Benzene

Benzene (according to IUPAC Benzen) is a liquid organic hydrocarbon with the molecular formula _C6H6. It has a characteristic aromatic odour, is colourless, highly flammable and burns with a strong sooty flame. It mixes with almost all organic solvents, but hardly with water. Benzene is the parent compound of the aromatic hydrocarbons.

Originally, benzene was obtained during coke production for the steel industry. Nowadays, it is mostly obtained through refinery and petrochemical processes such as steam cracking as a by-product in gasoline, ethylene and p-xylene production. In addition, there are specific industrial production routes for benzene such as the dealkylation of toluene.

Benzene is an important building block for the petrochemical industry, with global demand exceeding 40 million metric tons per year in 2013. It is converted into numerous downstream products through addition, substitution and oxidation reactions. Its derivatives and downstream products such as ethylbenzene, cumene, cyclohexane, alkylbenzenes, chlorobenzenes, nitrobenzene and maleic anhydride are further processed into diverse products such as paints, pharmaceuticals, insecticides and plastics for various industries. Furthermore, it is contained in motor gasoline, although in most countries the content is limited. Benzene is carcinogenic and toxic.

Nomenclature

The name benzene was first used by Justus von Liebig in 1834. Liebig changed Eilhard Mitscherlich's name from 1833, who had referred to benzene as gasoline. In the Anglo-Saxon and French-speaking world, however, Mitscherlich's adapted name (French: benzène, English: benzene) continued to be used.

Since the systematic chemical nomenclature uses the suffix -ol for alcohols, the historically used name benzene, which is most commonly used in German, is misleading; the name benzene was determined by IUPAC to be the official nomenclature for this hydrocarbon.

History

In the second half of the 17th century, Johann Rudolph Glauber, who also discovered Glauber's salt (sodium sulfate), discovered benzene during the distillation of coal tar. The composition was unknown to him, however, and he called it a "subtle and lovely oleum." In 1825, the English physicist Michael Faraday discovered benzene in illuminating gas when he isolated it from liquid residues that separated from the gas phase when whale oils were burned in London street lamps. He therefore suggested the name "pheno" (Gr. phainein = to shine). This name is part of the term phenyl group, which in organic chemistry denotes the atomic group -C6H5.

A year later, this oil was identified as a hydrocarbon. In 1834, the German chemist Eilhard Mitscherlich obtained benzene from benzoic acid and calcium oxide; he also converted benzene to nitrobenzene, azobenzene and benzenesulfonic acid. He named the substance "benzine" because of its relationship to benzoic acid. He also created the correct molecular formula _C6H6. In the same year, "gasoline" was renamed benzene by Justus von Liebig. In 1845, the English chemist Charles Mansfield isolated benzene from coal tar while working under the supervision of August Wilhelm von Hofmann.

A long scholarly dispute smoldered over the correct structural formula of benzene. Initial ideas such as the Prisman structure proposed by Albert Ladenburg, that of benzvalene, dicyclopropenyl and James Dewar's Dewar benzene turned out to be wrong. It was not until 1861 that the Austrian chemist Johann Josef Loschmidt, then still a school teacher, formulated some possible structural formulae for benzene, which the German chemist and professor of chemistry August Kekulé adopted in 1865 - possibly as a suggestion for his Kekulé structural formula. According to legend, this idea came to Kekulé in a dream. He dreamt of a snake biting its own tail. Kekulé describes this in his speech on the 25th anniversary of the benzene ring in 1890. The six monkeys that alternately grasped each other's feet with either both or one hand and thus formed the ring structure are based on a joke made in 1886 at a beer evening of the German Chemical Society.

Kekulé's structure was the first to account for the experimental finding that all carbon atoms in benzene are equivalent. However, it did not explain all the peculiarities of benzene, such as its unusually low reactivity compared to olefins. The absence of an addition reaction with bromine water, as would be expected from the Kekulé structural formula, remained puzzling. The proof of the equivalence of the hydrogens in the benzene molecule was obtained between 1869 and 1874. In 1872 Kekulé formulated his "oscillation hypothesis" of a permanent change of place of single and double bonds.

It was not until the 20th century that the phenomenon of delocalized electron clouds, which give the benzene molecule a special stability, could be demonstrated via X-ray structure analysis. In 1925, Armit and Robinson introduced the simplified notation with the concentric circle in the formula, which is intended to express that all bonds are absolutely equivalent and that no localizable double bonds exist. A purely hypothetical form with three fixed double bonds and three single bonds would be called "1,3,5-cyclohexatriene".

In 1849, the industrial production of benzene based on hard coal began. It was handled carelessly until campaigns finally revealed the dangers of benzene over 100 years later, when its toxicity became known.