Atom Economy, Expanding Boundaries to Incorporate Upstream Reactions

In this work, we address one of the overall shortcomings of atom economy and the limited scope of the boundaries chosen. While atom economy is not the only measure of a process' sustainability, it is a valid comparison from a materials usage view. With this in mind, many publications in green chemistry focus on atom economy as a method for estimating the environmental impacts of a reaction scheme to reach a desired product. The choice of which reactions are considered in the atom economy analysis, or the system boundaries, plays as great of an importance as it does in performing life cycle assessments. Most applications of atom economy begin their analysis with manufactured intermediates and follow a short chain of reactions to the final products. This ignores any upstream processing that was done to create the intermediates and may skew results to incorrectly favor one of the proposed reaction schemes. Here, we develop a simple expression for atom economy that aggregates overall reaction stoichiometries for a chain of reactions leading from raw materials to the final products. We find that by expanding the boundaries to incorporate upstream manufacturing of raw materials, the overall atom economy changes substantially. In order to fully utilize atom economy, system boundaries must encompass upstream reactions in order to prevent sub-optimal materials usage. In this paper, we calculate the overall atom economy of two reaction schemes for synthesizing 1,3-propanediol. This system was chosen as a case study to highlight the choice of system boundaries. By implementing the aforementioned equation to include the upstream processing reactions it is found that the atom economy of each process is no longer 100% as had been reported previously. Introduction Most chemists have traditionally measured the efficiency of a reaction by the percentage yield . In an effort to foster awareness of the atoms of reactants that are incorporated into the desired product and those that are wasted (incorporated into undesired products), the concept of atom economy (AE) was first developed by Barry Trost in 1991. There are many papers on atom economy [3-6] that only address the atom economy of reactions within a limited system boundary. By limiting the scope of the calculation, atom economy can falsely identify favorable reaction synthesis routes. Due to the numerous different routes that can be utilized to produce a single product, a more thorough atom economy calculation should be used. Fig.1 depicts an expanded version of typical atom economy calculations. Many atom economy calculations calculate only either the final or last two steps of a synthesis in order to prove that one route is more favorable than another. However, using a boundary that encompasses all of the reactions, it can be seen that exanding the boundaries could lead to different results. In the case where only the last two steps for each synthesis are considered, the bottom route in Fig. 1 appears more favorable, but the answer could change by moving to expanded boundaries as shown. Although this is a simplified example, it highlights the problem of non-optimal choices that can be made by limiting system boundaries. B