Carbon compounds containing unsaturated but stable ring structures are called aromatic compounds. For example, benzene, C6H6, is an aromatic hydrocarbon. In aromatic compounds there are conjugated double bonds which are delocalised. For example, in benzene there are three delocalised carbon-carbon double bonds.
The geometry around a carbon atom involved in a double bond and two single bonds is trigonal planar or triangular planar. This is shown in the figures below
Figure 1 - Delocalisation of double bonds in benzene
Why is benzene not 1,3,5-cyclohexatriene?
In benzene the length of all the six carbon-carbon bonds is exactly the same, intermediate between lengths of C-C and C=C bonds.
The reactions of benzene are not characteristic of alkenes: benzene undergoes substitution reactions more easily than the addition reactions which are typical of unsaturated compounds such as alkenes and alkynes.
Why are benzene and other aromatic compounds so stable?
The high stability of benzene and other aromatic compounds is due to the delocalisation of the double bonds (more exactly due to the delocalisation of pi-electrons associated with the double bonds).
The word aromatic is used to describe any system that is stabilized by a ring containing delocalised conjugated double bonds.
Figure 2 - A bonding model
Which reaction (addition or substitution) do aromatic compounds undergo more easily and why?
Aromatic compounds undergo substitution reactions more easily than addition reactions because of the delocalisation of the double bonds.
Representation of the benzene molecule:
A hexagonal ring with a circle inside the ring is used to represent the benzene molecule. Each corner of the hexagon represents a carbon atom to which a hydrogen atom is attached. Each carbon is bonded to two other carbon atoms. The circle inside the hexagon represents the delocalisation of the pi-electrons associated with the double bonds.
Naming of aromatic compounds
In the IUPAC system of nomenclature, monosubstituted alkylbenzenes are named as derivatives of benzene. For example, methylbenzene (commonly known as toluene), ethyl benzene, 2-propylbenzene (or isopropylbenzene), etc. With two or more substituents, a numbering system is used.