Benzene (C6H6), the simplest aromatic hydrocarbon, was first isolated in 1825 by English chemistMichael Faraday from the oily residues left from illuminating gas. In 1834 it was prepared from benzoic acid (C6H5CO2H), a compound obtained by chemical degradation of gum benzoin, the fragrant balsamexuded by a tree that grows on the island of Java, Indonesia. Similarly, the hydrocarbon toluene(C6H5CH3) received its name from tolu balsam, a substance isolated from a Central American tree and used in perfumery. Thus benzene, toluene, and related hydrocarbons, while not particularly pleasant-smelling themselves, were classified as aromatic because they were obtained from fragrant substances.Joseph Loschmidt, an Austrian chemist, recognized in 1861 that most aromatic substances have formulas that can be derived from benzene by replacing one or more hydrogens by other atoms or groups. The term aromatic thus came to mean any compound structurally derived from benzene. Use of the term expanded with time to include properties, especially that of special stability, and eventually aromaticity came to be defined in terms of stability alone. The modern definition states that a compound is aromatic if it is significantly more stable than would be predicted on the basis of the most stable Lewis structural formula written for it. (This special stability is related to the number of electronscontained in a cyclic conjugated system; see below Arenes: Structure and bonding.) All compounds that contain a benzene ring possess special stability and are classified as benzenoid aromatic compounds. Certain other compounds lack a benzene ring yet satisfy the criterion of special stability and are classified as nonbenzenoid aromatic compounds.