Benzene
Benzene (C6H6) is an aromatic compound. It was discovered in 1825 by Michael Faraday who found it in an oily residue in street lighting.

It is colourless
Sweet smelling
Highly flammable
Component of petrol
Carcinogen
It is found in cigarette smoke
The group for benzene is phenyl.

The molecule
It is a hexagonal ring consisting of six carbon atoms. Every carbon is joined to two other carbon atoms and one hydrogen atom. It is a hydrocarbon called an aromatic compound or arene. Benzene's structure can be shown in two ways. It doesn't react in the same way as alkenes despite having the alkene suffix.

Aromatic Compounds
This was a name typically used for benzene derivatives. This is because many sweet smelling compounds contained a benzene ring. Odourless compounds are also found to have a benzene ring, despite the name given to them as aromatic. There are many aromatic compounds which can be synthesised from benzene.

August Kekule
Kekule established a possible structure of benzene. Despite the molecular formula of C6H6 suggesting there are many double and triple bonds, Kekule disagreed with this. This is because compounds with multiple bonds are very reactive, but benzene is not.

The Kekule Model
He suggested in 1865 that the model was based again on a six carbon ring, but with alternating single and double bonds. He claimed he thought of this whilst he was day-dreaming about a snake catching its own tail.

Against the Kekule Model

The lack of reactivity. Kekule's model had double bonds, this means that it should decolourise bromine water in an electrophilic addition reaction. But: benzene doesn't undergo electrophilic addition reactions. Also benzene doesn't decolourise bromine water, under normal conditions. Therefore it lead scientists to believe that benzene doesn't have any double bonds.
Bond lengths. X-ray diffraction allows bond lengths to be measured. In 1929, the lengths of the bonds in benzene were measured to all be 0.139nm. This bond length is between the length of a single bond, 0.153nm, and a double bond, 0.134nm.
Hydrogenation enthalpies. The Kekule structure can be given the name cyclohexane-1,3,5-triene - this indicates the location of the double bonds. Given this, it would be expected that benzene would have an enthalpy change of hydrogenation which is three times higher than that of cyclohexene. The enthalpy change of hydrogenation is -120kJ/mol for cyclohexene. Therefore it would be expected that benzene would have an enthalpy change of hydrogenation of -360kJ/mol. However, the actual enthalpy change of hydrogenation for benzene is -208kJ/mol - 152kJ/mol less than expected. This means the actual structure is more stable than the Kekule model expected.

This evidence against the Kekule structure lead scientists to propose the delocalised model of benzene.

The delocalised model of benzene
The features of the model:

It is planar, cyclic and hexagonal hydrocarbon with six carbon atoms and six hydrogen atoms.
Every carbon uses 3 of 4 of its electrons to bond to two other carbons and one hydrogen.
Each carbon has one electron in a p-orbital at right angles to the plane of the bonded carbon and hydrogen atoms.
The adjacent p-orbitals overlap sideways, above and below the plane of carbon atoms to form a ring of electron density.
This creates a system of pi bonds.
The six electrons in the pi bonds are said o be delocalised.