Yield (chemistry)

In chemistry, the yield of a reaction is understood to be the amount of product obtained. It is particularly common to speak of a “yield” when a reaction was carried out with the intention of producing a certain product, i.e. in synthetic chemistry . Often the stated yield does not relate to a single reaction , but rather to a series of steps which lead from the starting compounds to the desired product, losses can occur in each step, in particular also during the purification of the product. In order for the stated yield to be meaningful, it must be related to a basis. It is customary to specify the quotient of the amount of substance of isolated product and the theoretical maximum (without any losses) amount of product substance that can be obtained in percent . A high yield is usually aimed for.

definition

The yield is usually expressed as the quotient of the amount of substance actually obtained and the theoretically maximum possible amount of product. Calculated in this way, it represents a stoichiometric ratio and is usually expressed in percent.

${\displaystyle {\text{Ausbeute}}={\frac {\text{Tatsächliche Stoffmenge an Produkt}}{\text{Maximal mögliche Stoffmenge an Produkt}}}\cdot 100\ \%}$

The theoretical maximum amount of product corresponds to the amount of substance of the scarcest starting material or a stoichiometric fraction or a multiple thereof.

Vogel's Textbook of Practical Organic Chemistry classifies the yields into the following categories:

yield	Rating
about 100%	Quantitatively
about 90%	Excellent
about 80%	Very good
about 70%	Well
below 70% to about 50%	Satisfying
under 50%	Bad

Alternative bases of reference

In the case of very incomplete reactions, however, yields can also be related to the recovered starting compounds. This information is common in the scientific literature for studies on the variability of a chemical reaction without optimizing the reaction conditions. Such information is often marked in the published tables with "based under recovered material" . Furthermore, in such investigations, one finds yields of non-isolated product-educt mixtures in the literature, the evaluation only being carried out with the aid of analytical methods such as gas chromatography or NMR spectroscopy . With such information it is not possible to reliably infer an actual yield of isolated product from the published value, since no practical problems had to be solved for the investigation.

meaning

For the (industrial) synthesis of chemicals, the yield is of great importance and considerable research effort is invested in optimizing processes for better yields. Poor yields not only mean high consumption of expensive chemicals, but also produce unnecessary waste. Waste in turn means additional disposal logistics and costs, sometimes associated with additional permits. In industry, the latter problem is increasingly countered by attempting to isolate the by-products - if they occur in sufficient quantities - and to try to utilize them economically.

In scientific research, very low yields must sometimes be accepted when complex compounds are synthesized for the first time and a large number of steps are required. In 1954 the first total synthesis of strychnine was achieved in 28 steps with a yield of 0.000 06%. Since then, the number of steps has been reduced to 10 and the yield increased to 1.4%.

example

The following example is intended to illustrate the calculation and significance of the yield.

background

Acetylsalicylic acid can be produced from benzene in a four-step process. Only the gross equations with the stoichiometrically consumed reagents are given below. (As usual in organic chemistry, the symbols “Ph” are used for a phenyl group and “Ac” for the acetyl group .)

1. ${\displaystyle {\text{Ph}}{-}{\text{H}}+2\ {\text{HSO}}_{3}{\text{Cl}}\longrightarrow {\text{Ph}}{-}{\text{SO}}_{2}{\text{Cl}}+{\text{HCl}}+{\text{H}}_{2}{\text{SO}}_{4}}$
   Benzene (written here as Ph – H) reacts with two equivalents of chlorosulfonic acid to form benzene sulfochloride , with hydrogen chloride gas and sulfuric acid being formed as further products.
2. ${\displaystyle {\text{Ph}}{-}{\text{SO}}_{3}{\text{H}}+2\ {\text{NaOH}}\longrightarrow {\text{Ph}}{-}{\text{OH}}+{\text{Na}}_{2}{\text{SO}}_{3}+{\text{H}}_{2}{\text{O}}}$
   Benzene sulphonyl chloride is converted into phenol with two equivalents of sodium hydroxide , whereby sodium sulphite and water are obtained.
3. ${\displaystyle {\text{Ph}}{-}{\text{OH}}+{\text{CO}}_{2}\longrightarrow {\text{HO}}{-}{\text{Ph}}{-}{\text{COOH}}}$
   Phenol in a Kolbe-Schmitt reaction with carbon dioxide to salicylic acid reacted.
4. ${\displaystyle {\text{HO}}{-}{\text{Ph}}{-}{\text{COOH}}+{\text{AcOAc}}\longrightarrow {\text{AcO}}{-}{\text{Ph}}{-}{\text{COOH}}+{\text{CH}}_{3}{\text{OAc}}}$
   Salicylic acid reacts with acetic anhydride to form acetylsalicylic acid and methyl acetate .

adoption

Assume that from 10.0 g of benzene (0.128  mol ) after steps 1 to 4, 8.62 g (0.0479 mol) of acetylsalicylic acid was prepared. The following amounts were used in each step or obtained after cleaning:

step	Output compound		Reagent		product
1	benzene	0.128 mol	3 equiv. Chlorosulfonic acid	0.384 mol	Benzenesulfochloride	0.0960 mol
2	Benzenesulfochloride	0.0960 mol	3 equiv. Sodium hydroxide	0.288 mol	phenol	0.0768 mol
3	phenol	0.0768 mol	Carbon dioxide, 500 kPa	-	Salicylic acid	0.0538 mol
4th	Salicylic acid	0.0538 mol	1.25 equiv. Acetic anhydride	0.0672 mol	Acetylsalicylic acid	0.0479 mol

invoice

A yield can be given for each of these partial steps, as well as for the entire synthesis. To do this, the maximum possible product quantities must first be calculated. Since the reagent was used in excess in each step (which is usually reasonable), each time at most as much product could have been created as the starting material used. The yields of the partial steps are accordingly:

${\displaystyle \eta _{1}={\frac {n({\text{Ph}}{\text{SO}}_{2}{\text{Cl}})}{n({\text{Ph}}{\text{H}})}}={\frac {0{,}0960~{\text{mol}}}{0{,}128~{\text{mol}}}}=75\,\%}$

${\displaystyle \eta _{2}={\frac {n({\text{Ph}}{\text{OH}})}{n({\text{Ph}}{\text{SO}}_{3}{\text{H}})}}={\frac {0{,}0768~{\text{mol}}}{0{,}0960~{\text{mol}}}}=80\,\%}$

${\displaystyle \eta _{3}={\frac {n({\text{HO}}{\text{Ph}}{\text{COOH}})}{n({\text{Ph}}{\text{OH}})}}={\frac {0{,}0538~{\text{mol}}}{0{,}0768~{\text{mol}}}}=70\,\%}$

${\displaystyle \eta _{4}={\frac {n({\text{AcO}}{\text{Ph}}{\text{COOH}})}{n({\text{HO}}{\text{Ph}}{\text{COOH}})}}={\frac {0{,}0479~{\text{mol}}}{0{,}0538~{\text{mol}}}}=89\,\%}$

The overall yield is calculated analogously

${\displaystyle \eta _{\text{ges}}={\frac {n({\text{AcO}}{\text{Ph}}{\text{COOH}})}{n({\text{Ph}}{\text{H}})}}={\frac {0{,}0479~{\text{mol}}}{0{,}128~{\text{mol}}}}=37\,\%}$

and it applies

${\displaystyle \eta _{\text{ges}}=\eta _{1}\cdot \eta _{2}\cdot \eta _{3}\cdot \eta _{4}}$

Since the total yield is the product of all partial yields, one way of optimizing is to reduce the number of partial reactions.

Criticism and consideration of errors in determining the yield

The exact determination of the isolated yield of a reaction is not trivial. In the pharmaceutical industry, the system of Good Manufacturing Practice (GMP) is established in order to be able to provide precise information; in most laboratories in other branches of industry or in universities and research institutes, errors are considered when calculating isolated yields - especially in the case of preparative yields Small-scale experiments (5-20 mg) - little common. The weighing errors alone can be significant, for example between ± 1.5% (weighing 100 mg) and ± 20% (weighing 3 mg). In addition, there are even more significant weighing errors in the widely used determination of the tare weight , which of course also has a corresponding effect as an even larger deviation in the determination of the isolated product quantity (net weight) and makes the calculation of the precise isolated yield of a chemical reaction even more difficult, especially with small batches.

In addition, the calibration status of the scales used is unknown in many laboratories .

literature

• L. Gattermann and H. Wieland: The practice of the organic chemist. 43rd edition de Gruyter, 1982, ISBN 3-11-006654-8 .
• HR Christians: Fundamentals of general and inorganic chemistry. 8th edition Otto Salle, 1985, ISBN 3-7935-5394-9 .
• HGO Becker et al .: Organikum - organic-chemical basic internship. 22nd edition. Wiley VHC, 2004, ISBN 3-527-31148-3 (quoted as: “Organikum”).

Individual evidence

1. ^ AI Vogel et al .: Vogel's Textbook of Practical Organic Chemistry. Prentice Hall, 5th edition, 1996, ISBN 978-0-582-46236-6 .
2. ↑ KPC Vollhardt, NE Schore: Organic Chemistry. (translated by H. Butenschön) 4th edition. Wiley VHC, 2005, ISBN 3-527-31380-X , p. 354.
3. ↑ Tab. 5.25, p. 364, Organikum.
4. ↑ Organikum, Table 5.66, p. 392.
5. ↑ Synthesis of acetylsalicylic acid (aspirin) from salicylic acid and acetic anhydride. Retrieved on October 19, 2010 ( minutes as PDF (139 kB) ).
6. ↑ Martina Wernerova, Tomas Hudlicky: On the Practical Limits of Determining Isolated Product Yields and Ration of Stereoisomers: Reflections, Analysis, and Redemption . Synlett, 2010, pp. 2701-2707.