Experimental evidence shows benzene cannot be explained by simple alternating double bonds, instead supporting a delocalised π system that accounts for its unusual stability and reactivity.

The Need for Experimental Evidence

Early structural models of benzene attempted to explain its formula, C₆H₆, using alternating single and double bonds. While these Kekulé structures account for valency, experimental measurements reveal discrepancies that cannot be explained by localised double bonds alone.

The delocalised model proposes that electrons in the π system are spread evenly over the entire ring, rather than confined between specific carbon atoms. Several independent experimental observations support this model and collectively provide strong evidence for delocalisation in benzene.

Carbon–Carbon Bond Lengths in Benzene

One of the strongest pieces of experimental evidence comes from precise measurements of carbon–carbon bond lengths using techniques such as X-ray diffraction.

In alkenes, a C=C double bond is shorter than a C–C single bond because increased electron density between the nuclei pulls the atoms closer together. Typical bond lengths are:

• C–C single bond: approximately 0.154 nm

• C=C double bond: approximately 0.134 nm

If benzene contained alternating single and double bonds, two distinct bond lengths would be observed. Instead, all six carbon–carbon bonds in benzene are found to be identical, each measuring approximately 0.139 nm, which is intermediate between single and double bond lengths.

This diagram highlights benzene’s planar hexagonal structure with six identical C–C bond lengths, providing direct structural evidence for a delocalised π system rather than alternating single and double bonds. Source

This uniform bond length provides direct structural evidence that the π electrons are not localised, supporting the delocalised benzene model rather than the Kekulé structure.

Enthalpy of Hydrogenation Evidence

Further evidence comes from studying the enthalpy of hydrogenation of benzene and comparing it with that of similar compounds.

Hydrogenation is the reaction of an unsaturated compound with hydrogen to form a saturated compound. For alkenes, this reaction releases a predictable amount of energy per C=C bond.

Cyclohexene, which contains one C=C bond, has an enthalpy of hydrogenation of approximately –120 kJ mol⁻¹. If benzene contained three isolated C=C bonds, its expected enthalpy of hydrogenation would be around –360 kJ mol⁻¹.

However, experimental measurements show that the enthalpy of hydrogenation of benzene is only about –208 kJ mol⁻¹.

This lower-than-expected value indicates that benzene is significantly more stable than predicted by the Kekulé model. The extra stability is referred to as delocalisation energy, arising from the evenly spread π electrons across the ring.

Stability and Delocalisation Energy

The difference between the expected enthalpy of hydrogenation (–360 kJ mol⁻¹) and the observed value (–208 kJ mol⁻¹) represents the additional stability of benzene due to delocalisation.

The diagram shows that benzene releases less energy on hydrogenation than expected for three C=C bonds, demonstrating additional stabilisation from delocalised π electrons. Source

This energy difference demonstrates that breaking the delocalised π system requires extra energy, making benzene thermodynamically more stable than compounds with localised double bonds.

Key implications of this stability include:

• Lower reactivity towards reactions that would disrupt the π system

• Preference for reactions that preserve delocalisation

• Greater resistance to processes typical of alkenes

Resistance to Addition Reactions

Alkenes typically undergo electrophilic addition reactions, where the π bond breaks to form two new σ bonds. If benzene contained three isolated double bonds, similar addition reactions would be expected.

Experimentally, benzene shows a strong resistance to addition reactions, such as addition of bromine or hydrogen, under normal conditions. Instead, benzene reacts primarily via electrophilic substitution, which preserves the delocalised π system.

This reaction scheme illustrates benzene’s resistance to electrophilic addition, showing substitution with bromine in the presence of a catalyst while preserving the delocalised π system. Source

This behaviour contrasts sharply with alkenes, which readily decolourise bromine water at room temperature. Benzene does not do so without a catalyst, providing further evidence that its bonding is fundamentally different.

Linking Experimental Evidence to the Delocalised Model

Each experimental observation supports the same conclusion:

• Equal bond lengths show electrons are evenly distributed

• Lower enthalpy of hydrogenation indicates enhanced stability

• Resistance to addition demonstrates reluctance to disrupt the π system

Together, these findings cannot be explained by alternating single and double bonds. They are fully consistent with a delocalised π system formed by overlap of p orbitals around the ring, producing a continuous region of electron density above and below the plane of the benzene molecule.

This experimental evidence underpins the modern understanding of benzene’s structure and explains both its unusual stability and distinctive chemical behaviour.