3.3.10 Aromatic Chemistry

Benzene has six π electrons delocalised over the ring, with six equivalent C–C bonds. The equal bond lengths and greater stability than a localised cyclohexatriene model support delocalisation.

When you draw or describe the model, keep the geometry and electron model together: each carbon is trigonal planar, each contributes a p orbital, and the continuous overlap gives a π system above and below the ring.

A localised cyclohexa-1,3,5-triene model would contain three ordinary C=C bonds. Real benzene is lower in energy because six π electrons are delocalised over the whole ring.

Use two observations as the clean evidence pair: all six C–C bonds have the same intermediate length, and hydrogenation is less exothermic than the three-isolated-C=C model predicts. Both point to one delocalised structure.

Compare the measured enthalpy of hydrogenation with the value expected for three isolated C=C bonds. Benzene is less exothermic to hydrogenate than that model predicts, showing extra delocalisation stability.

Take the question’s hydrogenation value for one ordinary C=C bond and multiply it by three to build the localised model. The measured benzene value is less negative; that energy gap is the thermochemical evidence for extra stabilisation.

Compare benzene with an alkene: addition would leave benzene without its stabilised delocalised π system. In electrophilic substitution the ring can restore that delocalisation when H⁺ is lost, so the aromatic system survives in the product.

That restoration of aromatic delocalisation is the key energetic reason benzene commonly substitutes where an alkene would add.

For electrophilic substitution at benzene, let the ring regain its delocalised π system at the end. The π electrons attack the electrophile, a cationic intermediate forms, then loss of H⁺ restores the aromatic ring.

Once the ring has lost H⁺ and regained delocalisation, stop at the monosubstituted product for the treatment required here. The mechanism should finish with the aromatic ring restored.

For nitration, I’d remember why the NO₂ group is useful as well as the reaction itself. Nitro compounds are useful intermediates, and reducing –NO₂ gives an aromatic amine such as phenylamine.

A useful sequence to know is benzene → nitrobenzene → phenylamine, with the second step being reduction. Here the nitro group is an intermediate that opens a route to a different functional group.

Friedel–Crafts acylation forms a C–C bond by electrophilic substitution using an acylating reagent with AlCl₃. The aromatic ring is restored after substitution, giving an aryl ketone.

If you are planning a synthesis, the useful feature is the new C–C bond between the ring and the acyl group. That lets Friedel–Crafts acylation extend the carbon skeleton while retaining the aromatic ring.

For nitration, you first generate NO₂⁺ from concentrated HNO₃ and H₂SO₄, then show benzene attacking the electrophile and losing H⁺ to restore delocalisation. For Friedel–Crafts acylation, AlCl₃ helps generate the acyl electrophile.

In both mechanisms, the ring attack forms a cationic intermediate and the final deprotonation restores the aromatic π system. Make that restoration visible; stopping at the cationic intermediate leaves the mechanism incomplete.