17.10 : 방향족 탄화수소 양이온: 구조적 개요

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.