Question

How do you identify aromatic rings?

Answer 1

An aromatic ring must be a planar, cyclic, system containing (4n + 2) π electrons in alternating single and double bonds with no intervening saturated carbon atoms.

Explanation:

Hückel's rule states that an aromatic system must have (`4n + 2`) π electrons (#n = 0, 1, 2 ,3, 4, …)

Examples

Thus, benzene is aromatic because it satisfies Hückel's rule.

`4n+ 2 = 6`

`n = 1`

The cyclopropenyl cation is also aromatic.

`4n + 2 = 2`

`n= 0`

A ring with 4 pi electrons is not aromatic

`4n + 2 = 4`

`n = 2/4 = 1/2`

n is a fraction.

Thus, cyclobutadiene doesn't follow Hückel's rule and is not aromatic.