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Aromatic Compounds (Overview)

Benzene: An Aromatic Compound

The compound which is today known as benzene was isolated for the first time in 1824 by Michael Faraday. He found it in the liquid residue that condensed from the gas phase when whale oil was burnt by the street lamps in London. Initially, the name "pheno", which derives from the Greek word "phainein" (= shine), was proposed.

Ten years later, in 1834, Eilhardt Mitscherlich determined the correct empirical formula of benzene: C6H6. Due to the chemical relationship it has with benzoic acid, he called it benzin. It was only later on that the name "benzin" was replaced by "benzene".

The relatively low number of hydrogens in proportion to the number of carbons is characteristic of aromatic compounds. Aromatic compounds are usually found tree and plant oils. While benzaldehyde is found in cherries and peaches, toluene may be isolated from the trunk of the South American Myroxylon balsamum tree, for instance. Benzene is obtained by distillation of coal. Due to their aromatic and often pleasant smell, the name "aromatic compounds" was given to the members of this family. Many natural products, such as the hormone oestrone and the opiate morphine, contain aromatic parts and synthetic products, such as the sedative drug valium, too.

Examples of naturally occuring and synthetic aromatic compounds

Fig.1
Benzene
Fig.2
Benzaldehyde
Fig.3
Toluene
Tab.1
Naturally occuring and synthetic aromatic compounds
OestroneMorphineDiazepam (valium)
Fig.4
Fig.5
Fig.6
Mouse
Fig.7
Mouse
Fig.8
Mouse
Fig.9

Further "daily" aromatic compounds are saccharine (a synthetic sweetener), benzopyrene (a carcinogenic compound, which is formed when meat is broiled), the anthraquinone dye 1,4-diaminoanthraquinone (a purple hair dye), and (2,4,5-trichlorophenoxy)acetic acid. This acetic acid derivative is a defolient that was used during the Vietnam War. In its synthesis the "Seveso poison" 2,3,7,8-tetrachlorodibenzo[b,e]-p-dioxin appears as a side product. This dioxin is also formed as a by-product when aromatic and chlorine-containing compounds are improperly burnt in an incineration plant.

Nowadays, the chemical meaning of the term "aromatic" is no longer correlated with the compound's smell. The term "aromatic" now applies to a family of compounds whose members are structurally related in that they contain an aromatic system. The chemical behavior of aromatic compounds is considerably different from that of aliphatic compounds.

The effects of exposure to benzene over a longer period of time are, among others, bone-marrow atrophy and leukemia. This is commonly the case even if the concentration of benzene is relatively small. Great precautions must therefore be taken when working with benzene.

Exercise: Benzene as an aromatic compound

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