17.10 : Aromatic Hydrocarbon Cations: Structural Overview

Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp³ carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp³ carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.

Removing one hydrogen from the intervening CH₂ group with both or one or no electrons generates cycloheptatrienyl cation, radical, and anion, respectively, thereby converting the sp³ carbon of cycloheptatriene to sp². As a result, the generated empty 2p orbital in cycloheptatrienyl cation and the p orbitals of other carbons in the ring forms a continuous loop of overlapping p orbitals. Therefore, compared to radical and anion, the cycloheptatrienyl cation with 6 π electrons and a continuous loop of overlapping p orbitals fulfills all Huckel’s criteria and is aromatic.