In organic chemistry, the term aromaticity is used to describe a cyclic (ring-shaped), planar (flat) molecule that exhibits unusual stability as compared to other geometric or connective arrangements of the same set of atoms. As a result of their stability, it is very difficult to cause aromatic molecules to break apart and to react with other substances. Organic compounds that are not aromatic are classified asaliphatic compounds—they might be cyclic, but only aromatic rings have especial stability (low reactivity).
Since one of the most commonly encountered aromatic systems of compounds in organic chemistry is based on derivatives of the prototypical aromatic compound benzene (an aromatic hydrocarbon common in petroleum and its distillates), the word “aromatic” is occasionally used to refer informally to benzene derivatives, and this is how it was first defined. Nevertheless, many non-benzene aromatic compounds exist. In living organisms, for example, the most common aromatic rings are the double-ringed bases in RNA andDNA. A functional group or other substituent that is aromatic is called an aryl group.
The earliest use of the term “aromatic” was in an article by August Wilhelm Hofmann in 1855. Hofmann used the term for a class of benzene compounds, many of which do have odors (aromas), unlike pure saturated hydrocarbons. Today, there is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds (how they smell), although in 1855, before the structure of benzene or organic compounds was understood, chemists like Hofmann were beginning to understand that odiferous molecules from plants, such as terpenes, had chemical properties we recognize today are similar to unsaturated petroleum hydrocarbons like benzene.
In terms of the electronic nature of the molecule, aromaticity describes a conjugated system often made of alternating single and double bonds in a ring. This configuration allows for the electrons in the molecule’s pi system to be delocalized around the ring, increasing the molecule’s stability. The molecule cannot be represented by one structure, but rather a resonance hybrid of different structures, such as with the two resonance structures of benzene.