By Samantha A. M. Smith, Member-at-Large for the GCI
Green Chemistry Principle number two, Atom Economy, focuses on metrics used to compare the efficiency of a reaction.1 However, Atom Economy doesn’t take into account solvents, reagents such as catalysts, drying agents, energy, or recyclability of any of the materials. Is it reasonable for an industry such as pharma to use such a metric? What about E-factor, which is a measure of process waste and, if “complete” (cEF = complete E-factor), also recyclability of solvents and catalysts? It’s known that the pharmaceutical industry generally has the highest E-factor values compared to petrochemicals, bulk, and fine chemicals, indicating more waste generated per mass of desired product.2 But if you wanted to compare your technology to already implemented pharmaceutical processes, where would you find such information?
Roschangar, Sheldon, and Senanayake created a new metric for such a purpose: the Green Aspiration Level™.3,4 This new metric allows one to compare an ideal process with the average commercial process in terms of environmental impact for the production of a pharmaceutical. Say you have an alternative product to Viagra™ and want to know if its production is more or less impactful. You could apply any of the existing metrics (including yield, atom economy, E-factor, and more, summarized in Table 1 of reference 3), or you could use the Green Aspiration Level™ (GAL). To do so, you determine the waste (Complete Environmental Impact Factor (cEF) or Process Mass Intensity (PMI)) and assess the complexity of the process, and use those to calculate the GAL, and in turn the Relative Process Greenness (RPG). From there, you can consult Table 1 (below) to determine the greenness rating of such a process.4
Waste and Complexity
Waste refers to a simple metric such as cEF or PMI (with reactor cleaning and solvent recycling excluded). Complexity of the process refers to the number of steps with no concession transformations, that is those that do not directly contribute to the building of the target molecules’ skeleton.5 The waste and complexity metrics require that the process starting materials are less than $100 USD/mol for proper comparison.
Green Aspiration Level™
Roschangar and coworkers have collected data on many commercial processes to develop an appropriate metric, and they currently use 26 kg of waste per kg of product as a standard based on their findings. This value is known as the average GAL, or tGAL.3,4
GAL = (tGAL) x Complexity
= 26 x Complexity
Relative Process Greenness
RPG = GAL/cEF
This metric is used as the comparison point for processes. The comparison can be done at different stages of development, either early or late development, and then again for those processes that are commercialized. In Table 1, there are minimum RPG values that will associate the process with an appropriate greenness percentile.
RPI = RPG(current) – RPG(early)
RPG can also be used to determine the improvement of a process. From early development, to late development, to commercialization, the difference in consecutive RPG values will give your Relative (Green) Process Improvement (RPI). In this case, the higher the number the better.It turns out the current commercial process for Viagra™ is actually quite efficient and is currently in the 90th percentile, exceeding the commercial average by 143% (RPG). The full process of determining and using this new metric, the Green Aspiration Level™, is described by Roschangar and coworkers in two very in-depth articles.3,4
1 Anastas, P. T., Warner, J. C. “Principles of green chemistry.” Green chemistry: Theory and practice (1998): 29-56.
2 Sheldon, R. A., Catalysis and Pollution Prevention, Chem. Ind. (London), 1997, 12–15.
3 Roschangar, F., Sheldon, R. A., Senanayake, C. H., Green Chem. 2015, 17, 752. DOI: 10.1039/C4GC01563K
4 Roschangar, F., Colberg, J., Dunn, P. J., Gallou, F., Hayler, J. D., Koenig, S. G., Kopach, M. E., Leahy, D. K., Mergelsberg, I., Tucker, J. L., Sheldon, R. A., Senanayake, C. H., Green Chem. 2017, 19, 281. DOI: 10.1039/c6gc02901a
5 Crow, J. M., “Stepping toward ideality”, Chemistry World, accessed July 13th, 2017. URL: https://www.chemistryworld.com/feature/stepping-toward-ideality/5190.article
Figures from Roschangar et al. 2015 reproduced with the permission of the Royal Society of Chemistry.