These notes explore benzene’s structure, comparing historical and modern models, explaining π-electron delocalisation, orbital overlap, and how structure influences stability, bond lengths, and chemical reactivity.

The Molecular Structure of Benzene

Benzene has the molecular formula C₆H₆ and is the simplest aromatic hydrocarbon. Early attempts to explain its structure struggled to account for its unusual stability and reactivity. Unlike alkenes, benzene does not readily undergo addition reactions, despite appearing to contain multiple carbon–carbon double bonds.

Each carbon atom in benzene is sp² hybridised, forming:

• Three σ (sigma) bonds

• A trigonal planar arrangement with bond angles of approximately 120°

Two of these σ bonds are formed with neighbouring carbon atoms, creating a hexagonal ring, and the third is formed with a hydrogen atom. This arrangement leaves one unhybridised p orbital on each carbon atom, perpendicular to the plane of the ring.

The Kekulé Model of Benzene

The earliest widely accepted model of benzene was proposed by August Kekulé in the 19th century. It represents benzene as a six-membered ring with alternating single and double carbon–carbon bonds.

This diagram shows the Kekulé model of benzene, drawn as a hexagon with alternating C–C and C=C bonds. It illustrates the early view of benzene as a cyclohexatriene with localised double bonds. Source

According to this model:

• There are three C=C double bonds and three C–C single bonds

• Double bonds are localised between specific carbon atoms

• Two equivalent Kekulé structures can be drawn by alternating the positions of the double bonds

This model successfully explains why benzene has six carbon atoms bonded in a ring and why each carbon forms three bonds. However, it fails to explain key experimental observations.

One major limitation is that the Kekulé model predicts two different carbon–carbon bond lengths, corresponding to single and double bonds. Experimental data show this is not the case.

Experimental Bond Length Evidence

Measurements using X-ray diffraction reveal that all six carbon–carbon bonds in benzene have the same length of approximately 0.139 nm. This value lies between:

• A typical C–C single bond (0.154 nm)

• A typical C=C double bond (0.134 nm)

This evidence contradicts the Kekulé model and suggests that bonding electrons are not confined between specific carbon atoms. A more accurate model is required to explain this equivalence.

The Delocalised Model of Benzene

The modern explanation of benzene’s structure is the delocalised model, sometimes referred to as the resonance model.

In benzene:

• Each carbon atom contributes one electron from its unhybridised p orbital

• These p orbitals overlap sideways around the ring

• This overlap creates a continuous π system above and below the plane of the ring

This figure illustrates how sideways overlap of six p orbitals forms a continuous π system above and below the benzene ring. The orbital diagram explains electron delocalisation; the electrostatic surface adds extra detail beyond syllabus requirements. Source

The six π electrons are delocalised over all six carbon atoms, meaning:

• All C–C bonds are equivalent

• Each bond has partial double bond character

• The structure is more stable than any arrangement with localised double bonds

This delocalisation is often represented by a hexagon with a circle inside, symbolising the shared π electrons.

This diagram compares resonance drawings with a delocalised representation of benzene. It reinforces that π electrons are shared across the entire ring rather than confined to individual double bonds. Source

p-Orbital Overlap and Stability

The delocalised model relies on effective p-orbital overlap. For this to occur:

• All carbon atoms must be coplanar

• p orbitals must be parallel and close enough to overlap

• Continuous overlap must exist around the entire ring

This arrangement allows π electrons to be spread evenly across the molecule. Delocalisation lowers the overall energy of benzene compared to hypothetical cyclohexatriene structures, making benzene thermodynamically more stable.

This increased stability is a defining feature of aromatic compounds and explains why benzene resists reactions that would disrupt its π system.

Reactivity Differences Explained by Structure

The Kekulé model predicts that benzene should behave like a typical alkene and undergo addition reactions readily. In reality, benzene shows very low reactivity towards addition reactions such as hydrogenation or halogen addition.

The delocalised model explains this behaviour:

• Addition reactions would break the continuous π system

• Loss of delocalisation would reduce stability

• As a result, these reactions are energetically unfavourable

Instead, benzene typically undergoes electrophilic substitution reactions, where:

• One hydrogen atom is replaced

• The aromatic π system is preserved

This contrast in reactivity between benzene and alkenes provides strong evidence in favour of the delocalised model over the Kekulé model.

Comparing the Two Models

Key differences between the models include:

• Bonding: Kekulé assumes localised double bonds; the delocalised model involves shared π electrons

• Bond lengths: Kekulé predicts alternating lengths; experimentally, all are equal

• Stability: Delocalisation accounts for benzene’s enhanced stability

• Reactivity: Only the delocalised model explains benzene’s resistance to addition reactions

The delocalised model is therefore accepted as the accurate description of benzene’s structure, fully consistent with experimental evidence and observed chemical behaviour.