Synthesis of speciality chemicals
Specialty Chemicals Chemical synthesis is involved with the construction of complicated chemical compounds from simpler ones. A synthesis usually is initiated for one of three reasons. The first reason is to attend an industrial requirement for a product. For example, ammonia is manufactured from nitrogen and hydrogen and is used to produce, among other things, ammonium sulfate, employed as a fertilizer; vinyl chloride is made from ethylene and is used in the generation of polyvinyl chloride (PVC) plastic. In general, a vast range of chemical compounds are produced for applications as fibres and plastics, pharmaceuticals, dyestuffs, herbicides, insecticides, and other products.
Second, an enormous quantity of compounds of considerable molecular complexity occur generally, in both living organisms and their degradation products; examples are proteins (in animals) and alkaloids (alkaline materials found in plants). The syntheses of these natural products must usually have been initiated in the context of the determination of the structures of the compounds; if a material is deduced to have a particular structure on the basis of its biochemical reactions and physical properties, then the determination that a compound synthesized by an unambiguous method for this structure is alike to the natural product provides confirmation of the efficacy of the assigned structure.
Third, a synthesis may be rolled out to receive a compound of specific structure that does not happen naturally and has not earlier been done. This sort of synthesis is performed in order to examine the properties of the compound and thereby test theories of chemical composition and reactivity.
Approach to synthesis
The variety of compounds that are capable of being synthesized is essentially limitless. In exercise, the synthesis of a preselected compound is made possible by particular functional groups undergoing transformations that, while they are dependent on the provisions applied to the compound, are largely free of the structure of the remaining part of the molecule. Thus, the blend of knowledge of the structure of the compound to be synthesized and knowledge of the general types of transformation that compounds undergo permits a synthesis to be prepared. The general approach, cut to its barest essentials, is to check the structure of the desired end product—for example, Z—and to deduce the structure of some (slightly simpler) compound—for example, Y—that should be able for transformation into Z by a reaction of known type. A possible precursor of Y is sought in similar manner, and in this way the chain of compounds is increased until a compound, A, is reached that is available for the work; the necessary transformations, beginning with A and closing with Z, are then carried out. Most individual steps in the classification result in a change in only one bond; some result in changes in two bonds at a time, but it is important for more extensive changes to occur.
Evaluation of a synthetic method
Three factors need to be borne in mind when evaluating a particular synthetic plan. The first is cost—of far greater importance in modern, large-scale synthesis than in laboratory work in which a particular synthesis may be brought out only once, as in the total synthesis of a generally occurring compound, and which in any case is likely to be on a comparatively small scale. The environmental influence of chemical syntheses has become an important consideration. Syntheses or processes that have a favorable environmental result, whether by use of safe and generally available reagents or by minimization of environmentally harmful waste products, have become an indispensable feature of so-called “green chemistry.”
Second, the yield in each step must be examined. A step in a synthesis may provide a very low yield of the desired product. For example, a proportion of the reactant may be changed into a different product by an alternative process that contests with the desired one; some of the product may undergo a consequent reaction; or some of the product may be lost in the separation processes required of its isolation in a pure state. The yield is usually determined, on a percentage basis, as the number of molecules of product achieved when 100 could in principle have been formed. A yield of about 80 percent or more is usually regarded good, but some transformations can prove so challenging to realize that even a yield of 10 or 20 percent may have to be accepted. The ultimate synthetic goal in a perfect synthesis is to accomplish 100 percent “atom efficiency,” in which all atoms of all reagents are included into the synthesized product without the production of any by-products.
Naturally, the yield of a process influences the cost of the product, because the shortfall of a 100 percent yield represents wasted material. In addition, yield can be of the utmost significance in determining whether a synthesis is a practical possibility, because the overall yield of a synthesis is the product of the yields of the individual steps. If these intermediate yields are often low, the ultimate product may not be attainable in the necessary amount of the available starting material.
Finally, consideration need to be given to the rate at which each step in the planned sequence occurs. In many instances, a desired result is possible in principle but in practice takes place so slowly as to be ineffective. It is then necessary to investigate whether the rate can be increased to a workable level by altering the conditions of the reaction—for example, by increasing the temperature or by appending an extra species, called a catalyst, that increases the rate without modifying the course of the reaction.
Isolation and purification of products
The product of synthesis is normally infected with reagents used in the synthesis, by-products, and possibly some unchanged opening material; these contaminants must be eliminated in order for a pure product to be achieved. In a multistep synthesis, it is normally advisable to purify the product from each step before progressing to the next.