Topics - Aromatic Hydrocarbons

Aromatic Hydrocarbons An aromatic hydrocarbon contains one or more planar sets of six carbon atoms. The simplest aromatic hydrocarbon is benzene and the set of six carbon atoms is called the benzene ring. The benzene ring is usually drawn with alternating double bonds. There is a reason for this. Each carbon atom has four electrons to share, one shares with the hydrogen atom, and one each to the two neighboring carbons leaving one extra to share with one of its neighboring carbon atoms (i.e. a double bond). Aromatic bonding is a type of bonding quite distinct from other types of multiple bonding, such as double or triple bonds. Two fused benzene rings produce the hydrocarbon naphthalene and three fused benzene rings produce the two isomers anthracene and phenanthrene. The hydrogen atoms on benzene, for example, can be substituted by a methyl group or ethyl group producing methyl benzene (toluene) or ethyl benzene. Aromatic hydrocarbons are present in relatively large quantities in coal-gas extracts. The gas is washed with oil and the ‘benzole’ distilled from the oil producing a ‘benzole mixture’. The benzole mixture contains about 70%v/v of benzene, about 20%v/v of toluene, about 5%v/v of the higher aromatics and about 5%v/v of aliphatic hydrocarbons, naphthenes and olefins (typical ofcoke oven benzole). The carbon/hydrogen ratio of the aromatics is high so their presence in coal gas give the gas its luminosity when burnt. Unlike aliphatic compounds which undergo nucleophilic substitution reactions, aromatic hydrocarbons undergo electrophylic substitution reactions.