The Chemistry of Benzene

A Hexagonal Molecule on which is Founded Aromatic Chemistry

The solution to the problem of a mysterious chemical. Delocalised electrons make the molecule reactive and dangerous

Benzene is one of the most interesting organic chemicals of all because it took so long for chemists to understand the structure. It was a mystery for so long but once its structure was determined it proved to be the key to a host of large molecules.

Discovery

The famous chemist, Michael Faraday discovered benzene in 1825 while distilling crude oil. Its empirical formula was found but its structure was not determined until 1865 when German chemist, Friedrich August Kekulé had a day-dream or “reverie” while thinking about the structure. He saw a snake curl round and bite its own tail, forming a ring structure. Kekulé immediately knew that benzene was a ring of six carbons connected by alternating double and single bonds (see image 2)

Hybridisation

The problem with this structure was that double and single bonds have different lengths, but X-ray crystallography studies have shown that the bond-lengths in benzene are the same. This led some scientists to suggest that the structure resonated between double and single bonds. Eventually Linus Pauling used his molecular orbital theories to propose a structure in which all the bonds are equal with a hybridised circular orbital above and below the benzene ring (see image 1).

Arenes

Benzene is the simplest of the group of organic chemicals known as arenes. The hydrogen atoms attached to each carbon in the ring can be substituted for other functional groups to form other chemicals many of which are important in industry for example. Benzene rings can be fused together to make structures such as naphthalene. Graphite is made up of a whole hexagonal network of carbon atoms in which all the hydrogen atoms have been replaced by other carbon atoms.

Lead-Free Petrol

One of the most common uses of benzene is as an additive to petrol or gasoline in cars. It improves what is known as the octane rating and reduces knocking. In the drive to reduce lead-containing additives in petrol benzene has been used as a replacement, although there are some health issues related to benzene itself. It is used in many parts of the chemical industry as a raw material in the manufacture of plastics, lubricants, drugs and pesticides.

Carcinogen

In the early twentieth century benzene was used as an after-shave lotion because of its pleasant smell, but it has since been discovered to be seriously hazardous to the health. It can be carried through the blood and damage bone marrow and red blood cells. It is also carcinogenic and can cause leukaemia.

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Orbitals of Benzene

Kekule Benzene