Solvents are used in several industrial processes, academic research, and in a large section of economic activities worldwide. The volume of these materials released into the environment is extremely concerning. Their generally high vapor pressure allow for easy dispersion in air, promoting contact with living organisms and the pollution of the environment. The high volume, the particular properties of each solvent and the volatility factor in high-risk solvents threaten animal and plant life, and the sustainability of our environment.
Multiple international treatises on the subject have generated guidelines to ban the usage of solvents like carbon tetrachloride and benzene due to their acute carcinogenic properties. However, the utilization of solvents, which border on “safe” and unsafe solvent – like methylene chloride and hexanes – have not been limited by means of any public policy or law enforcement – even when there is evidence of their negative effects on human health. Although contradictory, this is at some level reasonable; their properties and applications are economically important that banning them may cause the collapse of many industries. Production of essentials like shoes, bathtubs, pharmaceuticals, polymer fabrication, manufacturing of light bulbs, extraction of cooking oils, glues, etc. are just a few examples that put us in this paradoxical situation of having to survive within the demands of modern life while diminishing our health and tilting the fragile equilibrium of the ecosystems we inhabit. In order to weaken the force of the boomerang of civilization flourishing, and the inevitable self-destruction that our society and way of life promotes, a lot of advances in green chemistry are occurring to specifically address the three alternatives to our solvent issues: solvent usage minimization, replacement with green alternatives, or elimination of solvents.
The utopian image of chemical transformations and processes is sketched in solventless conditions. However, many processes require a solvent of some kind in order to be successful, other alternatives exist. Solvent minimization involves either recycling solvents for reuse or manufacturing protocol modifications to carry out reactions in highly concentrated solutions. Currently the research efforts at the green chemistry area are focused on the design and search of the materials to replace the usual and toxic solvents like methylene chloride for aqueous solvents, supercritical or dense phase fluids, ionic liquids, liquid polymers, switchable solvents, CO2-expanded liquid, immobilized solvents and fluorous solvents.
Unfortunately many researchers are hesitant to replace solvents with green alternatives due to the sometimes complex modifications that must be undertaken. If we take as an example CO2-based solvents, the equipment and operator training would undergo a radical change. The chemical synthesis laboratory of an average academic institution would have to be remodeled in some cases in order to do chemistry regularly with these new alternatives. Sadly, transitions like these will take time and resources. Without being overly pessimistic the transition could take more than a generation of scientists in a standard academic lab to phase out wash bottles of acetone and rotoevaporators. The desire for more feasible replacements have been achieved by slight modifications of the structure of conventional solvents in order to retain solvent properties without retaining the harmful effects of the replaced solvent. For instance, retaining properties such as hydrogen-bond accepting/donating ability, polarity, and polarizability, but avoiding such hazards as flammability, peroxide formation, and persistency. Some examples of green alternatives include ethyl lactate (low toxicity, biodegradable, renewable), γ-valerolactone (low toxicity, biodegradable, renewable), 2-methyltetrahydrofuran (renewable), and cyclopentylmethylether (does not form peroxides, low solubility in water).
Because of the known hazards associated with many solvents, the transition to green alternatives has already begun to be installed in the laboratories and minds of future principal investigators and industrial project managers; therefore, as a researcher it is imperative that a conscious effort is made to eliminate the use of hazardous solvents and make “greener” processes in the laboratory. An excellent starting point is a table Pfizer has developed a table to aid in selecting commercially available solvents in order to make synthesis “greener”. The table is color coordinated organize desirable solvents (green), acceptable solvents (yellow), and undesirable solvents (red).
Other ways to evaluate the ”greenness” of a solvent include three major factors not normally considered by researchers when choosing a solvent: 1) environmental impact of the solvent, 2) environmental impact of the manufacturing process to produce the solvent, 3) and the environmental impact of the energy usage during solvent production. Examples of the kinds of environmental impacts solvents are known to have include flammability, toxicity, smog formation, and inhalation hazards; the last two are closely related to the volatility of the solvent.
While solvents are still a significant environmental issue and the ideal solution for this complex problem is being formulated, an important contribution from the solvent user is to make wise choices every day in lab. Using Pfizer’s table as a guide or reducing the amount of solvent in our reactions is as simple as turning off the lights in an empty room or riding a bike to campus in order to reduce energy use. However, the elimination or greening of solvents is even greater due to the impact, persistence, and hazards these substances contribute to the environment. Think about the difference that can be made if the scientific community embraces green practices in their pursuit of research/design objectives. Remember that the smallest reaction with the smallest amount of a hazardous solvent still negatively impacts our environment and our community.