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An aromatic hydrocarbon (sometimes referred to as an arene) is a hydrocarbon of which the molecular structure incorporates one or more planar sets of six carbon atoms that are connected by delocalized electrons numbering the same as if they consisted of alternating single and double covalent. Aromatic hydrocarbons contain carbon and hydrogen atoms only.

Benzene (C₆H₆) is the least complex aromatic hydrocarbon, and it was the first one named as such. Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one to each of the two neighboring carbon atoms which leaves one electron to share with one of the two neighboring carbon atoms, thus creating a double bond with one carbon and leaving a single bond with the other, which is why some representations of the benzene molecule portray it as a hexagon with alternating single and double bonds.

After benzene, aromatic hydrocarbons can be polycyclic (also called condensed aromatic systems) (Figure A-3):

Figure A-3 Examples of aromatic hydrocarbons showing the alternating double bonds and single bonds.

Other depictions of the structure portray the hexagon with a circle inside it, to indicate that the six electrons are floating around in delocalized molecular orbitals the size of the ring itself. This represents the equivalent nature of the six carbon–carbon bonds all of bond order 1.5; the equivalency is explained by resonance forms. The electrons are visualized as floating above and below the ring, with the electromagnetic fields they generate acting to keep the ring flat.

The general properties of aromatic hydrocarbons are: (i) they display aromaticity, (ii) the carbon–hydrogen ratio is high, (iii) they burn with a strong sooty yellow flame because of the high carbon–hydrogen ratio, and (iv) they undergo electrophilic substitution reactions and nucleophilic substitution reaction.

See also: Alkenes, Alicyclic Hydrocarbons, Aliphatic Hydrocarbons.