Structure and Bonding in Aromatic Compounds

The Benzene Molecule

The structure of benzene, a fundamental concept in organic chemistry, represents a cornerstone in understanding aromatic compounds. 

Electron Delocalisation, Bond Length and Energies

The modern idea of benzene, developed through various experimental observations, shows a planar hexagonal molecule where all carbon-carbon bond lengths are equal. This equality of bond lengths is due to the delocalization of π electrons across the ring structure.

Experimental evidence shows that benzene’s C-C bond lengths (about 1.39 Å) are intermediate between those of typical carbon-carbon single bonds (C-C, about 1.54 Å) and double bonds (C=C, about 1.34 Å). This uniformity supports the idea of delocalized electrons rather than localized double and single bonds. Additionally, benzene is more stable than would be predicted for a molecule with three double bonds, indicating delocalization of electrons contributes to its stabilization.

π-Electron Cloud

The unhybridized p orbitals on each carbon atom overlap with those on adjacent carbon atoms, creating a continuous loop of overlapping p orbitals above and below the plane of the carbon atoms. This overlapping forms a delocalized π electron system shared across all six carbon atoms, rather than being localized in double bonds between specific carbon atoms. The delocalization of these electrons provides extra stability to the benzene molecule, known as resonance energy or delocalization energy.  See the following diagrams to further understand what this electron cloud looks like.

The symmetrical structure of benzene is a consequence of the interplay between the σ and π electrons in the molecule. The symmetrical σ frame acts in conjunction with the delocalized π frame to enforce the regular hexagon.

Implications of sp2 hybridisation

The sp2 hybridization and resulting delocalized π electron system make benzene less reactive toward addition reactions that would disrupt its aromatic system. Instead, benzene typically undergoes electrophilic aromatic substitution reactions, where a hydrogen atom is replaced by an electrophile, preserving the aromatic ring structure.  The sp2 hybridization and the π electron cloud also influence benzene's physical properties, such as its planar geometry, equal bond lengths (intermediate between single and double bonds), and ability to absorb ultraviolet light, which contributes to its UV-visible spectroscopy characteristics.