Aromatic Chemistry: Benzene and Electrophilic Substitution

Benzene (C₆H₆) is the parent aromatic hydrocarbon and one of the most thoroughly studied molecules in organic chemistry. Its six carbon atoms form a planar regular hexagon, and its six π electrons are delocalised in a continuous cloud above and below the ring plane. That delocalisation is not a cosmetic detail: it confers an extra ~150 kJ mol⁻¹ of thermodynamic stability beyond what an alternating-double-bond model would predict, and it dictates the entire reactivity profile of aromatic compounds at A-Level and beyond. This lesson contrasts the historical Kekulé alternating-bond picture with the modern delocalised model, marshals the experimental evidence that decides between them (bond lengths, enthalpy of hydrogenation, lack of bromine-water decolourisation, X-ray crystallography), and then unpacks the three set-piece electrophilic substitution reactions on the AQA specification — nitration, halogenation, and sulfonation. We close with the qualitative substituent-effect framework that determines whether the next electrophile attaches ortho/para or meta to an existing group. The full Friedel–Crafts treatment lives in the next lesson (L4); here we lay the structural and mechanistic foundation that everything else builds on.

Spec mapping (AQA 7405): This lesson maps to §3.3.10 (aromatic chemistry — the structure of benzene and electrophilic substitution). It builds directly on §3.1.3 (bonding, including delocalised π systems) and on the alkene-addition mechanisms taught in the AS organic foundations course (§3.3.4) — those contrast lessons are essential reading. Forward links: L4 of this course (Friedel–Crafts alkylation and acylation, full carbocation rearrangement treatment); §3.3.11 (phenols and aromatic amines, which depend on directing effects introduced here). Refer to the official AQA specification document for the exact wording of each section.

Assessment objectives: Recall of the Kekulé vs delocalised models, the planar regular hexagonal geometry, and the standard reagents/conditions/electrophiles for nitration, bromination, and sulfonation are AO1 items. Writing the full curly-arrow mechanism, including the Wheland (cyclohexadienyl cation) intermediate and the restoration of aromaticity step, is AO2 and is a near-certain feature of Paper 2. Rationalising why benzene resists addition (using the enthalpy of hydrogenation evidence quantitatively) and predicting regioselectivity for substituted benzenes (toluene vs nitrobenzene, ortho/para vs meta directors) is AO3 reasoning — and the discriminator between B and A* responses.

Benzene: From Kekulé to the Delocalised Model

The Kekulé Proposal (1865)

The German chemist whose name attaches to the model — we'll write it with the standard accent throughout, though it appears unaccented in older textbooks — proposed in 1865 that benzene was a six-membered ring of carbon atoms with alternating single and double bonds: cyclohexa-1,3,5-triene. He later suggested the structure rapidly interconverted between two equivalent forms (the "Kekulé forms") to account for the lack of any detectable difference between adjacent C–C and C=C bonds. This was a remarkable insight for its time and gave organic chemistry a working structural picture for nearly seventy years. But four independent lines of evidence ultimately showed that the alternating-bond picture is wrong as a description of the real electronic structure — it survives today only as a useful bookkeeping device for counting electrons in mechanisms.

Evidence for Delocalisation

(1) C–C bond lengths are all identical. X-ray crystallography and electron diffraction both give every C–C bond in benzene as 139 pm (0.139 nm). For comparison: a "pure" C–C single bond (in ethane) is 154 pm; a "pure" C=C double bond (in ethene) is 134 pm. Benzene's 139 pm sits exactly between these values — every bond has the same partial-double-bond character. An alternating Kekulé structure would predict bonds alternating between 134 and 154 pm, and a regular hexagon would distort into an elongated form. It does not.

(2) Enthalpy of hydrogenation is much less exothermic than predicted. The measured enthalpy of hydrogenation of one isolated C=C bond in cyclohexene is:

cyclohexene + H₂ → cyclohexane; ΔH = −120 kJ mol⁻¹

If benzene really were cyclohexa-1,3,5-triene with three independent C=C bonds, the enthalpy of full hydrogenation to cyclohexane should be:

predicted ΔH(C₆H₆ + 3H₂) = 3 × (−120) = −360 kJ mol⁻¹

The experimental value is:

C₆H₆ + 3H₂ → C₆H₁₂; ΔH = −208 kJ mol⁻¹

Benzene is therefore (−208) − (−360) = +152 kJ mol⁻¹ more stable than the Kekulé model predicts. This 152 kJ mol⁻¹ is the delocalisation energy (sometimes called resonance energy or aromatic stabilisation energy). It is the quantitative thermochemical signature of aromaticity and is the single most-cited piece of evidence on every A-Level mark scheme.

(3) No reaction with bromine in tetrachloromethane. Alkenes — including cyclohexene — decolourise bromine water (or bromine in CCl₄) almost instantaneously via electrophilic addition. Benzene does not. Drop bromine onto benzene at room temperature in the absence of a halogen-carrier catalyst, and nothing happens: the orange colour persists. The π system is genuinely less reactive than a localised C=C bond. If benzene had three independent double bonds, the molecule should decolourise three equivalents of bromine; it decolourises none.

(4) X-ray crystallography confirms a planar regular hexagon. Direct structural determination, first achieved by Kathleen Lonsdale in 1929, shows benzene to be a flat, six-fold-symmetric ring (all bond angles 120°, all bond lengths equal). There is no detectable bond-length alternation even at very low temperatures. The hexagonal symmetry is real, not an average over rapidly interconverting Kekulé forms.

The Modern Delocalised Model

The modern bonding picture for benzene runs as follows:

Each carbon is sp² hybridised — three sp² hybrid orbitals point at 120° in the ring plane, forming the C–C and C–H σ-bond framework.
Each carbon has one remaining unhybridised 2p orbital, perpendicular to the ring plane.
The six 2p orbitals overlap sideways with both neighbours simultaneously, producing a continuous π electron cloud with density above and below the ring plane.
Six π electrons (one from each carbon) occupy this delocalised system.
The skeletal symbol — a circle inside the hexagon — represents this delocalised π cloud. The alternating-line "Kekulé" symbol is still widely used in mechanisms because curly arrows are easier to draw with localised bonds; both notations refer to the same real molecule.

Key Definition: Aromaticity in the A-Level sense is the extra thermodynamic stability that arises when a planar, cyclic, conjugated π system contains the right number of π electrons (six, for benzene). A more general criterion — Hückel's (4n+2) rule — is signposted in the Misconceptions section below; you don't need it for AQA marks, but it is the framework you'll meet at undergraduate level.

Electrophilic Substitution: The General Pattern

Benzene is electron-rich (six π electrons spread over a small ring), so it attracts electrophiles — but it almost never undergoes addition. Instead, it substitutes one ring H for the incoming group. The reason is straightforward thermochemistry: addition would destroy the aromatic system and forfeit the ~152 kJ mol⁻¹ delocalisation energy; substitution preserves it. Every electrophilic substitution mechanism on the AQA specification follows the same three-step template:

Generate the electrophile (E⁺). This usually requires a catalyst that activates an otherwise unreactive reagent (concentrated H₂SO₄ activates HNO₃; AlCl₃/FeBr₃ "halogen carriers" activate Cl₂/Br₂). The catalyst is regenerated at the end of the cycle.
Electrophile attacks the π cloud. A curly arrow runs from the delocalised π system of the ring to the electrophile, forming a new C–E σ bond. This produces a Wheland intermediate (also called a σ-complex, cyclohexadienyl cation, or arenium ion): the C bonded to E becomes sp³, the positive charge is delocalised over the five remaining ring carbons, and aromaticity is temporarily lost. This is the rate-determining step.
Loss of H⁺ restores aromaticity. A curly arrow from the C–H bond on the sp³ carbon back into the ring reforms the delocalised six-electron π system; the H leaves as a proton, usually picked up by the catalyst's conjugate base to regenerate the catalyst.

The energetic cost of going through the Wheland intermediate (where aromaticity is briefly lost) is what makes the rate-determining step the electrophilic attack — but the restoration of aromaticity in step 3 is what makes the overall reaction strongly thermodynamically downhill, and it is why substitution always wins over addition for benzene.

Reaction 1: Nitration

Overall equation: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O (benzene → nitrobenzene)

Reagents: concentrated HNO₃ and concentrated H₂SO₄.

Conditions: below 50 °C (typically 25–50 °C, warmed in a water bath). Above this temperature, mono-substitution is followed by a second nitration to give 1,3-dinitrobenzene, then 1,3,5-trinitrobenzene; controlling the temperature is the key practical skill.

Electrophile: the nitronium ion, NO₂⁺.

Generation of the electrophile. Concentrated H₂SO₄ is a stronger acid than HNO₃, so it protonates HNO₃ on the OH group; water is then displaced, leaving the linear nitronium cation:

HNO₃ + H₂SO₄ → H₂NO₃⁺ + HSO₄⁻ H₂NO₃⁺ → NO₂⁺ + H₂O

Net: HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

Mechanism (the three-step template applied):

Curly arrow from the benzene π cloud to the nitrogen of NO₂⁺ — forms the new C–N bond. The ring now carries +1 charge, delocalised over the remaining five carbons (the Wheland intermediate).
Curly arrow from the C–H bond on the sp³ ring carbon back into the ring — H⁺ leaves, aromaticity is restored, and the product is nitrobenzene.
The departing H⁺ is captured by HSO₄⁻ to regenerate H₂SO₄: H⁺ + HSO₄⁻ → H₂SO₄. Sulfuric acid is therefore a true catalyst (consumed in step 1, regenerated in step 3).

Why nitration matters. Nitrobenzene is the gateway to aromatic amines: reduction with tin metal in concentrated HCl (followed by NaOH workup) — or with H₂ over a nickel catalyst — gives phenylamine (aniline), C₆H₅NH₂. Phenylamine is the starting material for diazonium salts (azo dyes), for many sulfa drugs, and for paracetamol. The dinitro and trinitro derivatives are explosives — 2,4,6-trinitrotoluene (TNT) is made by sequential nitration of methylbenzene.

Reaction 2: Halogenation (Chlorination and Bromination)

Overall equations:

C₆H₆ + Cl₂ → C₆H₅Cl + HCl (benzene → chlorobenzene) C₆H₆ + Br₂ → C₆H₅Br + HBr (benzene → bromobenzene)

Reagents: the halogen (Cl₂ or Br₂) plus a halogen-carrier catalyst — AlCl₃ or FeCl₃ for chlorination; AlBr₃ or FeBr₃ for bromination. The catalyst must match the halogen (you do not use AlCl₃ with Br₂, because halide exchange would give a mixture).

Conditions: anhydrous, room temperature or gentle warming, dark glass to prevent radical pathways. Most importantly, benzene does not react with bromine water — unlike alkenes. This is the standard exam test that distinguishes an aromatic ring from a true C=C double bond.

Electrophile: a halogen cation, Cl⁺ or Br⁺ (more rigorously, a strongly polarised halogen δ⁺–δ⁻–MX₃ complex that behaves as a cation).

Generation of the electrophile. The Lewis-acidic metal halide accepts a lone pair from one halogen atom, polarising the X–X bond and effectively producing X⁺ and MX₄⁻:

Br₂ + FeBr₃ → Br⁺ + FeBr₄⁻ Cl₂ + AlCl₃ → Cl⁺ + AlCl₄⁻

Mechanism (template again):

Curly arrow from the π cloud to the halogen cation — forms the new C–X bond and generates the Wheland intermediate.
Curly arrow from the C–H bond on the sp³ carbon back into the ring — H⁺ is lost, aromaticity restored, the aryl halide is formed.
H⁺ + FeBr₄⁻ → HBr + FeBr₃ (or H⁺ + AlCl₄⁻ → HCl + AlCl₃) — the halogen carrier is regenerated.

The forward link to Friedel–Crafts chemistry (L4 of this course) is structural: alkylation and acylation use exactly the same halogen-carrier activation, but with R–Cl or RCO–Cl in place of Cl₂. The intermediate is then an alkyl or acyl carbocation that substitutes onto the ring by the same three-step mechanism.

Reaction 3: Sulfonation

Overall equation: C₆H₆ + H₂SO₄ → C₆H₅SO₃H + H₂O (benzene → benzenesulfonic acid)

Reagents: concentrated sulfuric acid; more commonly, fuming sulfuric acid (oleum) — H₂SO₄ saturated with dissolved SO₃ — at moderate temperatures (40–80 °C). Sulfonation is reversible (an unusual feature for electrophilic aromatic substitution), so the reaction can be driven backwards by heating with dilute aqueous H₂SO₄ — a useful synthetic trick for installing then removing a temporary directing group.

Electrophile: sulfur trioxide, SO₃, acting via its protonated form HSO₃⁺ (often written as the polarised species ⁺SO₂(OH)). In oleum the active electrophile is SO₃ itself, since the sulfur is already electron-deficient.

Mechanism (template):

Curly arrow from the benzene π cloud to the sulfur of SO₃ (or HSO₃⁺) — forms a new C–S bond and produces the Wheland intermediate.
Loss of H⁺ from the sp³ ring carbon restores aromaticity, giving the sulfonate ion C₆H₅SO₃⁻.
Protonation of the sulfonate gives benzenesulfonic acid, C₆H₅SO₃H.

Sulfonic acids are strong acids (comparable to H₂SO₄), and the sulfonyl chloride derivatives are the starting points for sulfa antibiotics — the first wave of antibacterials, developed in the 1930s. Detergent chemistry also relies heavily on aromatic sulfonates (long-chain alkylbenzenesulfonates are the active surfactants in most household detergents).

The Wheland Intermediate in Detail

All three reactions above go through a positively charged, non-aromatic intermediate where one ring carbon has become sp³ and the +1 charge is delocalised over the remaining five carbons. This is the Wheland intermediate (also: σ-complex, arenium ion, cyclohexadienyl cation). A faithful drawing shows:

five sp² carbons retaining p-orbital overlap (the partial π system that holds the positive charge);
one sp³ carbon, now bonded to four groups (two ring carbons, H, and the new electrophile);
a delocalised + charge, often drawn with three resonance structures (positive charge on the two ortho and one para positions relative to the sp³ carbon) or — equivalently — with a dashed semicircle and an explicit +.

The intermediate is higher in energy than benzene (aromaticity is lost) but lower in energy than the transition state that produces it (it is a real, isolable species in some cases — Olah's superacid work in the 1960s let chemists generate and characterise stable arenium ions, the cornerstone of his 1994 Nobel Prize). For A-Level purposes the key points are: (a) one ring carbon is sp³; (b) the + is delocalised over the other five; (c) restoring aromaticity by loss of H⁺ is the thermodynamic driving force for the second step.

Substitution vs Addition: A Quantitative Argument

If benzene underwent addition (analogous to ethene → 1,2-dibromoethane), the delocalised π system would be destroyed and the molecule would forfeit the ~152 kJ mol⁻¹ of resonance stabilisation. Substitution, by contrast, breaks one C–H bond (≈ 460 kJ mol⁻¹ in the ring) and forms one C–E bond (≈ 300–400 kJ mol⁻¹ depending on E), but preserves the aromatic π system. The net thermochemistry is comparable between the two pathways, but the transition-state energy for the substitution mechanism (which restores aromaticity in step 2) is much lower than for addition (which would lock the molecule out of the aromatic system permanently). Aromaticity is therefore preserved kinetically and thermodynamically. Every aromatic substitution mechanism in A-Level Chemistry is ultimately a consequence of this 152 kJ mol⁻¹ stabilisation energy — make sure you can quote the number and the cyclohexene-trimer comparison.

Substituent Effects: Activating and Deactivating Groups

When the ring already carries a substituent, two questions arise: (a) is the ring more or less reactive than benzene itself, and (b) where on the ring does the next electrophile go? AQA expects qualitative familiarity with the answers; the deep mechanistic treatment is L4.

Activating groups push electron density into the ring, making it more reactive than benzene and directing the next electrophile to the 2 (ortho) and 4 (para) positions:

–OH, –OR (alkoxy), –NH₂, –NHR, –NR₂ — strong activators (lone pair donation into the π system).
–R (alkyl, including –CH₃ in toluene/methylbenzene) — weak activator (σ-donation via hyperconjugation).

Deactivating groups pull electron density out of the ring, making it less reactive than benzene and directing the next electrophile to the 3 (meta) position:

–NO₂, –CN, –SO₃H — strong deactivators.
–COOH, –COR (ketone/ester carbonyl), –CHO — strong deactivators.
–CF₃, –⁺NR₃ — strong deactivators (no lone pair available).
The halogens (–F, –Cl, –Br, –I) are an interesting exception: they are weakly deactivating but ortho/para-directing, because they withdraw inductively (σ) but donate via lone pairs (π).

Worked example: nitration of toluene (methylbenzene). Methylbenzene reacts with the HNO₃/H₂SO₄ system about 25 times faster than benzene, and the products are predominantly 2-nitromethylbenzene (~58%) and 4-nitromethylbenzene (~37%) — the ortho/para combination — with only trace 3-nitromethylbenzene (~5%). The methyl group activates the ring (hyperconjugation pushes electron density from the C–H bonds of CH₃ into the π system) and stabilises Wheland intermediates where the + is ortho or para to it.

Worked example: chlorination of methylbenzene. Br₂/FeBr₃ or Cl₂/AlCl₃ on toluene gives 2- and 4-chloromethylbenzene as the major products, again reflecting the ortho/para-directing methyl group. The rate is enhanced (by a factor of about 350 for chlorination) compared with benzene itself.

The rules are mechanistically explained by stabilisation of the Wheland intermediate: activating groups stabilise the + charge when it falls ortho or para to them (because those positions adjoin the substituent and benefit from its donation); deactivating groups destabilise the + charge at ortho/para, so meta becomes the least-bad option. L4 develops these intermediates with full resonance diagrams.

Practical Skills: Performing a Nitration Safely

The nitration of benzene (or, more commonly in school labs, of methylbenzene) is a classic A-Level demonstration but a genuinely hazardous one. The key practical-skills points:

Aromatic Chemistry