Criteria for Aromaticity and the Hückel 4n + 2 Rule

Benzene, cyclobutadiene, and cyclooctatetraene are represented as a hybrid of two equivalent structures.

However, unlike benzene, cyclobutadiene and cyclooctatetraene are unstable and not aromatic.

To account for these differences, Erich Hückel outlined a set of structural features underlying aromaticity, known as Hückel's rule or the 4n + 2 rule.

Hückel's rule states that an aromatic compound must be cyclic and planar, with an unhybridized p orbital on each carbon.

The p orbitals must overlap to form a continuous loop of (4n + 2) π electrons, where n is a positive integer.

Benzene fulfills all the criteria for Hückel's rule, establishing its aromaticity.

In contrast, cyclooctatetraene assumes a non-planar tub conformation with no overlap between adjacent p orbitals. Additionally, it lacks (4n + 2) π electrons and is therefore not aromatic.

Similarly, cyclobutadiene is cyclic, planar, and has a continuous overlap of p orbitals. However, it lacks (4n + 2) π electrons and loses aromaticity.