91Ó°ÊÓ

The aromaticity of a molecule is decided by Huckel’s $\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2}\mathbf{π}$rule. According to this rule, an aromatic compound is a planar, conjugated system that contains a total of$\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2}\mathbf{π}$delocalized electrons ( where n is an integer). Thus, aromatic compounds contain 2,6,10,14, and 18…., pi electrons. Molecules withelectrons are not aromatic systems. These systems are called antiaromatic and are often very unstable.

To make a 1, 3, 5, 7-cyclononatetraene aromatic compound, abstract a proton from the labeled${\text{CH}}_{\text{2}}$carbon as shown. This will generate cyclononatetraenyl anion, which is a planar aromatic molecule with 10 electrons.

1,3,5,7-cyclononatetraene cyclononatetraenyl anion

(8 electrons) (10 electrons)

Conversion of 1,3,5,7-cyclononatetraene to aromatic substance