Green Chemistry

A chemical reaction involves reactants (either elements or compounds made of several elements) changing due to chemical bonds breaking and/or new bonds forming to make a new arrangement of atoms (the product). Each element has a unique mass. Relative to one another, the mass of hydrogen is 1, carbon is 12, and so on. Green Chemistry has developed a set of equations to measure the amount of waste each chemical reaction produces. One of the most widely used is Atom Economy (Trost, 1991). Atom Economy measures the mass of the atoms in the product compared to the reactants. The optimum Atom Economy is 100%, where all the atoms in the reactants are theoretically incorporated into the product. This is elegantly demonstrated by the work of the 2022 winners of the Nobel Prize in Chemistry. Professors Morten Meldal and Barry Sharpless first described a series of reactions in which there is no loss of atoms into by-products. These reactions became to be known as click chemistry because the reactants fit together like toy building blocks. Professor Carolyn Bertozzi shared the Nobel prize for her work applying click chemistry to biochemistry.

It is important to say that click chemistry is not waste-free. These reactions are performed by dissolving the reactants in a solvent, which is later disposed of. The reaction may not reach completion, and so some reactants remain. Atom economy is a theoretical calculation that does not factor in these considerations. For this reason it is also helpful to measure the total waste formed in a reaction. E-factor (short for environmental-factor) compares the total mass of waste to the mass of product made (including solvents and other chemicals not directly involved in the reaction). Ultimately, chemists will consider a multitude of metrics to evaluate how green a reaction is, including Atom Economy and E-factor, but also energy efficiency and other attributes guided by the principles of Green Chemistry.