Benzene: the delocalised model

Kekulé proposed alternating single and double bonds. The modern delocalised model is different: benzene is a planar, regular hexagon in which each carbon is bonded to two carbons and one hydrogen by σ bonds, and contributes its remaining p electron to a delocalised π system — a ring of electron density above and below the plane of the molecule.

Three pieces of evidence for delocalisation:

• Bond lengths. All six C–C bonds are identical (0.139 nm) — between a single bond (0.153 nm) and a double bond (0.134 nm). Kekulé’s structure would show two different lengths.
• Enthalpy of hydrogenation. Cyclohexene releases −120 kJ mol⁻¹, so Kekulé benzene (3 double bonds) should release 3 × −120 = −360 kJ mol⁻¹. The measured value is only −208 kJ mol⁻¹. Benzene is therefore 152 kJ mol⁻¹ MORE STABLE than the Kekulé model predicts — the delocalisation (resonance) energy.
• Reactivity. Benzene does not decolourise bromine water. It has no localised region of high π electron density to induce a dipole in Br₂, so it does not undergo electrophilic addition the way an alkene does.

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Nitration and Friedel–Crafts

Benzene reacts by electrophilic substitution — an electrophile replaces a hydrogen, which preserves the stable delocalised π system.

The general mechanism, in two steps: the delocalised π electrons attack the electrophile, forming an unstable positively charged intermediate in which delocalisation is partially broken; then the C–H bond breaks and its electron pair restores the full delocalised ring, releasing H⁺.

1. Nitration — concentrated HNO₃ with a concentrated H₂SO₄ catalyst at 50 °C (above 50 °C you get dinitration). The electrophile is the nitronium ion, NO₂⁺:

Nitration matters: nitrobenzene is the route to phenylamine (and hence to dyes), and to explosives such as TNT.

2. Friedel–Crafts — needs a halogen carrier (AlCl₃) to generate a strong enough electrophile.

• Acylation: CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻. The acylium ion substitutes into the ring to give a ketone (phenylethanone). The AlCl₃ is regenerated from AlCl₄⁻ + H⁺ → AlCl₃ + HCl.
• Alkylation: CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄⁻, giving methylbenzene.

Why acylation is more useful: it forms a C–C bond to a carbonyl carbon, which can then be reduced or attacked further — so it is a key chain-lengthening step in synthesis.

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Match the reagent to the product

Tap a reagent on the left, then its product on the right.

Preparing amines

Route 1 — from a halogenoalkane and excess ammonia (in ethanol, heated in a sealed tube). This is nucleophilic substitution:

Why the excess is essential: the primary amine produced is itself a nucleophile — in fact a better one than ammonia — so it attacks more halogenoalkane, giving a secondary amine, then a tertiary amine, then a quaternary ammonium salt. A large excess of ammonia makes it more likely that the halogenoalkane meets an ammonia molecule first, maximising the yield of the primary amine.

Route 2 — reduction of a nitrile (with LiAlH₄, or H₂ over a nickel catalyst). This also adds a carbon to the chain:

Route 3 — aromatic amines, by reducing a nitroarene with tin and concentrated HCl, followed by NaOH (to liberate the free amine from its salt):

Quaternary ammonium salts with a long hydrocarbon chain are cationic surfactants — used in fabric conditioners and hair products, because the positive head binds to negatively charged fabric or hair surfaces.

Why some amines are stronger bases

An amine is a base because the lone pair on the nitrogen accepts a proton. So the more available and electron-rich that lone pair is, the stronger the base.

• Primary aliphatic amines (e.g. ethylamine) are STRONGER bases than ammonia. The alkyl group is electron-releasing (positive inductive effect), so it pushes electron density onto the nitrogen. The lone pair is more available, and the resulting ion is stabilised.
• Aromatic amines (phenylamine) are MUCH WEAKER bases than ammonia. The nitrogen lone pair is partially delocalised into the benzene ring’s π system, so it is far less available to accept a proton.
• Amides (e.g. ethanamide) are not basic at all in the same way — the nitrogen lone pair is delocalised into the adjacent C=O group.

The exam sentence: "The alkyl group is electron-releasing, so the electron density on the nitrogen is increased and the lone pair is more available to accept a proton." For phenylamine, swap in "the lone pair is delocalised into the ring, so it is less available."

Sort each compound by its basicity

Tap a compound, then tap how basic it is compared with ammonia.

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Electrophiles, catalysts and basicity

• Name the electrophile. Nitration: NO₂⁺. Friedel–Crafts acylation: the acylium ion RCO⁺. Alkylation: the carbocation R⁺. Say where it comes from as well.
• The catalyst is regenerated. In Friedel–Crafts, AlCl₄⁻ + H⁺ → AlCl₃ + HCl. Show that step — it is often worth a mark on its own.
• Benzene substitutes, it does not add — because substitution preserves the delocalised π system.
• Basicity is about the AVAILABILITY of the lone pair. Alkyl groups push electron density in (more basic); a benzene ring pulls it out into the ring by delocalisation (much less basic).

Temperature control in nitration matters: keep the mixture at 50 °C. Above that, a second nitro group substitutes and you get dinitrobenzene — which is exactly how TNT is made, and exactly what you do not want if you are after mononitrobenzene.

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The big ideas to know

That is AQA 3.3.10 and 3.3.11. Press Finish.