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The aromatic hydrocarbons include benzene and the alkyl derivatives of benzene, a six sided ring drawn with alternating single and double bonds or with a circle in the middle.
The Benzene Ring
Any compound with the benzene ring is an aromatic compound.
The term "aromatic" came to be because earlier compounds found with the rings had pleasant fragrances, but it turns out that the ring has nothing to do with smell.
An aromatic compound like asprin is entirely odorless.
Since the benzene ring looks very much like a triene (alkene with three double bonds), it should be expected that its properties would be like one, but it is not.
It does not undergo the addition reactions that alkenes do, and it is unusually stable to oxidizing agents.
The reason why is because the benzene ring is a resonance structure (see Resonance: When Lewis Structures Fail).
benzene (resonance structures)
The carbon atoms, in fact, are not connected by alternating single and double bonds, but they all are connected by hybrid partial double bonds.
Overlapping p orbitals causes delocalization (See Delocalized Molecular Orbitals) of bonds, giving stability to the ring system.
If benzene undergoes an addition reaction, that would break the stabilizing electron network.
Instead, benzene prefers to react by substitution.
Benzene will react by substitution with chlorine, bromine, and nitric acid, provided that a Lewis acid catalyst is present.
C6H6 + Cl2 ==============> C6H5-Cl + HCl
C6H6 + Br2 ==============> C6H5-Br + HBr
C6H6 + HNO3 ==============> C6H5-NO2 + H2O