In an addition reaction, two substances called reagents react with each other to form a product. Addition reactions are distinguished from other types of reactions by their lack of byproduct; the main product is the only substance produced by the reaction. These reactions are used to create compounds that could not be created in any other way.
An addition reaction is made possible by the presence of double or triple bonds in one of the reagent molecules. By decreasing the strength of the bond to a single or double bond, the reaction makes room in the molecular structure for another bond. Thus, the two molecules combine into one by forming a new bond at this location.
One important addition reaction is the Diels-Alder reaction, which is also called diene synthesis. Two professors of chemistry, Kurt Alder and Otto Diels, developed this synthesis in 1928. The discovery of this process earned the two scientists a joint award of the 1950 Nobel Prize for chemistry.
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The discovery of the original Diels-Alder reaction was followed by the expansion of research into diene synthesis, which led to the development of many different reactions in the same class. All of the diene synthesis reactions form molecules with carbon rings in their structures, but some of the reactions that fall into this class, called hetero Diels-Alder reactions, substitute other atoms into the ring. In Aza Diels-Alder reactions, for example, the addition reaction forms a molecule containing a ring composed of five carbon atoms and one nitrogen atom. Oxo Diels-Alder reactions create carbon rings that contain an oxygen atom.
The original Diels-Alder addition reaction combines one molecule with four carbon atoms with another molecule with two carbon atoms. The four-carbon molecule is called the diene, and the other molecule is called the dienophile because of its propensity to react with dienes. Each of the carbon atoms in the dienophile bonds with one of the end carbon atoms in the diene chain. This forms the ring of six carbons.
These reactions are important because they allow scientists to create natural substances in the lab that could not be recreated previously. Camphor, for example, is made synthetically using a Diels-Alder reaction. The process is also used to make resins and oils that mimic the products made by animals and plants in the natural world. Generally, the synthesized substances are fairly stable.