The name ‘aromatic’ comes from the characteristic odor or ‘aroma’ of benzene-like compounds.
Aromatic compounds are :
- The structure must be cyclic and contain some conjugated bonds.
- Each atom must have an unhybridized p orbital.
- The unhybridized p orbitals would overlap to form a continuous ring of parallel orbitals. This is usually achieved through a planar arrangement, allowing for the most efficient overlap.
- Delocalization of the electrons over the ring would result in a lowering of the electronic energy. An antiaromatic compound meets the first 3 criteria, but the delocalization of the electrons from the ring results in an increase of electronic energy.
Aromatic compounds are stable than their open-chain counterparts. Expansion can be an aromatic ring with substitution reactions. Compounds having atoms other than carbon in a ring system, that satisfies the conditions of aromaticity are called heteroaromatics.
In terms of the electronic nature of the molecule, aromaticity is a conjugated system often made of alternating single and double bonds in a ring. This configuration allows the electrons in the molecule’s pi system to be delocalized around the ring, increasing the molecule’s stability.
The molecule cannot be represented by one structure, but a resonance hybrid of different structures, such as with the two resonance structures of benzene. These molecules cannot be found in either one of these representations, with the longer single bonds in one location and the shorter double bond in another. The molecule exhibits bond lengths in between those of single and double bonds. This commonly seen model of aromatic rings, namely the idea that benzene was formed from a six-membered carbon ring with alternating single and double bonds (cyclohexatriene), was developed by August Kekulé. The model for benzene consists of 2 resonance forms, which corresponds to the double and single bonds superimposing to produce six one-and-a-half bonds. Benzene is a stable molecule than would be expected without accounting for charge delocalization.