Yields in chemical reactions

There are several reasons why the actual yield is usually less than the theoretical yield in chemical reactions. Many reactions do not completely use up the limiting reactant. Instead, some of the products turn back into reactants so that the final result is a mixture of reactants and products. In many cases the main product must go through additional steps to purify or separate it from other chemicals. For example, banana flavoring must be distilled, or isolated based on its boiling point. Solid compounds, such as sugar, must be recrystallized. Some of the product may be lost in the process. There also may be other reactions, called side reactions, that can use up reactants without making the desired product. 

The problem-solving LAB on page 372 will help you understand the importance of percent yield in chemical reactions and the kind of factors that may determine the size of the percent yield. 

In this chapter, you will study factors that affect reversible changes, notably those in chemical reactions. You will learn how to determine the amounts of reactants and products that are present when their proportions no longer change. You will also learn how to make qualitative predictions about the ways that chemists can change these proportions. Finally, you will see how industrial chemists apply their knowledge of reversible changes to increase the yield of chemicals that are important to society. 

Radiation chemistry highlights the importance of the role of the solvent in chemical reactions. When one radiolyzes water in the gas phase, the primary products are H atoms and OH radicals, whereas in solution, the primary species are eaq , OH, and H" [1]. One can vary the temperature and pressure of water so that it is possible to go continuously from the liquid to the gas phase (with supercritical water as a bridge). In such experiments, it was found that the ratio of the yield of the H atom to the hydrated electron (H/eaq ) does indeed go from that in the liquid phase to the gas phase [2]. Similarly, when one photoionizes water, the threshold energy for the ejection of an electron is much lower in the liquid phase than it is in the gas phase. One might suspect that a major difference is that the electron can be transferred to a trap in the solution so that the full ionization energy is not required to transfer the electron from the molecule to the solvent. 

Esters are common components in cosmetics and skin-care products. They can be synthesized from fatty acids and alcohols using either chemical or enzymatic reactions. The chemical reactions are normally catalysed by acid catalysts. Enzymatic synthesis is carried out under milder conditions and therefore it provides products of very high purity. A range of esters such as isopropyl palmitate and isopropyl myristate are now produced industrially using enzymatic synthesis. The reactions are carried out in solvent-free systems using an immobilised lipase as catalyst. In order to get high yields in the reactions, water is removed continuously. 

It is essential that the specific rotation of the product and the pure enantiomer be measured in the same solvent, at the same wavelength and temperature, and at a similar concentration if possible both measurements should be made at the same time. Optical yields in enzymic reactions carried out under laboratory conditions approach 100 per cent an asymmetric chemical synthesis may be regarded as promising if the optical yield ranges upwards from 20 per cent. 

In optimization of the process of obtaining novocaine, FRFE 24"1 with generating ratio X XjXzXj was chosen for the basic design of experiments. The system factors are x,-time of reaction, min x2-temperature, °C x3-surplus of sodium salt of paraa-minobenzoic acid, % and X4-concentration of sodium salt of paraaminobenzoic acid, %. The system response is the yield of chemical reaction %. Outcomes of basic experiment with application of method of steepest ascent are shown in Table 2.191. 

The research of Paul Brumer and his colleagues addresses several fundamental problems in theoretical chemical physics. These include studies of the control of molecular dynamics with lasers.98 In particular, the group has demonstrated that quantum interference effects can be used to control the motion of molecules, opening up a vast new area of research. For example, one can alter the rate and yield of production of desirable molecules in chemical reactions, alter the direction of motion of electrons in semiconductors, and change the refractive indices of materials etc. by creating and manipulating quantum interferences. In essence, this approach, called coherent control, provides a method for manipulating chemistry at its most fundamental level.99... 

Identification of segregation by chemical methods. Partial segregation can be studied through its influence on the conversion and yield of chemical reactions. For instance, let us denote by Xmacro and micro t ie limiting extents of reaction one would observe in a well macromixed reactor. If the reactor is partially segregated X = gXmacro + (1 - g) X. ... 

The effect of coalescence and break-up of droplets on the yield of chemical reactions was studied by Villermaux (33). Micromixing effects may occur even in batch reactors if there is a drop size distribution and mass-transfer control. Although practical rules for the design and scale-up of liquid-liquid reactors are available as Oldshue showed in the case of alkylation (152), many problems remain unsolved (.5) mass transfer effects, high hold-up fractions (> 20 %), large density differences, high viscosities, influence of surfactants. 

Micromixing phenomena are the processes whereby different chemical species which are supposed to mix and react are coming into contact at the molecular scale. The result of imperfect micromixing is local unhomogeneity of the reacting mixture and this causes differences in the conversion and yield of chemical reactions, especially when portions of the fluid having reacted at different instants are mixed together. Fast reactions, combustions, precipitations, polymerizations may particularly be affected by these phenomena. 

The percentage yield of chemical reactions is extremely important in industrial chemistry and the pharmaceutical industry. For example, the synthesis of certain drugs involves many sequential chemical reactions. Often each reaction has a low percentage yield. This results in a tiny overall yield. Research chemists, who generally work with small quantities of reactants, may be satisfied with a poor yield. Chemical engineers, on the other hand, work with very large quantities. They may use hundreds or even thousands of kilograms of reactants A difference of 1% in the yield of a reaction can translate into thousands of dollars. 

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