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.

Procedure

Cycloheptatriene is another neutral monocyclic unsaturated hydrocarbon with an odd number of carbon atoms.

It has six π electrons, fulfilling Hückel's 4n + 2 rule.

However, the intervening sp³ carbon disrupts the delocalization of these π electrons; therefore, neutral cycloheptatriene is not aromatic.

The conversion of an sp³ carbon into an sp² carbon by removing one hydrogen with both, one, or no electrons generates a cation, a radical, and an anion, respectively.

As observed, only the cycloheptatrienyl cation has the required number of (4n + 2) π electrons.

Moreover, the generation of an empty 2p orbital facilitates a continuous overlap of p orbitals. Therefore, the cycloheptatrienyl cation is aromatic.

The aromaticity of the cation can be substantiated from the seven resonance structures, the symmetrical distribution of the positive charge in the electrostatic potential map, and the Frost diagram of the cation, with all π electrons in the bonding molecular orbitals.