The chemistry of aromatics

More accurate scientific tests showed many of these substances to be benzene derivatives. After the concept of aromaticity had been extended in the middle of the 19th century to include first all benzene derivatives, Emil Erlenmeyer suggested using the word "aromatic" for all compounds having similar properties to benzene. The decisive criterion for him was that the compound should tend more towards substitution reactions than towards additional ones.

In 1931 the scientist Erich Hückel achieved a breakthrough in defining the aromaticity of a substance. Named after him, "Hückel's rule" enables aromatic and anti-aromatic compounds to be distinguished on the basis of the number of π-electrons.

Besides classification, the structure of aromatics – particularly that of benzene – also represents a challenge for the scientific world.

The crucial breakthrough here was achieved by Friedrich August Kekulé of Stradonitz in 1865. He introduced a ring-like structure of benzene, the carbon atoms being interchangingly linked together via simple and double bonds.

Based on the structure developed by Kekulé, Robinson in 1925 proposed the ring symbol for benzene: The π-electrons being delocalised, the benzene molecule is represented as lying between two bordering structures. This delocalisation of bonds at the same time determines the characteristic stability of the benzene molecule - which also distinguishes the other aromatics.

In today's chemical industry, the BTX aromatics (benzene, toluene and xylene) - largely produced from petroleum - are especially important. Everyday, they help meet fundamental human needs, such as health, hygiene, housing and food; and they are also strong in areas as diverse as transport, high technology, sports, leisure – and even art restoration and crime detection! For more on the uses of aromatics, see our section Aromatics in everyday life.