Synthesis

FIELDS OF STUDY: Organic Chemistry; Inorganic Chemistry; Chemical Engineering

ABSTRACT

The process of synthesis and its importance in chemistry-related fields are described. Synthesis is the use of specific reactions to alter and elaborate a molecular composition and structure in order to obtain a desired compound.

The Value of Chemical Synthesis

Synthesis of a specific chemical compound is in principle the same as baking a cake. In both cases, specific materials are combined in the proper proportions and subjected to certain environmental conditions of temperature and pressure. Ideally, the materials combine in the desired manner to produce the desired result: in the case of a cake, something tasty to eat; in the case of chemical synthesis, the result may be a life-saving drug.

Chemical synthesis is a very valuable process economically, as it provides a vast array of useful materials that do not occur naturally. In biology, synthesis reactions are important to the maintenance of life, from the biosynthesis of glucose by green plants to the biosynthesis of DNA. Regardless of where they occur, all synthesis reactions obey the same basic rules of that govern chemical interactions, such as the law of the conservation of mass.

Historically, synthesis reactions were instrumental in the development of the science of chemistry from the pseudoscience of alchemy. The observation that combining various pure materials in the same way produced the same results and products led directly to the fundamental chemical laws of definite proportions and multiple proportions and, later, to the modern theory of atomic structure.

General Concepts of Synthesis

In a simple decomposition reaction, a reactant material, AB, breaks down into its component materials, A and B. At its simplest, synthesis, the opposite of decomposition, occurs by addition reactions in which combining material A with material B produces the product material AB:

A + B → AB

Other simple addition combinations are also possible, such as A + 2B, 2A + 3B, and so on. Such combinations clearly illustrate the law of multiple proportions. In more complex syntheses, a variety of reaction types may be used in a sequential or stepwise manner in order to maintain complete control over the direction of the synthesis and the products obtained at each step.

Organic compounds offer the greatest variety of possible structures, and synthesis reactions involving them are subject to very stringent rules determined by the molecular orbital interactions in each particular chemical reaction. As a result, undesirable side reactions are more likely to occur, consuming reactants without producing the desired products. Various functional groups can react in different ways, and it is common practice to use what is called a protecting group to prevent one part of a molecule from taking part in a reaction when it is not supposed to. The protecting group is added to a reactive functional group in order to convert it into a nonreactive group. After the reaction is complete, the protecting group can then be removed, regenerating the original functional group.

Typically, synthesis reactions are monitored by determining the yield of the desired product. An ideal synthesis reaction would produce a 100 percent yield, with all of the starting reactants converted entirely and only into the desired product. In practice, this is almost unheard of, especially in multistep synthesis procedures, in which the reduction of yield in each step is a cumulative effect. A five-step procedure, for example, may have an good yield of 80 percent in each individual step, but after the five steps have been completed, the overall yield of desired product would be only 0.80⁵, or a mere 32 percent.

Synthesis in Chemical Engineering

Many important materials, especially various plastics and resins, are produced on an industrial scale. The very common plastics polyethylene and nylon are produced by simple addition chain reactions, in which individual molecules of the reactants bond to each other in a head-to-tail manner. The formation of polyethylene, for example, can be described by the reaction equation

nC₂H₄ → −(CH₂CH₂)_n−

in which the coefficient n indicates an indeterminate number of ethylene molecules. The structure of nylon polymers is more complex, since nylon is derived from two different types of compounds. Other synthetic polymers have even more complex structures due to bond formation between different molecules. Such compounds are typically used as matrix resins in advanced composite materials from which the most advanced aircraft and other constructs are made.

PRINCIPAL TERMS

• chemical reaction: a process in which the molecules of two or more chemical species interact with each other in a way that causes the electrons in the bonds between atoms to be rearranged, resulting in changes to the chemical identities of the materials.
• decomposition reaction: a chemical reaction in which a single reactant breaks apart to create several products with smaller molecular structures.
• law of conservation of mass: the principle that all atoms present at the beginning of a process must also be present after the completion of the process; matter can be neither created nor destroyed, only changed from one form to another.
• product: a chemical species that is formed as a result of a chemical reaction.
• reactant: a chemical species that takes part in a chemical reaction.

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