Aromaticity | Requirements for aromaticity | Aromaticity in benzene

AROMATICITY

Aromaticity is the property that imparts special stability to a molecule and such molecules are said to be "aromatic". In this sense, aromatic molecules are characterized by extra stability. 

Aromaticity in a molecule is because of certain structural features, meaning there are some requirements or pre-requisites for a molecule to be aromatic.

CYCLIC STRUCTURE

An aromatic molecule will always have a cyclic structure. Open chain molecules can never have aromaticity.

CONJUGATED SYSTEM

This cyclic structure must have a conjugated system of pi or non-bonding electrons. Conjugation means the presence of an alternate double and single bond system. A pair of non-bonded electrons at an adjacent position to the conjugated system will also be involved in conjugation.

PLANARITY

The cyclic conjugated molecule must be planar, meaning all the atoms in the ring must be in one plane. This is important for effective overlap of p orbitals during delocalization of electrons. When the atoms are in one plane, the p orbitals are parallel to each other and thus can overlap effectively.

SUSTAINED RING CURRENT

Current is the flow of electrons. Ring current here implies to the delocalized electrons which change their positions. Thus a sustained ring current means complete delocalization of pi or n electrons within the ring, involving all the atoms in the ring.

HUCKLE'S RULE

An aromatic molecule must obey the Huckle's rule, which states that, "There must be 4n+2 delocalized (pi or n) electrons inside the ring, where n is any whole number.

Thus if n = 0, 1, 2, 3,.....

4 (0) + 2 = 2

4 (1) + 2 = 6

4 (2) + 2 = 10  and so on.

Therefore, if 2, 6, 10....... electrons are involved in conjugation within a ring, the molecule is said to obey the Huckle's rule

CLOSED LOOP OF DELOCALIZED ELECTRONS

The delocalized electrons must form a closed loop within the ring. Delocalization must involve all the atoms present in the ring.

ELECTRONS IN BONDING MOLECULAR ORBITALS

All the electrons must occupy the bonding molecular orbitals.

CONCLUSION

If a molecule fulfils all of the above requirements, it will aromatic and will be highly stable. Stable enough to resist any addition reactions on the ring. Failure to fulfil any of the above will make the molecule either non-aromatic or anti-aromatic.

Benzene is an example of aromatic molecules.