Green chemistry is a redefinition of how chemistry and chemical engineering should be approached and practiced and, as such, has the potential to apply to all fields of chemistry.
In 1998, Anastas and Warner published 12 principles which outlined their idea of what makes a chemical process or product greener. Over the past 20 years, there has been many more contributions to define green chemistry, but no single definition or unified framework has been agreed upon1. The most promising efforts stem from green chemistry metrics. Leaving value-based conceptions behind, green chemistry metrics aims at quantifying different aspects of a chemical process to determine whether or not it is environmentally friendly.
Green chemistry can be defined most rigorously using metrics. One of the biggest challenges in green chemistry is in defining what makes a chemical process or product green5. Still open to debate even today are questions such as, “What is green?” and “How green?”. Various metrics - each representing a different approach to quantifying the ‘greenness’ of a process - have been proposed over time but no unified framework has yet been agreed upon. The complexity of chemical processes is partly to blame because it makes it difficult to formulate clear and simple-to-use metrics. Metrics that have already been formulated encompass the environmental impacts of mass and energy, hazardous substance, and life cycles4. Below are a few of the most notable metrics, each presenting a different perspective on green chemistry.
In 1991, Barry Trost, a synthetic organic chemist, introduced the concept of atom economy. Atom economy calculates how many atoms from the reactants remain in the final product2. It a simplified, even simplistic, representation which aims to evaluate how green a chemical reaction is. As a standalone metric, it does not offer a complete picture. However, it is a historically important metric that helped spread awareness of green engineering principles.
Reaction mass efficiency uses atom economy, yield, and stoichiometry (the ratios of products and reactants in a chemical process) to calculate the proportion of the mass of the reactants that remain in the final product. Reaction mass efficiency focuses on the impact of the use of materials rather than on the impact of waste. Similar to atom economy, it fails to include other materials than reactants such as reagents, solvents, and catalysts1. This is why process mass intensity was introduced.
Process mass intensity calculates the mass intensity of a reaction and takes all materials (reactants, reagents, catalysts, solvents) into account. It is the ratio of the total mass used in the reaction by the mass of the final product. This metric aimed to combat the general lack of efficiency of pharmaceutical processes.
The E-factor has a similar approach. This metric is the ratio of total waste (kg) generated by a chemical process to the mass (kg) of the product created. It is a simple and intuitive metric that can be used in all industries. Its main weakness is the lack of clarification of what constitutes waste.
Effective mass yield is defined as the percentage of the mass of the chemical product relative to the mass of all "non-benign" materials used for its synthesis. This metric gives the proportion of the final mass of the product that is made from non-toxic materials. It is the first metric that factors in the toxicity of all components (reactants and reagents). As with the E-Factor calculation, the definition of “non-benign” is imprecise.
Molar efficiency is the ratio of the moles of final product and moles of all reaction materials (reactants, additives, solvents, catalysts). This metric is mostly useful in evaluating the efficiency in discovery medicinal chemistry.
Introduced in 19971, step economy establishes the direct connection between the number of steps in a chemical process and its greenness. While seemingly obvious, this metric pioneered this type of holistic thinking.
Following the same logic, pot economy states that the fewer pots (or reaction vessels) that are used for a multicomponent reaction, the greener the process will be.
Each metric is a unique way of quantifying the greenness of a chemical process and develops unique green chemistry methodologies and approaches. Each metric is valid in its own limited scope of definition. They are most valuable when combined together to form a systematic approach to the evaluation of the greenness of a chemical process or product. While no unified framework has yet been agreed upon, when used together, metrics are complementary and offer a powerful set of green chemistry tools.