The Need for Alternative Solvents

RSC Green Chemistry Book Series Alternative Solvents for Green Chemistry By Francesca M. Kerton Francesca M. Kerton 2009 

Solvent Boiling pointl°C Flash pointj°C TLV-TWA lppm Hazards Green  

Ethyl lactate 154 46 Not yet established Irritant, flammable Renewable 

Dichloromethane 40 none 100 Toxic, harmful, suspected carcinogen  

Water 100 none Not applicable Renewable, non-flammable, cheap 

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Another difficulty that has to be overcome is the traditional approach of many food processing organisations and the apprehension of working at high pressure. As the need for alternative solvents develops and more experimental data on a wide variety of products become available this difficulty should become less severe. 

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

Much of the current interest in using analytical-scale SFE systems comes from the need to replace conventional liquid solvent extraction methods with sample preparation methods that are faster, more efficient, have better potential for automation, and also reduce the need for large volumes of potentially hazardous liquid solvents. The need for alternative extraction methods is emphasized by current efforts to reduce the use of methylene chloride as an extraction fluid for environmental sample preparation [158]. The potential for applying SFE to a wide variety of environmental and biological samples for both qualitative and quantitative analyses is widely described in reviews [159-161] and the references therein. Analytical-scale SFE is most often applied to relatively small samples (e.g., several grams or less). 

If the product is needed on a small scale, up to 100 g, the reaction can be scaled out rather than scaled up. The excellent reproducibihty together with automation can easily produce up to 100 g overnight. Bose et al. have described an alternative for minor scale-up where the use of the microwave-assisted organic reaction enhancement (MORE) technique reduces the need for organic solvents and increases atom economy by improving product selectivity and chemical yield, thus, minimizing the need for larger scale-up. 

In this respect, fluorous-phase operation is similar to temperature-regulated phase transfer catalysis (see Section 2.3.5) and to special versions of soluble polymer-bound catalysis (see Chapter 7). Alternatively, the temperature-dependent solubilities of solid fluorous catalysts in liquid substrates or in conventional solvents containing the substrates could eliminate the need for fluorous solvents. 

Initially, multiple-step processes evolved, which could be conducted sequentially in a single reactor. These processes alternate the addition of phosphoric anhydride, water, alcohol, and hydrogen peroxide in a complex sequence to produce high MAP esters. To fill the need for a solvent to maintain reasonable viscosity throughout the process but avoid the necessity of its removal, the phosphate ester mixture itself could be used as a solvent [32,33]. 

Glass pH electrodes possess outstanding selectivities and the need for alternative H" ISEs might seem superfluous. Nonetheless, the contamination in biological media and high membrane resistances—particularly for micro glass sensors—pose problems (7). The pH behaviour of PVC sensor membranes comprising eleven different neutral ionophores in conjunction with KTpClPB and solvent mediator in the usual proportions is of some interest (53,54). 

Whereas the use of conventional fast atom bombardment (FAB) in the analysis of polymer/additive extracts has been reported (see Section 6.2.4), the need for a glycerol (or other polar) matrix might render FAB-MS analysis of a dissolved polymer/additive system rather unattractive (high chemical background, high level of matrix-, solvent- and polymer-related ions, complicated spectra). Yet, in selected cases the method has proved quite successful. Lay and Miller [53] have developed an alternative method to the use of sample extraction, cleanup, followed by GC in the quantitative analysis of PVC/DEHP with plasticiser levels as typically found in consumer products (ca. 30 %). The method relied on addition of the internal standard didecylphthalate (DDP) to a THF solution of the PVC sample with FAB-MS quantitation based on the relative signal levels of the [MH]+ ions of DEHP and DDP obtained from full-scan spectra, and on the use of a calibration curve (intensity ratio m/z 391/447 vs. mg DEHP/mg DDP). No FAB-matrix was added. No ions associated with the bulk of the PVC polymer were observed. It was... 

The need for multiple desolvation of the metal ion in some systems may provide a barrier to complex formation which is reflected by lower formation rates - especially for inflexible macrocycles such as the porphyrins. Because of the high energies involved, multiple desolvation will be unlikely to occur before metal-ion insertion occurs rather, for flexible ligands, solvent loss will follow a stepwise pattern reflecting the successive binding of the donor atoms. However, because of the additional constraints in cyclic systems (relative to open-chain ones), there may be no alternative to simultaneous (multiple) desolvation during the coordination process. 

The need for special facilities for work involving neutron activation analysis and radiochemical measurements has been referred to above in Section 4.3.6. Other safety factors may also influence your choice of method. For example, you may wish to avoid the use of methods which require toxic solvents, such as benzene and certain chlorinated hydrocarbons, or toxic reagents, such as potassium cyanide, if alternative procedures are available. Where Statutory Methods have to be used, there may be no alternative. In such cases, it is essential that staff are fully aware of the hazards involved and are properly supervised. Whatever method is used, the appropriate safety assessment must be carried out before the work is started. Procedures should be in place to ensure that the required safety protocols are followed and that everyone is aware of legislative requirements. 

Commonly used methods for the determination of petroleum hydrocarbon contamination in soil are modifications of Environmental Protection Agency method 418.1, which use sonication or a Soxhlet apparatus for analyte extraction and either infrared spectrometry [5] or gas chromatography with flame ionization detection [6-7] for extract analysis. Regardless of the analytical method following the extraction, both modifications use Freon-113, which has been implicated as a cause of ozone depletion. Therefore, alternative methods are being sought for the determination of hydrocarbon contamination in environmental samples that reduce the need for this halogenated solvent. 

After all we all need more efficient and less hazardous chemical processes. And for that reason we all need the use of alternative solvents. 

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