Module 7 : Benzenes and Substituted Benzenes

Lecture 17 : Benzene and Related Compounds - I

7.4 Aromaticity

Aromatic compounds are those that meet the following criteria

Cyclopentadiene is not aromatic because of the presence of sp3 hybridized carbon atom. The cyclopentadienyl anion is aromatic because it has an uninterrupted ring of p-orbital and (4n+2) π-system. The cyclopentadienyl cation is antiaromatic as it has (4n) π-system.

Cycloheptatrienyl cation and cyclooctatetraene dianion are aromatic compounds because they have uninterrupted ring of p-orbital and (4n+2) π-system.

Cyclopropenone and cycloheptatrienone are stable aromatic compounds. We know that the double bond in carbonyl (C=O) group is polarized to give partial positive charge on the carbon atom and partial negative charge on the oxygen atom. So cyclopropenone and cycloheptatrienone are considered to be aromatic as it obeys (4n+2) π-rule. But the same reason makes cyclopentadienone to be antiaromatic and it is unstable, rapidly undergoes a Diels-Alder dimerization.