aromatic

Aromatic describes a class of chemical compounds that exhibit unusual stability due to delocalized electrons in a cyclic, planar arrangement. In modern chemistry, the term refers to aromaticity, a property of systems in which pi electrons are delocalized over a ring. The word originated in the 19th century from the observation that many of these compounds had distinctive odors, but the smell is not a reliable indicator of aromaticity.

Aromaticity is characterized by criteria such as being cyclic, planar, and fully conjugated, with a closed loop of p orbitals that contains a number of pi electrons following Hückel's rule (4n + 2). In monocyclic rings, this typically means 6, 10, 14, ... pi electrons. Benzene is the prototypical example with six pi electrons. The concept extends to heteroaromatics (pyridine, furan) where heteroatoms contribute lone pair electrons. Rings with 4n pi electrons are antiaromatic and unstable, while nonplanar or nonconjugated rings are non-aromatic.

In addition to the chemical sense, aromatic is also used to describe substances with a pleasant or notable smell, such as many essential oils. Not all fragrant compounds are aromatic in the chemical sense, and many aromatic substances lack odor. Aromaticity also informs studies of polycyclic aromatic hydrocarbons and organometallic systems known as metalla-aromatics, reflecting the broad use of the term in chemistry.