Benzene is the simplest aromatic hydrocarbon, with the molecular formula C6H6. In 1865, German chemist F.A. Kekulé proposed that benzene has a hexagonal structure with alternating single and double bonds, which has been used ever since. According to the hybrid orbital theory, the six π electrons in the benzene molecule are a whole, evenly distributed above and below the ring plane, and the structure of benzene can also be represented by a hexagon with a circle in it (see “Aromatic Hydrocarbons”). The benzene molecule has a special aromatic ring structure. Hydrocarbons containing a benzene ring structure are called aromatic hydrocarbons, and compounds containing a benzene ring structure are called aromatic compounds.