Yield (chemistry)
In chemistry, is meant by the yield of a reaction, the amount of product recovered. It is particularly common to speak of a "yield", when a reaction was carried out with the intention to produce a specific product, that is, in synthetic chemistry. Often, the reported yield is not related to a single reaction, but to a series of steps that lead from the starting materials into the desired product, in each step - there may be losses - especially in the purification of the product. Thus, the reported yield is meaningful, it must be related to a base. It is common to specify the ratio of the molar amount of isolated product and the theoretical maximum (without any loss ) to be recovered product molar percentage [ %]. Typically, a high yield is desired.
- 3.1 Background
- 3.2 adoption
- 3.3 invoice
Definition
The yield is usually expressed as the ratio of the amounts of real product recovered and the theoretical maximum amount of product. Based on this calculation, it represents a stoichiometric ratio size and is usually expressed in percent.
The theoretical maximum amount of product which corresponds to the amount of substance of the scarcest of the reactant or a stoichiometric fraction or multiples.
Vogel's Textbook of Practical Organic Chemistry classifies the yields in the following categories:
Alternative tracing
However, yields can also be related to the recovered starting compounds at very incomplete reactions. This specification is common in the scientific literature for studies on the variability of a chemical reaction without optimization of the reaction conditions. Such information is often characterized in the published tables with " based under recovered material". Further can be found in the literature for such studies, yields of non-isolated product reactant mixture wherein the analysis was performed only by using an analytical method such as gas chromatography or NMR spectroscopy. With such information it is not possible from the published value reliably close to an actual yield of isolated product, as for the study had to be solved no practical problems.
Importance
For the (industrial) synthesis of chemicals, the yield of great importance and considerable research effort is being invested in optimizing processes to better yields down. Poor yields not only mean a high consumption of expensive chemicals, but also produce unnecessary waste. Waste disposal logistics and, in turn, imply additional costs, partially connected to the additional permissions. In industry, the latter problem is exacerbated countered that even the byproducts - as they occur in sufficient quantity - to isolate and economically exploit attempts.
In scientific research needs to be taken sometimes to be content with very low yields when complex compounds are first synthesized and a variety of steps is required. Thus succeeded about 1954, the first total synthesis of strychnine in 28 steps with a yield of 0.000 % 06. Since then, the number of steps 10 is reduced and the yield can be increased to 1.4%.
Example
The following example will illustrate the calculation and significance of the yield.
Background
Acetylsalicylic acid can be prepared in a four step process from benzene. In the following, only the gross equations are given with the stoichiometrically consumed reagents. ( As usual in organic chemistry, the symbols "Ph" for a phenyl group and " Ac " are used for the acetyl group. )
Assumption
Suppose it were of 10.0 g of benzene ( 0.128 mol ) after step 1 to 4 8.62 g ( 0.0479 mol) of acetylsalicylic acid produced. Here, at each step following amounts were used, or derived after cleaning:
Account
For each of these sub-steps, a yield can be specified, as well as for the entire synthesis. For this, the maximum possible amounts of product must be calculated. Since in each step, the reagent ( as it usually makes sense) was used in excess, each time could exceed the amounts of product arise as starting material was used. The yields of the individual steps are therefore:
The yield over all calculated from analogous
And it is
Since the total yield is the product of all the part yields, is a way to optimize and to reduce the number of reaction steps.
Criticism and error analysis for the determination of 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 [ English: Good Manufacturing Practice ( GMP) ] established in order to give precise details can, in most laboratories of other industries or universities and research institutes are error considerations in calculations of isolated yields - particularly in preparative experiments on a small scale (5-20 mg) - not widespread. But the weighing error can be significant, for example, between ± 1.5% ( weighing 100 mg) and ± 20 % ( weighing 3 mg). In addition there are more substantial weighing errors in widely used, determine the tare weight, this will have repercussions well as complying with even larger deviation in the determination of the isolated product quantity (net weight) and the calculation of precise isolated yield of a chemical reaction more difficult, especially in small batches.
In addition, in many laboratories the calibration status of the scales used is unknown.