Atom economy describes the conversion

Conversion (chemistry)

In chemistry, the phrase conversion has several meanings* specifically the property 'X' related to the yield 'Y' by multiplication with the selectivity 'S', i.e. the mathematical definition X * S = Y, all calculated on a molar basis; e.g...

 efficiency of a chemical process in terms of all atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s involved (desired products produced). In an ideal chemical process, the amount of starting materials or reactants equals the amount of all products generated (see stoichiometry

Stoichiometry

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

) and no atom is wasted. Recent developments like high raw material (such as petrochemicals

Petroleum

Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

) costs and increased sensitivity to environmental concerns have made atom economical approaches more popular. Atom economy is an important concept of green chemistry

Green chemistry

Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances...

 philosophy, and one of the most widely used ways to measure the "greenness" of a process or synthesis

Green Chemistry Metrics

Green chemistry metrics measures efficiency in a chemical process. Having made a green chemistry improvement to a chemical process, it is important to be able to quantify the change. By quantifying the improvement, there is a tangible element or benefit from the new technology introduced...

.

Atom economy can be written as:

% atom economy =

Note that atom economy can be poor even when chemical yield is near 100%, see for instance the Cannizzaro reaction

Cannizzaro reaction

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position...

 or the Wittig reaction

Wittig reaction

The Wittig reaction is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide to give an alkene and triphenylphosphine oxide....

. If the desired product has an enantiomer

Enantiomer

In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

 the reaction needs to be sufficiently stereoselective even when atom economy is 100%. A Diels-Alder reaction

Diels-Alder reaction

The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...

 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.
The Gabriel synthesis

Gabriel synthesis

The Gabriel synthesis is named for the German chemist Siegmund Gabriel. Traditionally, it is a chemical reaction that transforms primary alkyl halides into primary amines using potassium phthalimide....

 of amines is an example of extremely low atom economy, as stochiometric quantities of phthalic acid

Phthalic acid

Phthalic acid is an aromatic dicarboxylic acid, with formula C6H42. It is an isomer of isophthalic acid and terephthalic acid. Although phthalic acid is of modest commercial importance, the closely related derivative phthalic anhydride is a commodity chemical produced on a large...

 derivatives are formed. In most cases, the atom economy of the Gabriel synthesis is <<50%.

Atom economy can also be adjusted if a pendant group is recoverable, for example Evans auxiliary groups. However, if this can be avoided it is more desirable, as recovery processes will never be 100%. Atom economy can be improved upon by careful selection of starting materials and a catalyst system.

Atom economy is just one way to evaluate a chemical process. Other criteria can include energy consumption, pollutants released and price.

Poor atom economy is common in fine chemicals

Fine chemicals

Fine chemicals are pure, single chemical substances that are commercially produced with chemical reactions for highly specialized applications. Fine chemicals produced can be categorized into active pharmaceutical ingredients and their intermediates, biocides, and specialty chemicals for technical...

 or pharmaceuticals synthesis, and especially in research, where the aim to readily and reliably produce a wide range of complex compounds leads to the use of versatile and dependable, but poorly atom-economical reactions. For example, synthesis of an alcohol is readily accomplished by reduction of an ester with lithium aluminum hydride, but the reaction necessarily produces a voluminous floc of aluminum salts, which have to be separated from the product alcohol and disposed of. The cost of such hazardous material disposal can be considerable. Catalytic hydrogenolysis of an ester is the analogous reaction with a high atom economy, but it requires catalyst optimization, is a much slower reaction and is not applicable universally.

Creating reactions utilizing atom economy

It is fundamental in chemical reactions of the form A+B→ C+D that two products are necessarily generated though product C may have been the desired one. That being the case, D is considered a byproduct. As it is a significant goal of green chemistry to maximize the efficiency of the reactants and minimize the production of waste, D must either be found to have use, be eliminated or be as insignificant and innocuous as possible. With the new equation of the form A+B→C, the first step in making chemical manufacturing more efficient is the use of reactions that resemble simple addition reactions with the only other additions being catalytic materials.