Definitions
In chemical reaction engineering, "yield", " conversion" and "selectivity" are terms used to describe ratios of how much of a reactant has reacted—conversion, how much of a desired product was formed—yield, and how much desired product was formed in ratio to the undesired product—selectivity, represented as X,S, and Y. According to the ''Elements of Chemical Reaction Engineering'' manual, yield refers to the amount of a specific product formed per mole of reactant consumed. In chemistry, mole is used to describe quantities of reactants and products in chemical reactions. The Compendium of Chemical Terminology defined yield as the " ratio expressing the efficiency of a mass conversion process. The yield coefficient is defined as the amount of cell mass (kg) or product formed (kg,mol)The use of kilogram-mole (kg-mol or g-mol)—the number of entities in 12 kg of 12C was replaced with the use of the kilomole (kmol) in the late 20th century. The kilomole is numerically identical to the kilogram-mole. The name and symbol adopt the SI convention for standard multiples of metric units—kmol means 1000 mol. related to the consumed substrate (carbon or nitrogen source or oxygen in kg or moles) or to the intracellular ATP production (moles)."PAC, 1992, 64, 143. (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) Compendium of Chemical Terminology In the section "Calculations of yields in the monitoring of reactions" in the 1996 4th edition of ''Vogel's Textbook of Practical Organic Chemistry'' (1978), the authors write that, " theoretical yield in an organic reaction is the weight of product which would be obtained if the reaction has proceeded to completion according to the chemical equation. The yield is the weight of the pure product which is isolated from the reaction." The chemist, Arthur Irving Vogel (1905 – 1966) was the author of textbooks including the ''Textbook of Qualitative Chemical Analysis'' (1937), the ''Textbook of Quantitative Chemical Analysis'' (1939), and the ''Practical Organic Chemistry'' (1948). In 'the 1996 edition of ''Vogel's Textbook'' , percentage yield is expressed as,In the section "Calculations of yields in the monitoring of reactions" ''Vogel's Textbook'' , the authors write that most reactions published in chemical literature provide the molar concentrations of a reagent in solution as well as the quantities of reactants and the weights in grams or milligrams(1996:33) According to the 1996 edition of ''Vogel's Textbook'' , yields close to 100% are called ''quantitative'', yields above 90% are called ''excellent'', yields above 80% are ''very good'', yields above 70% are ''good'', yields above 50% are ''fair'', and yields below 40% are called ''poor''. In their 2002 publication, Petrucci, Harwood, and Herring wrote that ''Vogel's Textbook'' names were arbitrary, and not universally accepted, and depending on the nature of the reaction in question, these expectations may be unrealistically high. Yields may appear to be 100% or above when products are impure, as the measured weight of the product will include the weight of any impurities. In their 2016 laboratory manual, ''Experimental Organic Chemistry'', the authors described the "reaction yield" or "absolute yield" of a chemical reaction as the "amount of pure and dry product yielded in a reaction". They wrote that knowing the stoichiometry of a chemical reaction—the numbers and types of atoms in the reactants and products, in a balanced equation "make it possible to compare different elements through stoichiometric factors." Ratios obtained by these quantitative relationships are useful in data analysis.Theoretical, actual, and percent yields
The percent yield is a comparison between the actual yield—which is the weight of the intended product of a chemical reaction in a laboratory setting—and the theoretical yield—the measurement of pure intended isolated product, based on the chemical equation of a flawless chemical reaction, and is defined as, The ideal relationship between products and reactants in a chemical reaction can be obtained by using a chemical reaction equation.Example
This is an example of an esterification reaction where one moleculePurification of products
In his 2016 ''Handbook of Synthetic Organic Chemistry'', Michael Pirrung wrote that yield is one of the primary factors synthetic chemists must consider in evaluating a synthetic method or a particular transformation in "multistep syntheses." He wrote that a yield based on recovered starting material (BRSM) or (BORSM) does not provide the theoretical yield or the "100% of the amount of product calculated", that is necessary in order to take the next step in the multistep systhesis. Purification steps always lower the yield, through losses incurred during the transfer of material between reaction vessels and purification apparatus or imperfect separation of the product from impurities, which may necessitate the discarding of fractions deemed insufficiently pure. The yield of the product measured after purification (typically to >95% spectroscopic purity, or to sufficient purity to pass combustion analysis) is called the ''isolated yield'' of the reaction.Internal standard yield
Yields can also be calculated by measuring the amount of product formed (typically in the crude, unpurified reaction mixture) relative to a known amount of an added internal standard, using techniques likeReporting of yields
In their 2010 '' Synlett'' article, Martina Wernerova and organic chemist, Tomáš Hudlický, raised concerns about inaccurate reporting of yields, and offered solutions—including the proper characterization of compounds. After performing careful control experiments, Wernerova and Hudlický said that each physical manipulation (including extraction/washing, drying over desiccant, filtration, and column chromatography) results in a loss of yield of about 2%. Thus, isolated yields measured after standard aqueous workup and chromatographic purification should seldom exceed 94%. They called this phenomenon "yield inflation" and said that yield inflation had gradually crept upward in recent decades in chemistry literature. They attributed yield inflation to careless measurement of yield on reactions conducted on small scale, wishful thinking and a desire to report higher numbers for publication purposes. Hudlický's 2020 article published in '' Angewandte Chemie''—since retracted—honored and echoed Dieter Seebach's often-cited 1990 thirty-year review of organic synthesis, which had also been published in ''Angewandte Chemie''. In his 2020 ''Angewandte Chemie'' 30-year review, Hudlický said that the suggestions that he and Wernerova had made in their 2010 ''Synlett'' article, were "ignored by the editorial boards of organic journals, and by most referees." Retracted.See also
* Conversion (chemistry) * Quantum yieldNotes
Further reading
* * * *References
{{reflist, 30em Stoichiometry Chemical reaction engineering Chemical synthesis Organic synthesis Biochemical engineering Chemical reactions