Aromatic: class of compounds that contain 6-membered benzene-like rings with 3 double bonds. Alkyl-sub benzenes are sometimes referred to as arenes; named in diff ways: smaller than the ring, arene is named as an alkyl-sub benzene, larger than the ring, phenyl-sub alkane, benzyl for group. Planar and has the shape of a regular hexagon. Benzene undergoes substitution reactions that retain the cyclic conjugation. Huckel 4n+2 rule: a molecule is aromatic only if it has a planar, monocyclic system of conjugation and contains a total of 4n+2 pi electrons, where n is an integer. Planar, conjugated molecules with 4n pi electrons are said to be antiaromatic because delocalization of their pi electrons would lead to their destabilization. For aromaticity, a molecule must be cyclic, conjugated and have 4n+2 pi electrons. Cyclopentadiene is one of the most acidic hydrocarbons because the anion formed by loss of h is so stable.