An arene is a hydrocarbon containing one or more benzene rings.

Benzene consists of a planar, regular hexagonal ring of six carbon atoms, with each carbon bonded to one hydrogen atom.

The main features of the delocalised model of benzene are:

1. Each carbon atom has 3 covalent bonds - one to a hydrogen atom and the other two to carbon atoms.
2. The fourth electron is in a p-orbital perpendicular to the plane of carbon atoms.
3. Adjacent p-orbitals overlap sideways above and below the plane of the carbon atoms to form a ring of electron density, known as a π-system.
4. The 6 electrons in this π-system are delocalised across all 6 carbon atoms in the ring.

Chlorobenzene

Aromatic compounds with more than one substituent are named by numbering the ring and listing the substituent groups using the lowest possible numbers.

For example, the skeletal formula of 1,4-dichlorobenzene is:

When benzene undergoes substitution reactions, the highly stable delocalised ring of π-electrons remains intact. Addition reactions into the benzene ring would disrupt the 'aromatic stabilisation' requiring a much greater activation energy.

The formula of benzene is C₆H₆.

The skeletal formula of benzene is:

Arenes typically undergo substitution reactions with electrophiles.

Alkenes react with bromine by electrophilic addition at room temperature, while benzene reacts with bromine by electrophilic substitution when heated in the presence of a catalyst.

The π-system of benzene is delocalised across the entire 6-membered ring so it has insufficient electron density to polarise the Br-Br bond. Heat and a catalyst are needed to initiate the reaction.

In contrast, the π-system of ethene is localised around the C=C bond so it is able to polarise the Br-Br bond sufficiently to initiate a reaction at room temperature without the need for a catalyst.