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Friedel-Crafts alkylation and acylation are the two named reactions that allow a carbon-substituent to be welded onto a benzene ring under Lewis-acid catalysis. Both were reported in 1877 (named only — no quoted words). Modern A-Level treatment focuses on the acylation route, because the resulting aryl ketone is a clean, single-product synthon for an enormous range of downstream transformations (reduction to alkyl chains, condensation, ring-closure). This lesson develops the full AlCl₃-catalysed mechanism, contrasts alkylation with acylation, and then turns to the most important conceptual extension at A-Level: how a substituent already on the ring controls where a second electrophile attacks. Activating groups (-OH, -NH₂, -OR, alkyl) direct ortho/para; strongly electron-withdrawing groups (-NO₂, -COR, -COOH, -CN, -SO₃H) direct meta; halogens are a special, deactivating-yet-ortho/para-directing case. Mastering these patterns is the gateway to retrosynthesis.
Spec mapping (AQA 7405): This lesson maps to §3.3.10 (aromatic chemistry — Friedel-Crafts reactions and the influence of substituents on further substitution). It builds directly on lesson 3 of this course (electrophilic substitution foundations — nitration, halogenation, sulfonation), is the prerequisite for lesson 5 (amines, where Friedel-Crafts acylation followed by reduction supplies the carbon skeleton), and reappears in lesson 8 (synthesis pathways and retrosynthetic analysis). Refer to the official AQA specification document for the exact wording of each section.
Assessment objectives: Recall of Friedel-Crafts conditions, reagents, and products — for both alkylation and acylation — is AO1. Writing full curly-arrow mechanisms for the AlCl₃-catalysed electrophile generation, the attack on benzene, and the regeneration of aromaticity is AO2 and is examined on Paper 2 essentially every cycle. Predicting the regiochemistry of further substitution (ortho/para vs meta) from the electronic character of an existing substituent, and rationalising why acylation is industrially and synthetically preferred to alkylation, are AO3 reasoning tasks that distinguish the top grade bands.
Friedel-Crafts alkylation introduces an alkyl group (-R) onto a benzene ring using a haloalkane and an anhydrous Lewis-acid catalyst — almost always anhydrous AlCl₃, though AlBr₃, FeCl₃, and BF₃ work analogously.
Overall equation:
C₆H₆ + R-Cl → C₆H₅-R + HCl (catalyst: anhydrous AlCl₃, reflux, anhydrous solvent)
For example, with chloromethane:
C₆H₆ + CH₃Cl → C₆H₅-CH₃ + HCl (methylbenzene)
With chloroethane:
C₆H₆ + CH₃CH₂Cl → C₆H₅-CH₂CH₃ + HCl (ethylbenzene)
The Lewis-acidic aluminium centre in AlCl₃ accepts a chloride lone pair from R-Cl, generating a tight ion pair:
R-Cl + AlCl₃ → R⁺ ··· AlCl₄⁻
In practice the electrophile is best represented as a highly polarised R-Cl-AlCl₃ complex rather than a fully free carbocation. For mechanism-drawing purposes at A-Level it is acceptable — and AQA mark schemes credit — drawing R⁺ as the attacking electrophile, with AlCl₄⁻ as the counter-ion that subsequently accepts H⁺.
The carbocation R⁺ is attacked by two of the six π-electrons of benzene. A curly arrow runs from the inside of the benzene ring to the carbocation, forming a new C-R σ-bond at one ring carbon. The carbon that has just been attacked becomes sp³ — it now bears R, H, and two adjacent sp² ring carbons. The remaining four π-electrons are delocalised over the other five ring carbons as a cyclohexadienyl cation. This positively charged, non-aromatic species is the Wheland intermediate (also called the σ-complex or arenium ion). It is conventionally drawn with a partial circle inside the ring covering five of the six carbons and a "+" inside or above the ring.
A chloride lone pair from AlCl₄⁻ removes the H atom on the sp³ ring-carbon. A curly arrow runs from the C-H bond into the ring, restoring the full six-electron π-system and the aromatic sextet. AlCl₃ is regenerated, and HCl is released as a by-product:
Wheland intermediate + AlCl₄⁻ → C₆H₅-R + HCl + AlCl₃
The catalyst AlCl₃ is regenerated and the loss of H⁺ (not loss of R⁺) is the thermodynamic driver — the energy gained by restoring aromaticity (resonance stabilisation ~150 kJ mol⁻¹ in benzene) is what makes substitution, not addition, the preferred outcome for arenes.
Alkylation has three serious limitations, which between them account for why acylation is the synthetically useful route at A-Level and beyond.
1. Polyalkylation. The product of monoalkylation (e.g. methylbenzene) is more reactive than benzene itself, because the alkyl group is electron-donating and activates the ring. The R⁺ electrophile therefore preferentially attacks the monoalkylated product, giving di-, tri-, and even hexa-alkylated mixtures. Controlling stoichiometry helps but does not eliminate the problem. Industrially, alkylation is run with a large excess of benzene to suppress polyalkylation by statistics.
2. Carbocation rearrangement. Primary and secondary carbocations rearrange to more stable carbocations via 1,2-hydride or 1,2-methyl shifts. For example, treating benzene with 1-chloropropane (CH₃CH₂CH₂Cl) and AlCl₃ yields mostly isopropylbenzene (cumene), not n-propylbenzene, because the primary n-propyl cation rearranges to the secondary isopropyl cation before attacking the ring. This makes the regiochemistry of the alkyl chain unpredictable.
3. Catalyst poisoning by water and by Lewis-basic substrates. AlCl₃ is destroyed by trace water (AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl). It also coordinates strongly to lone-pair-bearing substrates such as anilines (-NH₂), making alkylation of anilines via the direct Friedel-Crafts route impossible without first protecting the amine.
Friedel-Crafts acylation introduces an acyl group (-COR) onto a benzene ring using an acyl chloride (RCOCl) — or, less commonly, an acid anhydride (RCO)₂O — with anhydrous AlCl₃ in a dry, inert solvent such as dichloromethane or nitrobenzene.
Overall equation:
C₆H₆ + RCOCl → C₆H₅-COR + HCl (catalyst: anhydrous AlCl₃, reflux, anhydrous solvent)
For example, with ethanoyl chloride:
C₆H₆ + CH₃COCl → C₆H₅-COCH₃ + HCl (phenylethanone, an aryl methyl ketone)
With benzoyl chloride:
C₆H₆ + C₆H₅COCl → C₆H₅-CO-C₆H₅ + HCl (diphenylmethanone, benzophenone)
The product is an aryl ketone (ArCOR). The carbonyl group is electron-withdrawing — it deactivates the ring towards a second electrophilic attack, which is the single most important reason acylation is preferred over alkylation: the reaction stops cleanly at the mono-substituted product.
AlCl₃ accepts a chloride lone pair from RCOCl, generating an acylium ion (R-C≡O⁺) and AlCl₄⁻:
R-CO-Cl + AlCl₃ → R-C≡O⁺ + AlCl₄⁻
The acylium ion is genuinely a discrete, isolable cation. Its stability comes from resonance: two resonance structures (R-C⁺=O ↔ R-C≡O⁺) place positive charge on either carbon or oxygen, with the right-hand structure dominant because it places the positive charge on the more electronegative atom while preserving the carbon octet via a triple bond. The triple-bond structure makes the carbon sp-hybridised; the C-O linkage is short (~115 pm, close to the C≡O distance in carbon monoxide). Crucially, the acylium ion does not rearrange — there is no shorter, more-stable cation to rearrange to. This eliminates the rearrangement problem that plagues alkylation.
As in alkylation, two of the six π-electrons of benzene attack the electrophilic carbon of the acylium ion. A new C-C σ-bond forms, the attacked ring carbon goes sp³, and the remaining four π-electrons delocalise over the other five carbons as a cyclohexadienyl cation. The Wheland intermediate now bears a -COR group on the sp³ carbon and a delocalised positive charge over the rest of the ring.
AlCl₄⁻ removes the H atom from the sp³ ring carbon. The C-H bond electrons fold back into the ring, restoring the aromatic π-system. AlCl₃ and HCl are released — but with one extra wrinkle for acylation that does not arise in alkylation.
The aryl ketone product contains a Lewis-basic carbonyl oxygen lone pair, which coordinates strongly to AlCl₃. In effect, the AlCl₃ that started as catalyst ends bound stoichiometrically to the product as an Ar-CO(AlCl₃)R adduct. This means acylation requires more than catalytic AlCl₃ — typically slightly over one molar equivalent. The AlCl₃ is liberated at the end of the reaction by an aqueous-acid work-up: the reaction mixture is poured onto dilute HCl(aq) or ice, which hydrolyses the AlCl₃-product complex and releases the free aryl ketone, with the Al going into solution as Al³⁺(aq) or hydrolysed to Al(OH)₃ and dissolved in the acid.
For alkylation, the alkyl product has no carbonyl and does not sequester AlCl₃, so the catalyst is genuinely catalytic in alkylation but is stoichiometric in acylation. This stoichiometric cost is the principal economic drawback of acylation and a target for modern green-catalyst research (see Going Further).
Three independent factors make acylation the preferred synthetic method:
1. No rearrangement. The acylium ion R-C≡O⁺ is the most stable cation accessible from the acyl chloride. There is no lower-energy isomer it could rearrange to. The acyl group is therefore installed with its carbon skeleton exactly as drawn — no chain isomerisation, no ring contraction, no methyl migration.
2. Mono-acylation. The aryl ketone product is deactivated relative to benzene (the -COR group withdraws π-electron density via resonance and σ-electron density via induction). The acylium electrophile preferentially attacks the more electron-rich starting benzene rather than the less electron-rich ketone product. Statistically, this means the reaction stops at one acylation: di- and poly-acylation are negligible without forcing conditions. This is the practical opposite of alkylation, where the alkyl product is more reactive than benzene and polyalkylation dominates.
3. Reduction to clean alkyl chains. The aryl ketone product can be reduced — the carbonyl C=O converted to -CH₂- — by two named, A-Level-signposted methods. The Clemmensen reduction (zinc amalgam, concentrated HCl, reflux) and the Wolff-Kishner reduction (hydrazine, KOH, high-boiling solvent, reflux) both convert ArCOR to ArCH₂R. Combining Friedel-Crafts acylation with one of these reductions therefore allows clean installation of a linear, unbranched alkyl chain — exactly the chain that direct alkylation cannot deliver because of rearrangement. The textbook synthesis of n-propylbenzene is therefore: benzene + propanoyl chloride + AlCl₃ → 1-phenylpropan-1-one → (Clemmensen) → n-propylbenzene. This two-step route is cleaner than the one-step alkylation that gives mostly cumene.
Practical-skills box (anhydrous conditions). Friedel-Crafts reactions are exceptionally moisture-sensitive. AlCl₃ reacts vigorously with water: AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl. A trace of water destroys the catalyst, releases HCl fumes, and stops the reaction. In practical use: dry the glassware in an oven at 120 °C overnight, run the reaction under a CaCl₂ guard-tube or under dry N₂, use freshly distilled DCM or nitrobenzene as solvent, store AlCl₃ in a sealed bottle (it fumes in moist air), and dispense it quickly. The reaction is typically run at 0 °C to reflux of the chosen solvent. The aqueous-acid quench at the end is poured carefully onto crushed ice — adding water directly to the AlCl₃-rich reaction mixture is exothermic and ejects HCl fumes.
Once a benzene ring carries a substituent, a second electrophilic attack does not happen at random. The substituent controls both the rate (activating vs deactivating) and the position (ortho/para vs meta) of the second attack. The mechanism behind both effects is the same: the substituent alters the electron density distribution in the Wheland intermediate, stabilising or destabilising attack at particular positions. A substituent that stabilises positive charge at the ortho and para positions of the Wheland intermediate is an ortho/para director; one that destabilises those positions (or selectively stabilises meta attack) is a meta director.
Groups with a lone pair on the atom directly attached to the ring (-OH, -NH₂, -OR, -NR₂) donate π-electron density into the ring via resonance. In the Wheland intermediate for ortho or para attack, one of the three resonance structures places the positive charge on the ring carbon bearing the substituent. The substituent's lone pair can then donate into that carbon, giving a fourth, particularly stabilising resonance structure in which the substituent atom carries the positive charge and is double-bonded to the ring (e.g. H-O⁺=C, with the lone pair fully delocalised). No equivalent resonance form exists for meta attack — in the meta Wheland, the positive charge never resides on the substituent-bearing carbon, so the lone pair cannot stabilise. Ortho and para Wheland intermediates are therefore lower-energy than meta, and attack proceeds faster and predominantly at those positions.
Alkyl groups (-CH₃, -CH₂CH₃) also activate ortho/para, but via σ-donation (hyperconjugation and weak inductive donation from the C-H σ-bonds) rather than π-resonance. The effect is smaller in magnitude but operates by the same logic: any electron donation that stabilises positive charge at the ipso, ortho, or para positions accelerates attack there. -OH and -NH₂ are strongly activating; -OR is moderately activating; alkyl is weakly activating.
Worked example — nitration of methylbenzene (toluene). Treating C₆H₅CH₃ with concentrated HNO₃/H₂SO₄ at 30 °C yields a mixture of approximately 60% 2-nitromethylbenzene (ortho), 37% 4-nitromethylbenzene (para), and only ~3% 3-nitromethylbenzene (meta). The para:ortho ratio is below the statistical 1:2 (two ortho positions vs one para) because of mild steric hindrance from the methyl group; with bulkier alkyl groups (e.g. tert-butylbenzene) the para product dominates.
Halogens are the textbook exception: they deactivate the ring overall (electrophilic substitution of chlorobenzene is ~30 times slower than benzene with HNO₃/H₂SO₄) but still direct ortho/para. The explanation is that two opposing electronic effects operate. The σ-inductive effect: halogens are more electronegative than carbon and withdraw σ-electron density from the ring (deactivating, by destabilising any Wheland intermediate). The π-resonance effect: each halogen has lone pairs that can donate into the ring (activating, ortho/para-directing in the same way as -OH or -OR). Induction dominates over resonance in determining the rate (halobenzenes react slowly), but resonance dominates over induction in determining the position (when reaction does occur, ortho/para attack is favoured because only those positions can be stabilised by halogen lone-pair donation). Halogens are the only common substituents where rate and regiochemistry point in opposite directions.
Worked example — nitration of bromobenzene. Treating C₆H₅Br with HNO₃/H₂SO₄ yields a mixture of approximately 37% 2-bromonitrobenzene (ortho) and 62% 4-bromonitrobenzene (para), with traces of meta. The para preference here is more pronounced than in toluene because the bromine atom is larger and more sterically demanding at the ortho position.
Groups with a π-bond between the ring-attached atom and a more electronegative atom (-NO₂, -COR, -COOH, -SO₃H, -CN) withdraw π-electron density from the ring by resonance. They also withdraw σ-electron density inductively. Both effects deactivate the ring overall (electrophilic substitution of nitrobenzene is ~10⁷ times slower than benzene). Crucially, the resonance withdrawal places partial positive charges at the ortho and para positions. In the Wheland intermediate for ortho or para attack, one resonance structure places the full positive charge on the carbon bearing the substituent — and that carbon already carries a δ⁺. Putting two positive charges on adjacent or coincident atoms is profoundly destabilising; this resonance structure is essentially excluded. Meta attack does not produce a Wheland resonance structure with positive charge on the substituent-bearing carbon, so meta is the least destabilised (least bad) option. The reaction therefore proceeds at meta, despite all positions being deactivated.
Worked example — nitration of nitrobenzene. Treating C₆H₅NO₂ with fuming HNO₃ and concentrated H₂SO₄ at 90 °C yields approximately 93% 1,3-dinitrobenzene (meta), 6% 1,2-dinitrobenzene (ortho), and 1% 1,4-dinitrobenzene (para). The meta selectivity is essentially complete at the precision typically reported in A-Level data.
| Substituent | Effect on rate | Director | Mechanism |
|---|---|---|---|
| -NH₂, -OH | Strongly activating | ortho/para | Lone-pair π-donation |
| -OR, -NHR | Moderately activating | ortho/para | Lone-pair π-donation |
| -R (alkyl) | Weakly activating | ortho/para | σ-donation (hyperconjugation, induction) |
| -F, -Cl, -Br, -I | Weakly deactivating | ortho/para | σ-withdraw > π-donation |
| -NO₂, -COR, -COOH, -SO₃H, -CN | Strongly deactivating | meta | π and σ withdrawal |
| -NR₃⁺, -CF₃ | Strongly deactivating | meta | σ-withdrawal (no resonance for -NR₃⁺) |
For the synthesis of 4-nitrobenzoic acid from benzene, you would acylate first (-COR is a meta director that becomes -COOH after oxidation, but starting from benzene the order matters for the para target): in practice the route is methylbenzene → para-nitromethylbenzene (nitration of the activated ring, mostly para over ortho) → 4-nitrobenzoic acid (KMnO₄ oxidation of the methyl). Choosing the order in which substituents are installed is the central retrosynthetic decision in aromatic chemistry — and it is examined directly in lesson 8.
The Friedel-Crafts toolkit and substituent-effect rules connect aromatic chemistry to almost every other module of A-Level Chemistry. The links below are the highest-traffic ones in past papers and in retrosynthesis problems.
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