aromaticity

Aromaticity (Huckel’s Rule)

Aromatic compounds contain an alternative double and single bond in its cyclic
structure and resemble to benzene in certain characteristic properties. This characteristic
behaviour is called Aromatic character or aromaticity.

Criteria for aromaticity:

On this basis of molecular orbital treatment of various aromatic compounds, it is
observed that the compounds are aromatic which fulfil the following criteria or
requirement:

1. An aromatic compound is flat to ensure maximum overlap of p-orbitals.
2. It must have p-orbitals parallel to each other so that a continuous ovelap of p-

orbitals is possible around the ring carbon atoms.
3. It must have cyclic clouds of delocalized π electrons above and below the plane

of the molecule. The π clouds must contain (4n + 2) π electrons system where n
is an integer, its value may be 0, 1, 2, 3, 4, - - - etc. this rule is known as Huckel’s
Rule.

Thus, according to Huckel’s Rule, the number of π-elctrons in an aromatic
compound may be,

When n = 0 (4n + 2) π = 2 π
1 6π
2 10 π
3 14 π
4 18 π

Benzene: it is cyclic and planar compound having three alternative double bonds. Thus,
each carbon of the ring has a p-orbital and contiuous cloud of 6 π electrons is present.
Thus follows the Huckel’s Rule.

(4n + 2) π = 6 π

now since here n= 1 the benzene obeys this rule and hence must be aromatic.