Illustrated Glossary of Organic Chemistry

Atom economy: A concept to measure the efficiency of a synthetic process, in terms of the number of atoms required in all the starting materials and reactants versus how many of these atoms are wasted (i.e., atoms that do not become part of the final product). A process which uses many atoms not found in the final product is said to have low atom economy, whereas a process in which most atoms of the starting material and reactants end up in the final product is said to have high atom economy. Processes having high atom economy are generally (but not always) preferable to processes having low atom economy.

Chemoselective oxidation of this aldehyde alcohol using an acetal protecting group is of low atom economy, because the atoms of the protecting group (plus the atoms of the reactants needed to install and remove the protecting group) are not included in the product.

PCC

Chemoselective oxidation of the same aldehyde alcohol using a chemoselective oxidizing agent (such as PCC) is of higher atom economy (and therefore generally more desirable than the first reaction shown above) because less atoms are used that are not part of the final reaction product.