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Spec Mapping — OCR H432 Module 6.1.1 — Aromatic compounds, covering the mechanism of electrophilic substitution as the cornerstone reactivity mode of benzene and its substituted derivatives, the nitration of benzene by a mixture of concentrated nitric and sulfuric acids (with generation of the nitryl/nitronium cation NO₂⁺ as the active electrophile and the Wheland sigma-complex as the rate-determining intermediate), the halogenation of benzene by chlorine or bromine in the presence of a halogen-carrier catalyst (FeBr₃, FeCl₃, AlBr₃, AlCl₃ as Lewis acids), the three-step curly-arrow mechanism with explicit regeneration of catalyst, and the conceptual link back to Lesson 10's delocalisation enthalpy explaining why substitution is energetically favoured over addition (refer to the official OCR H432 specification document for exact wording).
Lesson 10 established why benzene resists addition — the delocalisation enthalpy of 152 kJ mol⁻¹ is too high a price to pay in a transition state. Lesson 11 builds the mechanism by which benzene does react: electrophilic aromatic substitution (EAS). The pattern is universal across aromatic chemistry — every reaction in the OCR specification (nitration here; halogenation here; Friedel-Crafts alkylation and acylation in Lesson 12; phenol's accelerated bromination, also Lesson 12) is a variation on the same three-step template. (i) Generate a strong electrophile (often via a Lewis-acid or Brønsted-acid catalyst); (ii) the benzene π-cloud attacks that electrophile, forming a high-energy Wheland (sigma) intermediate in which one ring carbon is sp³ and aromaticity is temporarily lost; (iii) loss of H⁺ from that sp³ carbon to a base restores aromaticity. Mastering the curly-arrow choreography here — two arrows in step (ii), one arrow in step (iii) — is the single most-rewarded mechanistic skill in OCR aromatic chemistry, examined in essentially every paper.
Key Mechanism — Electrophilic Aromatic Substitution (3 steps):
- Generate the electrophile E⁺ from the reagent and a catalyst (Brønsted acid for nitration, Lewis acid for halogenation and Friedel-Crafts).
- π-attack on E⁺ — the benzene π-cloud attacks E⁺ using one curly arrow from the ring (going to E⁺), with a second curly arrow from the C–E bond? No: a second arrow is not needed here because the electrophile has no leaving group attached. The result is a Wheland intermediate (σ-complex) in which one ring carbon is sp³ (bearing both the original H and the new E), the ring has lost aromaticity, and the positive charge is delocalised over the remaining five carbons. Two curly arrows are drawn for nitration step ii: π-cloud → N of NO₂⁺ forming the C–N bond, and (for some mark schemes) a counter-arrow from the N–O bond if NO₂⁺ is drawn with a double bond. For halogenation: π-cloud → δ⁺ Br with simultaneous Br–Br bond → Br⁻FeBr₃ on the way to FeBr₄⁻ (two arrows).
- Loss of H⁺ — one curly arrow from the C–H sigma bond on the sp³ ring carbon back into the ring; H⁺ leaves to a base (HSO₄⁻ for nitration, FeBr₄⁻ for bromination); aromaticity restored; catalyst regenerated.
Benzene's chemical stability arises from the continuous delocalised π-system described in Lesson 10 — six π-electrons spread over six sp² carbons in a planar ring, conferring a thermodynamic stabilisation of ≈ 152 kJ mol⁻¹ relative to a hypothetical cyclohexa-1,3,5-triene. Addition would destroy this arrangement: adding Br₂ across one C=C would convert two ring carbons from sp² to sp³, breaking the continuous p-orbital overlap and converting the aromatic ring into a non-aromatic cyclohexadiene. The transition state for such an addition would have lost most of the delocalisation enthalpy already — making the activation energy prohibitively high at room temperature.
Substitution, by contrast, replaces one of the six ring hydrogens with a new group (–NO₂, –Cl, –Br, –R, –COR …) while keeping the ring intact in the final product. The π-system is briefly disrupted during the Wheland intermediate (one carbon becomes sp³), but it is restored in the final step when H⁺ leaves and aromaticity returns. The net product still has the delocalised aromatic ring — so the 152 kJ mol⁻¹ stabilisation is preserved overall.
| Reaction type | Hypothetical example | What happens to benzene ring? | Energetic cost |
|---|---|---|---|
| Addition | C₆H₆ + Br₂ → C₆H₆Br₂ | Destroys aromatic ring (does not occur at room T) | ≈ 152 kJ mol⁻¹ delocalisation enthalpy lost |
| Substitution | C₆H₆ + Br₂ → C₆H₅Br + HBr | Preserves aromatic ring (this is what occurs, with catalyst) | None — aromaticity restored in product |
This principle governs almost all benzene chemistry: electrophilic substitution is the major reaction type, not electrophilic addition. The activation energy for substitution (≈ 80 kJ mol⁻¹ with halogen carrier) is well below the addition barrier.
Electrophilic substitution has three steps that repeat across all reactions:
The electrophile (E+) is formed from the reagent, often using a catalyst to make a strong enough electrophile. Examples:
The pi-electrons of benzene attack the electrophile. A pair of electrons leaves the delocalised system and forms a new covalent C-E bond. This creates a positively charged intermediate in which the ring has partial delocalisation around the remaining five carbons. This intermediate is sometimes drawn as a Wheland intermediate or sigma-complex:
The ring is now NOT aromatic - it has lost 2 of its 6 pi electrons to the new C-E bond. This intermediate is high in energy and wants to return to the aromatic form.
The carbon now carrying E also carries an H (from the original C-H of benzene). This H is lost as H+, with both electrons in the C-H bond returning to the ring to restore the aromatic pi-system. The final product is a substituted benzene (E replaces H).
The overall result: one H on benzene replaced by E, with the ring still aromatic.
graph LR
A["Benzene C6H6<br/>aromatic"] --> B["Electrophile E+<br/>formed from catalyst"]
B --> C["Attack on ring<br/>sigma complex<br/>NOT aromatic"]
C --> D["Loss of H+<br/>to regenerate catalyst<br/>and aromatic ring"]
D --> E[Substituted benzene C6H5E]
Nitration is the introduction of a nitro group -NO2 onto the benzene ring. The electrophile is the nitronium ion, NO2+.
Step 1: Generate NO2+
Sulfuric acid is a stronger acid than nitric acid, so it protonates HNO3:
HNO3 + H2SO4 -> H2NO3+ + HSO4-
The protonated nitric acid then loses water to form the nitronium ion:
H2NO3+ -> NO2+ + H2O
Combined: 2 H2SO4 + HNO3 -> NO2+ + H3O+ + 2 HSO4-
The nitronium ion NO2+ is a linear cation (O=N+=O) and is a very strong electrophile because of the positive charge on nitrogen.
Step 2: Attack on Benzene
The benzene ring attacks NO2+:
C6H6 + NO2+ -> C6H5(H)(NO2)+ (sigma complex)
Step 3: Loss of H+
C6H5(H)(NO2)+ -> C6H5NO2 + H+
The H+ is recaptured by HSO4- to regenerate H2SO4 (acid catalyst).
C6H6 + HNO3 -> C6H5NO2 + H2O (with H2SO4 catalyst and 50 degrees C)
Product: nitrobenzene, a pale yellow oil with a characteristic almond smell.
Nitrobenzene is a key industrial intermediate. It can be reduced (using tin and HCl, or H2/Ni, or Sn/H+) to give phenylamine (aniline):
C6H5NO2 + 6 [H] -> C6H5NH2 + 2 H2O
Phenylamine is the starting material for dye manufacture (e.g. azo dyes) and for many pharmaceuticals. Nitration is also the first step in the industrial production of TNT (2,4,6-trinitromethylbenzene, an explosive).
Halogenation is the introduction of a halogen (Cl, Br, I) onto the benzene ring. The electrophile is the halogen cation X+, formed by the action of a halogen carrier (Lewis acid catalyst) such as AlCl3, FeCl3, AlBr3, or FeBr3.
Br2 and Cl2 are not electrophilic enough on their own to attack benzene (they can react with alkenes because alkenes are more reactive, but not with the stable aromatic ring). The halogen carrier polarises the X-X bond, generating a more reactive X+ species:
Cl2 + AlCl3 -> Cl+ + AlCl4- (or an ion pair Cl-AlCl4)
Br2 + FeBr3 -> Br+ + FeBr4- (or an ion pair Br-FeBr4)
The aluminium or iron acts as a Lewis acid - it accepts a lone pair from one of the halogen atoms, which effectively pulls one Cl (or Br) away with both electrons and leaves behind a bare Cl+ (or Br+).
Step 1: Generate Br+
Br2 + FeBr3 -> Br+ ... [FeBr4]-
Step 2: Attack on Benzene
C6H6 + Br+ -> sigma complex (same form as in nitration, with Br+ attached and a positive charge delocalised over the ring)
Step 3: Loss of H+
sigma complex -> C6H5Br + H+
The H+ reacts with FeBr4- to regenerate FeBr3 and produce HBr:
H+ + FeBr4- -> HBr + FeBr3
C6H6 + Br2 -> C6H5Br + HBr (with FeBr3 catalyst, room temperature)
Product: bromobenzene, a colourless liquid.
For chlorination:
C6H6 + Cl2 -> C6H5Cl + HCl (with AlCl3 or FeCl3 catalyst)
Product: chlorobenzene.
The OCR mark scheme for a nitration mechanism awards marks for: (i) the equation generating NO₂⁺ (sometimes with H₂SO₄ as a separate equation), (ii) the curly-arrow step showing π-attack on the N of NO₂⁺, (iii) the Wheland intermediate with explicit + charge, sp³ carbon bearing H and NO₂, and partial delocalisation marks (often a dashed semicircle in the ring), (iv) the curly arrow from C–H back into the ring with loss of H⁺, (v) the final nitrobenzene product, and (vi) the catalyst-regeneration equation.
| Step | Equation / description | What the arrows show |
|---|---|---|
| 1a | HNO3+H2SO4→H2NO3++HSO4− | H⁺ transferred from H₂SO₄ to HNO₃ (Brønsted acid–base; sulfuric is the stronger acid) |
| 1b | H2NO3+→NO2++H2O | Loss of water gives the linear nitronium cation (O=N⁺=O) |
| 1 (combined) | HNO3+2H2SO4→NO2++H3O++2HSO4− | Net electrophile generation |
| 2 | Benzene + NO₂⁺ → Wheland σ-complex | Two curly arrows. Arrow A: from the π-cloud (mid-ring) to the N of NO₂⁺, forming the new C–N σ-bond. Arrow B: from one N=O bond of NO₂⁺ back onto O (to accommodate electron flow as N now has four bonds). Result: sp³ C bearing H and NO₂; positive charge delocalised over the five remaining ring carbons; aromaticity temporarily lost |
| 3 | σ-complex → C₆H₅NO₂ + H⁺ | One curly arrow. From the C–H σ-bond on the sp³ carbon back into the ring (this restores the aromatic sextet); H⁺ leaves to HSO₄⁻ |
| 4 | H++HSO4−→H2SO4 | Catalyst regenerated. H₂SO₄ is not consumed overall — it functions as both a Brønsted acid (step 1) and a base (step 4) |
graph LR
A[Benzene + NO2+] --> B["Step 2: π-cloud attacks N<br/>forms C-N sigma bond<br/>Wheland intermediate"]
B --> C["Step 3: lose H+ to HSO4-<br/>aromaticity restored<br/>catalyst regenerated"]
C --> D[Nitrobenzene C6H5NO2]
Nitrobenzene is a key industrial intermediate. Reduction with tin and concentrated HCl (or alternatively H₂/Ni, or Sn/H⁺) gives phenylamine (also known by its older name aniline):
C6H5NO2+6[H]⟶C6H5NH2+2H2O
Phenylamine is the founding material of the synthetic dye industry — the diazonium chemistry developed by Perkin and Hofmann in the 1850s–1880s gave the first synthetic dyes (mauveine, fuchsin, the azo dyes) and remains central to dye and pigment manufacture. Multiple nitration of methylbenzene (toluene) under forcing conditions gives 2,4,6-trinitromethylbenzene (TNT) — an industrial explosive whose synthesis we mention here only as a textbook context (refer to standard industrial-chemistry references for the full preparation procedure rather than reproducing it here).
Halogenation is the introduction of a halogen (typically Cl or Br) onto the benzene ring. The electrophile is the halogen "cation" X⁺ — more accurately, a polarised δ⁺X–δ⁻X·MX₃ species generated by the action of a halogen-carrier catalyst (a Lewis acid).
Br₂ and Cl₂ on their own are not sufficiently electrophilic to attack benzene. They can react with alkenes (alkene π-electrons are localised and easily polarisable — Br₂ in CCl₄ adds across cyclohexene's C=C in seconds at room temperature) but not with benzene (delocalised π-electrons less easily polarised). The benzene π-cloud cannot polarise the Br–Br bond enough to generate a sufficiently electrophilic δ⁺ end. The halogen carrier provides the missing activation: a Lewis acid (FeBr₃, FeCl₃, AlBr₃, AlCl₃) accepts a lone pair from one halogen atom, withdrawing electron density and polarising the X–X bond strongly:
Br-Br+FeBr3⟶Brδ+−Brδ−⋯FeBr3
The δ⁺ Br end is now electrophilic enough to be attacked by the benzene π-cloud, and the δ⁻ Br end leaves with the catalyst as a tetrahalogenometal anion FeBr₄⁻.
Halogen-carrier catalysts are also called Lewis-acid catalysts because Lewis-acid behaviour (lone-pair acceptance) is the activating mechanism. AlCl₃ for chlorination, AlBr₃ or FeBr₃ for bromination are the standard choices in A-Level reactions; FeCl₃ is also accepted by OCR mark schemes for chlorination.
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