Bonding in Benzene

Specification Reference Organic chemistry, Aromatic chemistry 3.3.10.1

Quick Notes

AQA A-Level Chemistry diagram of benzene showing planar hexagonal ring

Benzene has a planar, cyclic structure with intermediate carbon bond lengths between single and double bonds.
Delocalised π-electrons (ring of delocalised electrons from the delocalisation of p electrons) increase benzene’s stability compared to the theoretical cyclohexa-1,3,5-triene (Kekule model).
Evidence from hydrogenation enthalpies shows benzene is more stable than expected.
Benzene undergoes substitution rather than addition reactions to preserve delocalisation and keep the ring of delocalised electrons.

Full Notes

Structure and Bonding in Benzene

Benzene (C₆H₆) is a planar, hexagonal molecule in which each carbon atom is bonded to two other carbon atoms and one hydrogen atom.

AQA A-Level Chemistry structure of benzene showing planar hexagon with alternating bonds

All C–C bond lengths in benzene are equal (~0.140 nm). This is between the length of a single bond (0.154 nm) and a double bond (0.134 nm).

Benzene’s structure cannot be represented accurately by alternating single and double bonds (as in the theoretical cyclohexa-1,3,5-triene proposed by Kekule which would also have a formula of C₆H₆).

AQA A-Level Chemistry comparison of C–C single, double, and benzene bond lengths

Instead, it has a delocalised system of π electrons, formed by the sideways overlap of unbonded p orbitals from each carbon atom.

AQA A-Level Chemistry diagram showing delocalised pi electrons above and below the benzene ring

These six p electrons form a continuous cloud of electrons above and below the plane of the carbon atoms, leading to uniform carbon-carbon bond lengths and a more stable structure.

Evidence for Delocalisation: Enthalpies of Hydrogenation

Enthalpies of hydrogenation show that benzene is more stable than expected - especially when compared to the theoretical cyclohexa-1,3,5-triene structure (Kekule model).

Less energy is released when benzene is hydrogenated and turned into cyclohexane than would be predicted for cyclohexa-1,3,5-triene.

AQA A-Level Chemistry data comparing hydrogenation enthalpies for cyclohexene, theoretical cyclohexa-1,3,5-triene, and benzene

Hydrogenation of cyclohexene (one C=C bond): ΔH = −120 kJ mol⁻¹

Expected hydrogenation of cyclohexa-1,3,5-triene (three C=C bonds):
ΔH = −360 kJ mol⁻¹
Actual hydrogenation of benzene: ΔH = −208 kJ mol⁻¹

This less exothermic enthalpy change for benzene indicates it is more stable than expected, due to delocalisation energy.

Matt’s exam tip

Make sure you can explain why we now propose the delocalised electron model of benzene - evidence of equal bond lengths, hydrogenation enthalpies and a tendency for substitution rather than addition reactions. Remember the theoretical molecule cyclohexa-1,3,5-triene doesn’t actually exist.

Why Benzene Undergoes Substitution Instead of Addition

Because benzene is an unsaturated hydrocarbon, we would expect it to react by addition reactions (like alkenes). However, it reacts by substitution reactions.

This is because because addition would disrupt the delocalised π-system, reducing stability whereas substitution maintains the delocalisation of electrons, keeping benzene stable.

AQA A-Level Chemistry schematic showing benzene preferring electrophilic substitution to preserve delocalisation

The electrophilic substitution mechanism and reactions of benzene that you need to know for AQA are outlined here.

Summary

Benzene is planar and cyclic with equal C–C bond lengths between normal single and double bonds.
Six p electrons from each carbon atom are delocalised over the ring, giving extra stability.
Hydrogenation enthalpies show benzene is significantly more stable than the theoretical cyclohexa-1,3,5-triene.
Benzene undergoes substitution instead of addition to preserve the delocalised π system.