Aromaticity can be defined as a property of conjugated cycloalkenes which helps in enhancing the stabilization of the molecule .This happens because of the delocalization of π electrons.
The Hückel's rule for aromaticity are:
- Molecule is cyclic
- Every atom of the ring must have one p orbital
- The molecule should be planar and every atom must have an sp2 hybridized orbital
- The molecule must have 4n+2 pi-bond electrons, where n is equal to any integer (0,1,2,3,…)
According to Hückel's Molecular Orbital Theory, a compound is stable if all the bonding molecular orbitals are completely filled with a pair of electrons. No anti-bonding orbital should be occupied.
Benzene has 6 π electrons. The first 2 π electrons fill up the lowest energy orbital, and the remaining 4 π electrons get filled in the succeeding energy level. In this manner ,all its bonding orbitals are filled, but none of the anti-bonding orbitals have any electrons. Therefore, the molecule shows exceptional stability
Aromatic compounds are highly stable compounds. On the other hand, Anti-aromatic compounds have an unusual instability. They have similar rules to aromaticity except the fact that the molecule has a closed loop of 4n pi-bond electrons.
Benzenoid and Non-benzenoid compounds
These molecules contain at least one benzene ring in their structure. A benzene ring has a cyclic structure and has six carbon atoms. It has a conjugated pi system containing alternate double and single bonds.
Since the molecule has double bonds because of the presence of a benzene ring, the molecule shows extra stability provided by the conjugated pi system. For example toluene
These molecules exhibit an aromatic behaviour without having any benzene nucleus . They have one or more rings fused but none of the rings is a benzene ring . The ring structure of these compounds might contain 5-7 etc number of carbons. The most common example is AZULENE . It is a system of two fused rings , one containing 7 and the other 5 carbons.
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