arenes

Arenes are hydrocarbons that contain at least one aromatic ring. The prototypical arene is benzene, C6H6. Aromatic rings are cyclic, flat structures with a conjugated system of p orbitals, which delocalize π electrons around the ring. According to Hückel's rule, aromatic systems contain 4n+2 π electrons, imparting unusual stability relative to nonaromatic rings.

In benzene and related arenes, the carbon atoms are sp2 hybridized, the ring is planar, and the carbon–carbon bonds are of intermediate length due to resonance. Arenes are typically nonpolar and relatively insoluble in water. They undergo substitution reactions that preserve the aromatic ring, such as electrophilic aromatic substitution (nitration, sulfonation, halogenation, Friedel–Crafts alkylation or acylation). Addition reactions are less favorable because they disrupt aromaticity.

Common monocyclic arenes include benzene, toluene, and xylene; polycyclic arenes include naphthalene, anthracene, and phenanthrene. The term can also apply to polycyclic aromatic hydrocarbons (PAHs) that contain fused arene rings. Some arenes are heteroaromatic in which one or more carbon atoms are replaced by heteroatoms (pyridine, furan) though these are sometimes distinguished as heteroaromatics.

Safety and environmental aspects: Benzene is a known human carcinogen; many PAHs are toxic and persistent pollutants. Arenes find use as solvents, chemical feedstocks, dyes, and in the production of polymers and pharmaceuticals.