Huckel’s rule of aromaticity and anti aromatic compounds

Huckel’s rule of aromaticity 

Huckel’s stated that the presence of (4n+2) numbers of delocalized π electrons in a flat or nearly flat cyclic system is the cause of aromaticity and it is called Huckel’s rule of aromaticity .

Where ,n = 1,2,3 …..etc, the whole number.

What is the condition of aromaticity ?

An organic species will be aromatic if the species satisfy the following conditions.

(I)The species  must be  planar 

(II)It  must  have  cyclic  structure 

(III)  The cyclic  structure must be stable 

(Iv) The  species must contain Huckel’s number of delocalized π electrons .

For example, benzene.

Benzene is a cyclic compound. It is a planar and stable structure. 

Benzene  contains of (4n+2) numbers of delocalized π electrons, which is a Huckel’s number. Hence , benzene is aromatic.

Besides benzene, there are a large number of benzenoid compounds , they are also obey  the Huckel’s rule  and hence aromatic.They are called benzenoid aromatic

What is non benzenoid aromatics ?

There are few conjugated monocyclic and poly cyclic systems  which contains  (4n+2) numbers  of delocalized π electrons and not benzenoid are known as non-benzenoid aromatic compounds.

Huckel’s rule predicts the [14] [18] and [22] annulene with (4n+2) π electrons when  n= 3 ,4, 5 respectively  have been found to be aromatic.

Aromatic compounds in biochemistry

Two amino acids phenylalanine and tyrosine  necessary for protein synthesis contain the benzene ring  are aromatic.

What is anti aromatic ?

Planar or non planar cyclic conjugated molecule  with 4n number of  π electrons and  related with high energies are called anti aromatic.

For example:

[4] annulene  that is cyclo butadiene  is a 4n molecule . It is a highly unstable compound and it is anti aromatic compound. 

Besides butadiene, [8], [16]  and [24] annulenes are anti aromatic . [10] annulene is  also anti aromatic , although it contains (4n+2) π electrons, because the cyclic [10] annulene is not planar.

Summary:

What is aromaticity ? what is Huckel's rule of aromaticity ?
What is anti aromatic ?
Aromatic compound in biochemistry .

Why phenol is acidic in nature-click here .