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Concept
Aromaticity
Summary
In chemistry, aromaticity means the molecule has cyclic (ring-shaped) structures with pi bonds in resonance (those containing delocalized electrons). Aromatic rings give increased stability compared to saturated compounds having single bonds, and other geometric or connective non-cyclic arrangements with the same set of atoms. Aromatic rings are very stable and do not break apart easily. Organic compounds that are not aromatic are classified as aliphatic compounds—they might be cyclic, but only aromatic rings have enhanced stability. The term aromaticity with this meaning is historically related to the concept of having an aroma, but is a distinct property from that meaning.
Since the most common aromatic compounds are derivatives of benzene (an aromatic hydrocarbon common in petroleum and its distillates), the word aromatic occasionally refers informally to benzene derivatives, and so 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 (Purine) in RNA and DNA. An aromatic functional group or other substituent is called an aryl group.
In terms of the electronic nature of the molecule, aromaticity describes a conjugated system often represented in Lewis diagrams as alternating single and double bonds in a ring. In reality, the electrons represented by the double bonds in the Lewis diagram are actually distributed evenly around the ring ("delocalized"), increasing the molecule's stability. Due to the restrictions imposed by the way Lewis diagrams are drawn, the molecule cannot be represented by one diagram, but rather a hybrid of multiple different diagrams (called resonance), such as with the two resonance structures of benzene. These molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another (see below). Rather, the molecule exhibits all equal bond lengths in between those of single and double bonds.
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