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Benzene: Introduction to Aromatic Compounds

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 fractional distillation, benzene, toluene, xylene and naphthalene, as well as many other aromatic compounds, are obtained.

Movie: Petroleum distillation

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).

Exercise

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