat reactants, solid-liquid phase-transfer catalysed and microwave-activated reactions, as well as gas-phase reactions [37-42]. However, not all organic reactions can be carried out in the absence of a solvent some organic reactions even proceed explosively in the solid state Therefore, solvents will still be useful in mediating and moderating chemical reactions and this book on solvent effects will certainly not become superfluous in the foreseeable future. 

Avoiding the use of organic solvents during MAOS of heterocycles lead to a clean, efficient and economical technology (Green Chemistry). Apart from this the safety is increased, workup is considerably simplified, cost is reduced, the use of toxic solvents can be avoided and the reactivities and sometimes the selectivities can also be enhanced without dilution. Due to these positive aspects of solvent free synthesis, there is a marked increase in the use of environmentally benign reagents and procedures. 

Rostamizadeh S, Ghaieni HR et al (2010) Clean one-pot synthesis of 1,2,4-oxadiazoles under solvent-free conditions using microwave irradiation and potassium fluoride as catalyst and solid support. Tetrahedron 66 494-497... 

PPG 425 has been used in the preparation of a range of benzaldehydes that were subsequently used in the solvent free synthesis of calix[4]resorcinarenes. The aldehydes are isolated in near quantitative yields by distillation from the PPG, which can be recycled. PPG has also been used in the indium metal mediated allylation of imines and sulfonylimines. Ultrasound was used for two reasons to clean the metal surface and to increase the solubility of the imine in PPG. The solvent was recycled three times, but a desire to develop an easier drying and recycling method was indicated. 

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