The core structural characteristic of aromatic hydrocarbons is that they possess a cyclic conjugated planar system that satisfies Hückel's rule. Specifically, this can be summarized as follows:
Overall skeletal characteristics: They contain a closed cyclic conjugated system. Broadly speaking, they are divided into benzene-based aromatic hydrocarbons (containing a benzene ring structure) and non-benzene aromatic hydrocarbons (not containing a benzene ring but satisfying the aromaticity condition). The core structure requires all ring-forming atoms to be essentially coplanar.
Carbon atom hybridization: Each carbon atom in the ring is sp² hybridized (a few special aromatic structures are sp hybridized). Each carbon atom retains one unhybridized p orbital. These p orbitals overlap and form delocalized conjugated large π bonds.
Electron rule (Hückel's rule): The total number of delocalized π electrons must satisfy the 4n+2 rule (n is a non-negative integer). For example, in a benzene molecule, n=1, there are 6 π electrons, which fully meets the rule requirements. This is the core condition for a stable aromatic structure.
The typical structure of benzene is as follows: the benzene molecule is a planar regular hexagon, with six carbon atoms forming a regular hexagonal framework. All carbon-carbon bonds have completely equal bond lengths (about 140 pm), which is between carbon-carbon single and double bonds. There is no alternating single and double bond structure. All bond angles are 120°, and the symmetry is extremely high.