The motivation for using metrics is the expectation that quantifying technical and environmental improvements can make the benefits of new technologies more tangible, perceptible, or understandable.
This, in turn, is likely to aid the communication of research and potentially facilitate the wider adoption of green chemistry technologies in industry.
For a non-chemist, an understandable method of describing the improvement might be a decrease of X unit cost per kilogram of compound Y.
For example, it would not allow a chemist to visualize the improvement made or to understand changes in material toxicity and process hazards.
For yield improvements and selectivity increases, simple percentages are suitable, but this simplistic approach may not always be appropriate.
For example, when a highly pyrophoric reagent is replaced by a benign one, a numerical value is difficult to assign but the improvement is obvious, if all other factors are similar.
A general problem is that the more accurate and universally applicable the metric devised, the more complex and unemployable it becomes.
A good metric must be clearly defined, simple, measurable, objective rather than subjective and must ultimately drive the desired behavior.
For companies that produce thousands of products, mass-based metrics may be the only viable choice for monitoring company-wide reductions in environmental harm.
In contrast, impact-based metrics such as those used in life-cycle assessment evaluate environmental impact as well as mass, making them much more suitable for selecting the greenest of several options or synthetic pathways.
[3] Atom economy was designed by Barry Trost as a framework by which organic chemists would pursue “greener” chemistry.
Economic and environmental costs of disposal of these waste make a reaction with low atom economy to be "less green".
This metric is a good simplification for use in the pharmaceutical industry as it takes into account the stoichiometry of reactants and products.
The atom economy calculation is a simple representation of the “greenness” of a reaction as it can be carried out without the need for experimental results.
However, this may not be considered as a "greener" method, as it implies a greater amount of the excess reactant remain unreacted and therefore wasted.
{\displaystyle {\text{Reaction mass efficiency}}={\frac {{\text{atom economy}}\times {\text{percentage yield}}}{\text{excess reactant factor}}}}
For example, these metrics could present a rearrangement as “very green” but fail to address any solvent, work-up, and energy issues that make the process less attractive.
Hudlicky defines it as “those by-products, reagents or solvents that have no environmental risk associated with them, for example, water, low-concentration saline, dilute ethanol, autoclaved cell mass, etc.”.
Until all toxicology data is available for all chemicals and a term dealing with these levels of “benign” reagents is written into the formula, the effective mass efficiency is not the best metric for chemistry.
Sheldon's analyses (see table) demonstrate that oil companies produce less waste than pharmaceuticals as a percentage of material processed.
The (currently) high profit margins within the sector mean that there is less concern about the comparatively large amounts of waste that are produced (especially considering the volumes used).
This table encouraged a number of large pharmaceutical companies to commence “green” chemistry programs.
[citation needed] The EcoScale metric was proposed in an article in the Beilstein Journal of Organic Chemistry in 2006 for evaluation of the effectiveness of a synthetic reaction.
Like the yield-based scale, the EcoScale gives a score from 0 to 100, but also takes into account cost, safety, technical set-up, energy and purification aspects.
These penalty points take into account both the advantages and disadvantages of specific reagents, set-ups and technologies.