The percentage of reactant atoms that end up in the desired useful reaction products is called the atom economy. The lower the atom economy, the less efficient the reaction. To calculate atom economy, you need to know the molecular masses of all the reactants and products in the reaction equation. It is important to note that atom economy and percent yield are two different things. Percentage yield is experimental and compares how much product you actually get compared to the theoretical maximum amount that could be made in the reaction. This is influenced by practical methods and how the reaction is carried out.
On the other hand, atom economy is theoretical and doesn’t depend on how the reaction is conducted. It measures how many of the atoms in the reactants are converted to the desired products based on the law of conservation of mass. It is a more accurate measurement of how many of the atoms in the starting materials are utilized.
It’s important to understand the difference between atom economy and percent yield so that you can be sure you are evaluating a reaction correctly. Percentage yield focuses on how much of the desired product is made whereas atom economy focuses on how many of the reactant atoms are used to make the useful product.
Rearrangement reactions have the best atom economies because they don’t involve elimination or addition and only result in the reorganization of atoms within a molecule. This is why they are considered green chemistry reactions. Unfortunately, most chemical reactions aren’t this efficient and often times a lot of atoms in the starting material end up as byproducts instead of being used to make the desired product. This is a huge problem especially when it comes to high-cost raw materials such as rare earth metals and fossil fuels.