# Aromatic Compounds (Overview)

## Occurence of Aromatic Hydrocarbons

Simple aromatic hydrocarbons come from two main sources: Coal and petroleum. Coal is a complex mixture of a large number of compounds, most of which are long-chain compounds. If coal is heated to about 1000 °C in the absence of air (oxygen), volatile components, the so-called tar oil, are stripped out. By , benzene, toluene, xylene and naphthalene, as well as many other aromatic compounds, are obtained.

Fig.1
Aromatic hydrocarbons from coal.

In contrast to coal, though petroleum consists largely of aliphatic compounds, it contains much fewer aromatic components. Industrial petroleum processing - that is, several subsequent fractional distillations (view movie above), cracking, reforming, and perhaps coking - eventually yields, among others, (pure) aromatic compounds. Cyclisations and dehydrogenations also occur during these processes. Heptane ($C7H16$), for instance, may be converted into toluene ($C7H8$).

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