aromatic compound, any of a large class of compounds that includes benzene and compounds that resemble benzene in certain of their chemical properties. Originally applied to a small class of pleasant-smelling chemicals derived from vegetables, it now encompasses a wide diversity of compounds with no connection to the sense of smell. Common aromatic compounds other than benzene include toluene, naphthalene, and anthracene (all of which are present in coal tar). Each of these compounds contains at least one ring that consists of six carbon atoms, each joined to at least two other carbon atoms, and each joined to adjacent carbon atoms by one single and one double bond. The resulting hexagonal structure is characteristic of many aromatic compounds. The general rule is that aromatic molecules have 4n + 2 delocalized electrons (where n is an integral number). Thus the number of delocalized electrons in benzene (6), naphthalene (10), and anthracene (14) agree with their aromatic character. On the other hand, the eight-membered cyclic compound with four alternating double bonds (cyclooctatetraene) is not aromatic and shows reactivity similar to alkenes. For this reason, the bonds in the aromatic ring are less reactive than ordinary double bonds; aromatic compounds tend to undergo ionic substitution (e.g., replacement of a hydrogen bonded to the ring with some other group) rather than addition (which would involve breaking one of the resonant bonds in the ring). Presence of the six-membered benzene ring is not essential for aromatic compounds; for example, furan, a heterocyclic compound that has a five-membered ring that includes an oxygen atom, has aromatic properties, as does pyridine, a heterocyclic compound whose six-membered ring includes a nitrogen atom.

Any of a large class of organic compounds whose molecular structure includes one or more planar rings of atoms, usually but not always six carbon atoms. The ring's carbon-carbon bonds (see bonding) are neither single nor double but a type characteristic of these compounds, in which electrons are shared equally with all the atoms around the ring in an electron cloud. The term was first applied circa 1860 to a class of hydrocarbons isolated from coal tar and distinguished by odours much stronger than those of other classes of hydrocarbons. In modern chemistry, aromaticity denotes the chemical behaviour, especially the low reactivity, of this class of molecules related to their bonding. The parent compound of this class is benzene (C₆H₆). Seealso hydrogenation.

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