Atom economy

Atom economy (atom efficiency) is the conversion efficiency of a chemical process in terms of all atoms involved and the desired products produced. Atom economy is an important concept of green chemistry philosophy, and one of the most widely used metrics for measuring the "greenness" of a process or synthesis.

Atom economy can be written as:

By the conservation of mass, the total molecular mass of the reactants is the same as the total molecular mass of the products. In an ideal chemical process, the amount of starting materials or reactants equals the amount of all products generated and no atom is wasted. However, in some processes, some of the consumed reactant atoms do not become part of the intended products. This can be a concern for raw materials that have a high cost or due to economic and environmental costs of disposal of the waste.

Atom economy is a different concern than chemical yield, because a high-yielding process can still result in substantial byproducts. Examples include the Cannizzaro reaction, where approximately 50% of the reactant aldehyde becomes the other oxidation state of the target, the Wittig reaction, where high-mass phosphorus reagents are used but ultimately become waste, and the Gabriel synthesis, which produces a stoichiometric quantity of phthalic acid

If the desired product has an enantiomer the reaction needs to be sufficiently stereoselective even when atom economy is 100%. A Diels-Alder reaction is an example of a potentially very atom efficient reaction that also can be chemo-, regio-, diastereo- and enantioselective. Catalytic hydrogenation comes the closest to being an ideal reaction that is extensively practiced both industrially and academically.

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