Green Chemistry Principle #2
Synthetic methods should be designed to maximize incorporation of all materials used in the process into the final product.
Contributed by Michael Cann, Ph.D., Professor of Chemistry, University of Scranton
The second principle of green chemistry can be simply stated as the “atom economy” of a reaction. Atom economy, which was developed by Barry Trost1, asks the question “what atoms of the reactants are incorporated into the final desired product(s) and what atoms are wasted?”
Traditionally, the efficiency of a reaction has been measured by calculating the percent yield. Let us assume that the following substitution reaction gives 100% yield. While this is admirable, we can shed more light on the efficiency of a reaction by calculating the “percent atom economy” as follows:
% Atom Economy = (FW of atoms utilized/FW of all reactants) X 100
= (137/275) X 100 = 50%
The percent atom economy is simply the formula weight of the desired product(s) (compound 4, 137 g/mol) divided by the sum of the formula weights of all the reactants (275 g/mol), which gives 50% in this case. Simply put, even if our percent yield is 100%, only half the mass of the reactants atoms are incorporated in the desired product while the other half is wasted in unwanted by-products. Imagine telling your mom you baked a cake and threw away half the ingredients! Thus chemists must not only strive to achieve maximum percent yield, but also design syntheses that maximize the incorporation of the atoms of the reactants into the desired product.
Principle #2 deals with the reactants. However, as those of us who have run a chemical reaction know, we usually use other materials such as solvents and separating agents during a synthesis. These materials usually make up the bulk of the material input, and thus we must also account for the waste that is produced from them. Stay “tuned” as you will see these discussed in subsequent Principles of Green Chemistry.
More Resources & Examples:
Atom Economy: A Measure of the Efficiency of a Reaction. Michael C. Cann and Marc E. Connelly; Real-World Cases in Green Chemistry; ACS, Washington, 2000.
1998 PGCCA Winner: Professor Barry M. Trost of Stanford University, "The Development of the Concept of Atom Economy."
1. The Atom Economy-A Search for Synthetic Efficiency; Barry M. Trost; Science 1991, (254), pp 1471-1477.