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Green Chemistry
Published May 28, 2023
Green chemistry is a philosophy focused on the design of chemical processes and products that eliminate or minimize the use and generation of hazardous substances.
When chemistry is conducted—when things are being made into other things—how might it affect the environment or public health? As we expend the planet’s resources to alter matter to our liking, how must we proceed so as to undertake a circular economy?
These questions we will find ourselves asking in a world of limited resources.
Following the philosophy of green chemistry requires upholding most—if not all—of its twelve principles.
Prevention is better than cure—preventing waste is preferable to managing waste after its development.
Principle 1 is the most fundamental of the principles of green chemistry. While it may exist merely as an idea, the following eleven principles provide guidance toward its realization.
Chemical reactions should be designed to maximize the incorporation of all reagents in the target compound.
Reactions consist of reactants and products. When it comes to mass-producing a compound, the reaction in which the compound forms will often produce several other compounds—the byproducts. Of all the products formed in this reaction, one is of interest—the target compound—and the others become waste.
Adopting principle 2 is as simple as asking, “How many atoms in the reactants will end up in the target compound, and how many of these atoms will end up as waste?” To embrace the concept of atom economy, and thereby reducing waste, is to maximize the percentage of atoms making up the reactants into the target compound.
Chemical processes should be designed to minimize toxicity to humans and the environment.
In some instances, it may not be practical to avoid using toxic substances. Though the toxicity of a chemical process is often associated with factors unrelated to the synthetic pathway leading to the target compound. These factors can be altered such that the chemical environment in which the synthetic pathway takes place is less hazardous.
Chemical products should be designed to reduce toxicity while maintaining their efficacy.
While principle 3 discusses altering the processes and surroundings of a chemical reaction, principle 4 discusses altering the reaction itself.
It can be challenging to find an alternative reagent or target compound that is less toxic and as functional as the substance that has already established its economical feasibility. The best approach to overcome such an obstacle is to promote the advancement of toxicology and molecular biology and their relation to chemical characterization.
Think of it this way: hazard is a design flaw. If a hazard arises with managing a chemical, the designer has failed.
Auxiliary substances—solvents and separation agents—should be made unnecessary wherever possible and innocuous when used.
Solvents and mass separation agents allow for many reactions to proceed; they provide a medium for mass and energy to transfer from reactant to product.
Solvents, in particular, cannot be disregarded, as they account for 50 to 80 percent of the total mass of a standard batch chemical operation. They are also the greatest contributor to the overall toxicity profile, ranking them the highest concern to process safety issues.
Complying with principle 5 could mean preserving more than half the mass of raw materials. Nonetheless, most reactions require solvents, so it is a matter of choosing a solvent that has the least toxicity, reduces energy requirements, and shows little to no interference with terrestrial and aquatic ecosystems.
The energy used throughout a chemical process should be minimized, as well as being recognized for its environmental and economic impacts. Reactions should be conducted at ambient temperature and pressure.
Fortunately, most reactions are conducted at ambient temperature and pressure, and the global energy sector is diligently making energy production more green and efficient as we approach the 22nd century. Regardless of the sustainability of energy production, energy use can be reduced, which warrants principle 6 to focus on other design parameters, such as arranging a synthesis to comprise the fewest number of steps, or using the lowest-cost starting materials.
A raw material—feedstock—should be renewable, rather than depleting.
Frankly, not all feedstocks can be renewable, and some alternatives that are renewable are not economically practicable.
There is hope for issuing renewable feedstocks as substitutes for non-renewable materials in the vast majority of economic sectors. For more than two decades now, the U.S. Department of Energy has had the vision to develop a “well-established, economically viable, bioenergy, and bio-based products industry.”
The world’s ecosystems produce 170 billion tons of plant biomass annually, and it is estimated that only 25 percent of annually-produced biomass affords enough resources for renewable starting materials to generate a bio-based economy.
Derivatization—blocking groups, protection, deprotection, modification of chemical processes—should be minimized, or avoided altogether if possible.
Derivatization is a common technique applied to the synthesis of organic compounds when several steps are required to obtain the target compound. Sometimes a starting material will react with an additional reagent in ways the chemist does not prefer. To prevent such an occurrence, the starting material could be “dressed up” to protect certain parts of its structure from being altered by the additional reagent. This protection leaves the reagent no other choice but to alter the parts of the starting material that are exposed.
The derivatization of chemical species requires additional reagents, leading to more steps and ultimately generating waste. If one’s goal were to minimize the number of steps required to obtain the target compound—principle 6—and reduce derivatives—principle 8—one would need a contribution so selective as to launch the starting material closer in structure to the target compound than any other method.
Even among the most complicated pathways lies the simplest of solutions: developing enzymes with specificity for target compounds is the best option to reduce derivatives. The potential of enzymes and their catalytic properties are described further in the following principle.
Catalytic reagents are superior to stoichiometric reagents.
A catalyst is a substance that enhances the rate of a reaction while remaining unchanged in its structure throughout the process. Because of its remarkable reactivity and invariable structure, a catalyst can be used sparingly and recycled indefinitely.
Catalysts, especially enzymes, have been paving the way for a bio-based economy, as they are highly effective at shaping substrates into those of astounding complexity.
A chemical product for the end consumer should be designed to break down at the end of its function into innocuous parts that do not persist in the environment.
Whereas principle 3, 4, 5, and 12 guide designers to reduce the hazards of compounds in the production phase, principle 10 emphasizes the design of the end-consumer chemical product.
Hazard—a potential source of harm—is a characteristic of a compound’s stoichiometry—the arrangement of atoms. Exposure is the contact between a compound and an organism. When a compound is characteristically hazardous and exposed to an organism, risk develops. Degradation can eliminate exposure and consequently prevent all risk associated with the compound or its parts.
Just how knowledge of toxicology can be used to design out molecular features underlying a molecule’s hazardous nature, mechanisms of degradation—biodegradation, hydrolysis, photolysis—can be designed into molecules to inhibit their environmental persistence.
Technologies that allow for real-time monitoring of chemical processes must be implemented for reactions that involve the formation of hazardous substances.
Principle 11 aligns closely with process analytical chemistry, a subdiscipline with attention to integrating real-time feedback of chemical processes. Technologies providing this real-time feedback can detect changes in process temperature, pH levels prior to reactions spiraling out of control, the poisoning of catalysts, and other signs of an approaching disaster.
Substances used in a chemical process should be chosen based on minimizing the likelihood of accidents, such as releases, fires, and explosions.
Principle 12 is customarily dubbed the “Safety Principle.”
Safety: the control of recognized hazards to achieve an acceptable level of risk. Principle 12 is often overlooked, since following through with most of the other principles results in safer conditions for both the labor force and consumers.
Sources
American Chemical Society. (2021). 12 Principles of Green Chemistry.