Benzene, also known as C6H6, PhH, and benzol, is an organic chemical compound that is a colorless and flammable liquid with a pleasant, sweet smell. Benzene is a known carcinogen. It is a minor, or additive, component of gasoline. It is an important industrial solvent and precursor in the production of drugs, plastics, gasoline, synthetic rubber, and dyes. Benzene is a natural constituent of crude oil, but it is usually synthesized from other compounds present in petroleum. Benzene is an aromatic hydrocarbon, and the second [n]-annulene ([6]-annulene).

History

Benzene was discovered in 1825 by the English scientist Michael Faraday, who isolated it from oil gas and gave it the name bicarburet of hydrogen. In 1833, the German chemist Eilhard Mitscherlich produced it via the distillation of benzoic acid (from gum benzoin) and lime. Mitscherlich gave the compound the name benzin. In 1845, the English chemist Charles Mansfield, working under August Wilhelm von Hofmann, isolated benzene from coal tar. Four years later, Mansfield began the first industrial-scale production of benzene, based on the coal-tar method.

Structure

The formula of benzene (C6H6) caused a mystery for some time after its discovery, as no explanation had been found that could account for all the bonds — carbon usually forms four single bonds and hydrogen one.

The chemist Friedrich August Kekulé von Stradonitz was the first to deduce the ring structure of benzene. An often-repeated story claims that after years of studying carbon bonding, benzene and related molecules, he dreamt one night of the Ouroboros, a snake eating its own tail, and that upon waking he was inspired to deduce the ring structure of benzene. However, the story first appeared in the Berichte der Durstigen Chemischen Gesellschaft (Journal of the Thirsty Chemical Society), a parody of the Berichte der Deutschen Chemischen Gesellschaft, which appeared annually in the late-19th century on the occasion of the congress of German chemists; as such, it is probably to be treated with circumspection.

While his (more formal) claims were well-publicized and accepted, by the early-1920s Kekulé’s biographer came to the conclusion that Kekulé’s understanding of the tetravalent nature of carbon bonding depended on the previous research of Archibald Scott Couper (1831-1892); further, the Austrian chemist Josef Loschmidt (1821-1895) had earlier posited a cyclic structure for benzene as early as 1862. The cyclic nature of benzene was finally confirmed by the eminent crystallographer Kathleen Lonsdale.

Benzene presents a special problem in that, to account for all the bonds, there must be alternating double carbon bonds:

Using X-ray diffraction, researchers discovered that all of the carbon-carbon bonds in benzene are of the same length, and it is known that a single bond is longer than a double bond. In addition, the bond length, the distance between the two bonded atoms in benzene is greater than a double bond, but shorter than a single bond. There seemed to be in effect, a bond and a half between each carbon.

This is explained by electron delocalization. In order to visualise this, one should consider the position of electrons in the bonds of benzene.

One representation is that the structure exists as a superposition of the forms below, rather than either form individually. This type of structure is called a resonance hybrid.

In reality, neither form really exists. Delocalisation must be explained using a higher level of theory than single and double bonds. The single bonds are formed with electrons in line between the carbon atoms - this is called σ (sigma) symmetry. Double bonds consist of a sigma bond and another, π bond. This second bond has electrons orbiting in paths above and below the plane of the ring at each bonded carbon atom. The π-bonds are formed from atomic p-orbitals above and below the plane of ring. The following diagram shows the positions of these p-orbitals:

Since they are out of the plane of the atoms, these orbitals can interact with each other freely, and become delocalised. This means that, instead of being tied to one atom of carbon, each electron is shared by all six in the ring. Thus, there are not enough electrons to form double bonds on all the carbon atoms, but the “extra” electrons strengthen all of the bonds on the ring equally. The resulting molecular orbital has π symmetry.

This delocalisation of electrons is known as aromaticity, and gives benzene great stability. This is the fundamental property of aromatic chemicals that differentiates them from non-aromatics.

To reflect the delocalised nature of the bonding, benzene may be depicted as a circle inside a hexagon in chemical structure diagrams:

As is common in diagrams of organic structures, the carbon atoms in the diagram above have been left unlabeled.

Benzene occurs sufficiently often as a component of organic molecules that there is a Unicode symbol with the code 232C to represent it.

[From Wikipedia]