What Factors Are Taken Into Consideration When Selecting a Solvent? – ACS GCI Pharmaceutical Roundtable (2024)
In order to gain a greater understanding of what makes the ‘best’ solvent for a given reaction, one must look deeper into the physicochemical properties of the solvent, as well as considering hazards (both human and environmental) and other considerations such as recoverability, renewability and any regulatory issues. (The criteria that are taken into consideration when assessing the environmental credentials of a solvent are explored inSolvent selection guides: Criteria for Solvent Selection).
Selection of a green solvent is complex as the following constraints all need to be taken into consideration. [1][2]
Chemical efficiency for the reaction (as well as downstream considerations of isolation/work-up etc.);
Safety/stability (flammability, flash point, resistivity, energy of decomposition, risk of peroxides);
Human health issues (acute, long-term and single target organ toxicity);
Environment (biodegradability, ecotoxicity, solubility in water, volatility, odour, life cycle analysis);
Quality (in the case of pharmaceuticals this is related to the risk of impurities in the drug substance);
Industrial Constraints (such as boiling point, freezing temperature, density, recoverability/recyclability);
Cost;
Compliance with legislation (e.g. regulations such as REACH are impacting on the use of some hazardous solvents, for example dipolar aprotics).
Safety/stability (flammability, flash point, resistivity, energy of decomposition, risk of peroxides); Human health issues (acute, long-term and single target organ toxicity); Environment (biodegradability, ecotoxicity, solubility in water, volatility, odour, life cycle analysis);
Compatibility: The solvent should be compatible with the other components of the system, such as the reaction vessel and any catalysts or reagents used. Safety: The solvent should be non-toxic and non-flammable, and should have a low vapor pressure to minimize the risk of fire or explosion.
Important solvent properties besides solvency include physical properties such as volatility, boiling point, surface tension, viscosity, and electrical resistance/conductivity. Other important criteria that I will not discuss here include cost, toxicity, flammability, and odor.
That is, solutes typically will dissolve best in solvents that have the most molecular similarities: polar solutes will dissolve better in polar solvents, and nonpolar solutes will dissolve better in nonpolar solvents.
Polar protic solvents can form hydrogen bonds with water to dissolve in water and are best for dissolving polar reactants such as ions. Nonpolar solvents are not capable of strong hydrogen bonds and are better used for dissolving nonpolar reactants such as hydrocarbons.
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