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Concept of Aromaticity

Aromaticity of compounds is based on certain rules. According to Huckel's rule, the number of pi electrons present should be a number denoted by the formula 4n + 2, where n is 0, 1, 2, 3, . . .. Hence, for compounds to be considered aromatic the number of pi electrons should be 2, 6, 10, . . .. In addition, the structure should also have a cyclic pi system to be aromatic. Atoms in the ring must have unhybridized p orbitals. These unhybridized p orbitals must overlap resulting in a cyclic pi system. For this overlap, the structure must have planar (or nearly planar) configuration. In other words, the atoms related or involved in the pi bond should be in the same plane.

Test whether you can recognize the aromaticity of the structures in the examples that follow:

Tips for testing aromaticity
 

Step 1 – Count the number of pi electrons.

Step 2 – See whether there is a cyclic p orbital overlap.

 

Predict the aromaticity of the compounds given below:
 

Compound 1

image\25813 ch 18.png

Number of pi electrons – 6

Cyclic overlap of p orbitals – Yes

Aromatic – Yes

 

Compound 2

image\25821 ch 18.png

 

Number of pi electrons – 4

Cyclic overlap of p orbitals – No

Aromatic – No
 

Compound 3

image\25830 ch 18.png

 

Number of pi electrons – 6

Cyclic overlap of p orbitals – Yes

Aromatic – Yes

Some examples of heterocyclic aromatic compounds.

image\Ch 18 page 242.png






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