Why is benzene so stable, and how does it react?
The structure and delocalised bonding of benzene, the evidence for delocalisation, electrophilic substitution reactions including nitration and halogenation, and a comparison of the reactivity of benzene with alkenes and of phenol with benzene.
A CCEA A-Level Chemistry answer on aromatic chemistry, covering the delocalised structure of benzene, the thermochemical and bond-length evidence for delocalisation, electrophilic substitution by nitration and halogenation, and the relative reactivity of benzene, alkenes and phenol.
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What this dot point is asking
CCEA wants you to describe the delocalised bonding in benzene, quote the evidence for it, write the mechanism and conditions for electrophilic substitution (nitration and halogenation), and compare the reactivity of benzene with alkenes and of phenol with benzene.
The structure of benzene
Electrophilic substitution
Benzene reacts by electrophilic substitution, keeping the delocalised ring intact.
Nitration: concentrated and concentrated at about . The electrophile is :
Halogenation: or with a halogen carrier (, ) that polarises the halogen to give the electrophile .
Reactivity comparisons
Examples in context
The industrial nitration of benzene to nitrobenzene is the first step toward aniline (phenylamine) dyes and toward TNT, and uses exactly the concentrated mixture above with careful temperature control to avoid dinitration. The phenol-versus-benzene contrast appears in CCEA practical work: phenol decolourises bromine water and forms a white precipitate of 2,4,6-tribromophenol instantly, whereas benzene does not react, demonstrating how the oxygen lone pair activates the ring.
Try this
Q1. State the type of hybridisation of each carbon atom in benzene. [1 mark]
- Cue. .
Q2. Give the electrophile responsible for the nitration of benzene and the equation for its formation. [2 marks]
- Cue. ; .
Q3. Explain why phenol is more reactive toward electrophiles than benzene. [2 marks]
- Cue. An oxygen lone pair is delocalised into the ring, increasing electron density and attracting electrophiles more strongly.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20194 marksDescribe the bonding in benzene and outline two pieces of evidence that support the delocalised model rather than the Kekule model.Show worked answer →
Each carbon in benzene is hybridised and forms three sigma bonds (to two carbons and one hydrogen). The remaining p orbital on each carbon overlaps sideways with its neighbours to form a ring of delocalised pi electrons above and below the plane.
Evidence 1, bond lengths: all six carbon-carbon bonds are identical at , between a single bond () and a double bond (). The Kekule model predicts alternating long and short bonds, which is not observed.
Evidence 2, enthalpy of hydrogenation: the measured value for benzene () is less exothermic than three times that of cyclohexene (). Benzene is about more stable than the Kekule structure predicts, the delocalisation (resonance) energy.
Markers reward (1) the delocalised pi system from p-orbital overlap, (2) equal bond lengths, (3) the enthalpy-of-hydrogenation comparison, (4) the conclusion of extra stability.
CCEA 20213 marksState the reagents and conditions for the nitration of benzene and explain why benzene undergoes substitution rather than addition.Show worked answer →
Reagents: concentrated nitric acid and concentrated sulfuric acid (the catalyst). Conditions: warm at about , keeping the temperature below to avoid multiple substitution.
The electrophile is , generated by .
Benzene undergoes substitution rather than addition because substitution restores the stable delocalised ring of pi electrons, whereas addition would permanently destroy the delocalisation and lose the associated stability. Markers reward (1) both concentrated acids and the temperature, (2) the electrophile, (3) preservation of the stable delocalised system as the reason for substitution.
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Sources & how we know this
- CCEA GCE Chemistry specification — CCEA (2016)