Aromatic compounds are a large class of unsaturated chemical compounds characterized by one or more planar rings of atoms joined by covalent bonds of two different kinds. The term "aromatic" originates from the past grouping of molecules based on odor, before their general chemical properties were understood. Today, the term "aromaticity" refers to the unique stability of these compounds due to particular bonding arrangements that cause certain π (pi) electrons within a molecule to be strongly held. Key features of aromatic compounds include:
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Cyclic structure: Aromatic compounds have a cyclic (ring-shaped) structure with pi bonds in resonance (those containing delocalized electrons) .
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Benzene ring: The most common aromatic compounds are derivatives of benzene, which is a six-membered ring with alternating double bonds.
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Substituents: Aromatic molecules with simple alkyl groups as substituents are named as derivatives of benzene. For instance, a benzene with an ethyl group attached to one of the carbons in the ring is simply called "ethylbenzene".
Aromatic compounds play key roles in the biochemistry of all living things. For example, the four aromatic amino acids histidine, phenylalanine, tryptophan, and tyrosine each serve as one of the 20 basic building-blocks of proteins. In industry, key aromatic hydrocarbons of commercial interest are benzene, toluene, ortho-xylene, and para-xylene, which are used to produce a range of important chemicals and polymers, including styrene, phenol, aniline, polyester, and nylon.
