Benzene – Delocalisation and Electrophilic Substitution
ChemistryOrganic ChemistryAges 16–17
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Sign in to playCompare the Kekulé model of benzene with a delocalised π system using enthalpies of hydrogenation (predicted −360, measured −208 kJ/mol), bond lengths and the bromine water test. The mechanism screen animates nitration, bromination and Friedel–Crafts alkylation and acylation with curly arrows: forming the electrophile, the arenium ion intermediate, loss of H⁺ and regeneration of the catalyst, with an energy profile.
Lesson: Arenes: the structure of benzene and electrophilic substitution
What it shows
Kekulé proposed alternating single and double bonds in benzene, but the evidence disagrees. Hydrogenating cyclohexene releases 120 kJ/mol, so three C=C bonds should release 360 kJ/mol; benzene releases only about 208 kJ/mol, so it is roughly 150 kJ/mol more stable. All six C–C bonds are 139 pm long, between a single (154 pm) and a double bond (134 pm), and benzene does not decolourise bromine water. Its six π electrons are delocalised over the ring. Benzene therefore reacts by electrophilic substitution, which keeps the delocalised system: the π electrons attack an electrophile, an arenium ion forms, and loss of H⁺ restores the ring.
How to use
On the Benzene structure screen, choose a Model and compare the bond lengths. Pick a molecule under Hydrogenation, predict ΔH, then press Add H₂ (measure ΔH) to reveal the measured value. Press Shake with bromine water to compare cyclohexene with benzene. On the Electrophilic substitution screen, choose a Reaction and use Play or Next step to follow the curly arrows and the energy profile.
Parameters you can change
- Screen Benzene structure, Electrophilic substitution
- Benzene model Kekulé, Resonance (two Kekulé structures), Delocalised π system
- Molecule hydrogenated Cyclohexene, Cyclohexa-1,3-diene, Benzene
- Substitution reaction Nitration, Bromination, Friedel–Crafts alkylation, Friedel–Crafts acylation
Questions to explore
- Why is the enthalpy of hydrogenation of benzene much less exothermic than three times that of cyclohexene?
- Which evidence shows that all six carbon–carbon bonds in benzene are identical?
- Why does benzene react with electrophiles by substitution rather than by addition?
