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Why is benzene so stable and how does it react?

The structure of benzene and the delocalised model. Evidence for delocalisation from enthalpies of hydrogenation and bond lengths. Electrophilic substitution reactions of benzene, including nitration and Friedel-Crafts acylation, with mechanisms and the role of the catalyst.

A focused answer to the AQA A-Level Chemistry 3.3.11 specification points on aromatic chemistry. Covers the delocalised model of benzene, thermochemical and bond-length evidence for it, and the electrophilic substitution mechanisms of nitration and Friedel-Crafts acylation.

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  1. What this dot point is asking
  2. The delocalised model
  3. Electrophilic substitution
  4. Try this

What this dot point is asking

AQA wants you to describe the delocalised model of benzene, give the thermochemical and bond-length evidence for it, and explain the electrophilic substitution mechanisms of nitration and Friedel-Crafts acylation, including how each electrophile is generated.

The delocalised model

In the Kekule model benzene was drawn with alternating single and double bonds. The accepted delocalised model says each carbon is sp2 hybridised, bonded to two carbons and one hydrogen in the plane, with one electron in a p-orbital perpendicular to the ring. These p-orbitals overlap sideways to form a ring of delocalised pi electrons above and below the plane, shared equally by all six carbons. This is why benzene is planar, regular and hexagonal, with all bond angles exactly 120∘120^\circ, rather than the irregular shape the Kekule structure would predict.

Electrophilic substitution

Benzene reacts by substitution, not addition, because this preserves the stable delocalised ring.

Nitration

The electrophile is the nitronium ion NO2+NO_2^+, generated from concentrated nitric and sulfuric acids:

HNO3+2H2SO4→NO2++2HSO4−+H3O+HNO_3 + 2H_2SO_4 \rightarrow NO_2^+ + 2HSO_4^- + H_3O^+

Friedel-Crafts acylation

The electrophile is an acylium ion RCO+RCO^+, generated from an acyl chloride and an AlCl3AlCl_3 halogen-carrier catalyst:

CH3COCl+AlCl3→CH3CO++AlCl4−CH_3COCl + AlCl_3 \rightarrow CH_3CO^+ + AlCl_4^-

The acylium ion substitutes onto the ring to form a phenyl ketone, and the AlCl3AlCl_3 catalyst is regenerated. Acylation introduces a C−CC-C bond, extending the carbon skeleton, which is useful in synthesis.

Both reactions share the same three-stage pattern: an electrophile is generated, the delocalised ring attacks it to form a positively charged intermediate in which the delocalisation is partly lost, and a proton is then lost to restore the aromatic system. Restoring the ring is the driving force, because the delocalised structure is about 150 kJ mol−1150\ \text{kJ mol}^{-1} more stable than a comparable molecule with localised double bonds, so benzene reacts by substitution (which keeps the ring) rather than addition (which would destroy it). This is the key contrast with alkenes, which add across the double bond because they have no aromatic stabilisation to preserve, and AQA frequently asks candidates to explain this difference in reactivity.

Try this

Q1. Give the formula of the electrophile in the nitration of benzene. [1 mark]

  • Cue. NO2+NO_2^+.

Q2. State two pieces of evidence that benzene has a delocalised structure. [2 marks]

  • Cue. Equal C-C bond lengths; enthalpy of hydrogenation less exothermic than predicted.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20185 marksBenzene reacts with a mixture of concentrated nitric acid and concentrated sulfuric acid. Give an equation for the formation of the electrophile, outline the electrophilic substitution mechanism, and explain how the sulfuric acid is regenerated.
Show worked answer →

Electrophile generation: HNO3+2H2SO4→NO2++2HSO4−+H3O+\text{HNO}_3 + 2\text{H}_2\text{SO}_4 \rightarrow \text{NO}_2^+ + 2\text{HSO}_4^- + \text{H}_3\text{O}^+. The electrophile is the nitronium ion NO2+\text{NO}_2^+.

Mechanism: the delocalised pi electrons of the ring attack NO2+\text{NO}_2^+ (curly arrow from the ring to the nitrogen), forming an unstable positively charged intermediate with the delocalisation partly disrupted. A C-H\text{C-H} bond then breaks, the proton is removed by HSO4−\text{HSO}_4^-, and the electrons return to restore the stable delocalised ring, giving nitrobenzene.

Catalyst regeneration: H++HSO4−→H2SO4\text{H}^+ + \text{HSO}_4^- \rightarrow \text{H}_2\text{SO}_4, so the sulfuric acid acts as a catalyst overall.

Markers reward the equation forming NO2+\text{NO}_2^+, the curly arrow from the ring, the intermediate, loss of H+\text{H}^+ to reform the ring, and regeneration of the sulfuric acid.

AQA 20213 marksThe enthalpy of hydrogenation of benzene is about −208 kJ mol−1-208\ \text{kJ mol}^{-1}, whereas three times the value for cyclohexene predicts about −360 kJ mol−1-360\ \text{kJ mol}^{-1}. Explain what this comparison tells you about the bonding in benzene.
Show worked answer →

If benzene contained three isolated C=C\text{C}=\text{C} double bonds, its enthalpy of hydrogenation would be about three times that of cyclohexene, around −360 kJ mol−1-360\ \text{kJ mol}^{-1}.

The actual value (−208 kJ mol−1-208\ \text{kJ mol}^{-1}) is much less exothermic, by about 150 kJ mol−1150\ \text{kJ mol}^{-1}, which means benzene is more stable (lower in energy) than the hypothetical molecule with three localised double bonds.

This extra stability is caused by the delocalisation of the pi electrons over the whole ring, so benzene has a delocalised structure, not alternating single and double bonds.

Markers reward the comparison of expected and actual values, the conclusion that benzene is more stable than predicted, and attributing this to delocalisation.

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