Difficulty: Intermediate | Prerequisites: Electrophilic aromatic substitution (EAS) basics, Lewis acids, resonance
Intramolecular Friedel-Crafts acylation is an electrophilic aromatic substitution where the electrophile (an acylium ion) and the aromatic ring are part of the same molecule, so the reaction closes a ring instead of adding a new substituent. The Lewis acid catalyst (usually AlCl3) activates the acyl chloride, and the tethered aromatic ring attacks to form a cyclic ketone. Five- and six-membered rings form readily; other sizes are rare.
Friedel-Crafts acylation
An electrophilic aromatic substitution in which an acyl group (R-C=O) is introduced onto an aromatic ring, using an acyl halide (or anhydride) and a Lewis acid catalyst. In simple terms, this means sticking a carbonyl-containing group directly onto a benzene ring.
Acylium ion (R-C≡O+)
The reactive electrophile generated when a Lewis acid (e.g. AlCl3) abstracts the halide from an acyl chloride. Think of it as a positively charged carbon that is hungry for electrons from the aromatic ring.
Lewis acid catalyst
A species that accepts an electron pair. In Friedel-Crafts reactions, AlCl3 or FeCl3 coordinate with the chlorine of the acyl chloride to generate the electrophile. Think of AlCl3 as the "key" that unlocks the acyl chloride's reactivity.
Intramolecular cyclisation
A ring-forming reaction where both reacting groups belong to the same molecule. The tether (the carbon chain connecting the aromatic ring to the acyl chloride) determines the ring size of the product.
Electrophilic aromatic substitution (EAS)
The broad reaction class in which an electrophile replaces a hydrogen on an aromatic ring. Friedel-Crafts acylation is one specific example.
Acyl chloride (acid chloride, R-COCl)
A carboxylic acid derivative where the -OH has been replaced by -Cl. It is the typical starting functional group for Friedel-Crafts acylation.
AlCl3 coordinates with the Cl of the acyl chloride, generating an acylium ion (or a highly polarised complex) tethered to the aromatic ring.
The electron-rich aromatic ring attacks the electrophilic acylium carbon. Because both groups are in the same molecule, this attack closes a ring.
A proton is lost from the ring carbon that was attacked, restoring aromaticity.
The product is a cyclic aryl ketone.
The intramolecular version follows the same mechanistic steps as intermolecular Friedel-Crafts acylation. The only difference is geometry: the electrophile is already connected to the nucleophilic ring by a chain of atoms, so the reaction is inherently favoured by proximity (an "effective concentration" argument).
Five-membered rings and six-membered rings form most readily (low ring strain, favourable transition-state geometry).
Six-membered ring formation is especially common.
Seven-membered rings can form but are less favoured.
Three- and four-membered rings are essentially never formed this way (too much angle strain in the transition state).
Count the atoms in the ring being formed: include the carbonyl carbon and the two ring carbons it bridges, plus the tether carbons between them.
If the aromatic ring has existing substituents, normal EAS directing rules apply.
Electron-donating groups (EDGs) activate the ring and direct ortho/para.
Electron-withdrawing groups (EWGs) deactivate the ring and direct meta.
Strongly deactivating groups (e.g. -NO2) can prevent Friedel-Crafts reactions entirely, because the ring is too electron-poor to attack the electrophile.
When the tether geometry allows cyclisation at more than one position, the product favoured is the one that forms the most stable ring size at the less-hindered position.
Example (a): Phenyl group with gem-dimethyl tether to acyl chloride
The substrate has a phenyl ring connected through a carbon bearing two methyl groups (gem-dimethyl) to a -CH2-C(=O)Cl chain. Count the atoms in the ring that would form: the acylium carbon attacks the ortho position of the phenyl ring, closing a six-membered ring. The product is a 2-indanone derivative (specifically 3,3-dimethyl-1-indanone), a five-membered cyclic ketone fused to the benzene ring. The gem-dimethyl groups end up on C-3 of the indanone.
Example (b): Cyclohexane bearing phenyl and acyl chloride
The substrate is a cyclohexane ring with a phenyl group and a -C(=O)Cl group on adjacent or nearby carbons. The acylium ion attacks the phenyl ring intramolecularly. The tether length through the cyclohexane framework favours a six-membered ring closure, producing a tetralone-type product (a cyclohexanone fused to the aromatic ring).
Example (c): Nitro-substituted ring with acyl chloride tethered to a second phenyl
This substrate has two aromatic rings: one bearing a -NO2 group and the other unsubstituted. The acyl chloride is positioned between them. The nitro group is a strong electron-withdrawing group that deactivates its ring toward EAS. Friedel-Crafts acylation therefore occurs on the unsubstituted (more electron-rich) phenyl ring, not the nitro-bearing ring. The cyclisation closes onto the electron-rich ring to give a cyclic ketone.
Students often confuse Friedel-Crafts acylation with Friedel-Crafts alkylation. Acylation does not suffer from carbocation rearrangements (the acylium ion is resonance-stabilised and does not rearrange). Alkylation does.
Students sometimes forget that Friedel-Crafts reactions fail on strongly deactivated rings. A ring bearing -NO2, -CF3, or -C(=O)R will not undergo the reaction because it cannot serve as a competent nucleophile.
A common error is miscounting the ring size. You must count every atom that ends up in the new ring, including the carbonyl carbon and the aromatic carbons at both attachment points.
Students occasionally draw the product without losing HCl. The final product is a ketone, not a protonated intermediate. Remember to restore aromaticity by removing the proton.
⚠️ Intramolecular Friedel-Crafts acylation is a favourite exam question because it tests three things at once: EAS mechanism, ring-size analysis, and directing effects.
⚠️ If a substrate has two aromatic rings and only one can react (e.g. one is deactivated by -NO2), you must identify which ring cyclisation occurs on. This is tested frequently.
⚠️ Expect to be asked to draw the cyclic product, including correct regiochemistry. If a ring has substituents, you need to show where the new bond forms relative to those substituents.
⚠️ Real-world application: Friedel-Crafts acylation is used industrially to build aromatic ketone scaffolds found in pharmaceuticals (e.g. anti-inflammatory drugs like ibuprofen synthesis involves a related F-C step) and in dye chemistry.
True or false: Friedel-Crafts acylation can occur on a nitrobenzene ring. (False. The -NO2 group deactivates the ring too strongly.)
Fill in the blank: The electrophile in Friedel-Crafts acylation is the ______ ion. (Acylium)
True or false: Intramolecular F-C acylation preferentially forms 5- and 6-membered rings. (True.)
Fill in the blank: The Lewis acid catalyst most commonly used for F-C acylation is ______. (AlCl3)
True or false: Unlike F-C alkylation, F-C acylation products do not undergo carbocation rearrangement. (True. The acylium ion is resonance-stabilised.)
Q: A substrate contains an acyl chloride tethered to a benzene ring by a three-carbon chain. What ring size forms in an intramolecular F-C acylation, and what functional group is in the product?
A: A six-membered ring forms (three tether carbons + the carbonyl carbon + two aromatic carbons = six atoms in the new ring). The product is a cyclic aryl ketone (a tetralone).
Q: A molecule has two aromatic rings, one bearing a para-NO2 group and the other unsubstituted. An acyl chloride is tethered between them. On which ring does intramolecular F-C acylation occur, and why?
A: It occurs on the unsubstituted ring. The nitro group is a powerful electron-withdrawing group that deactivates its ring toward electrophilic aromatic substitution. F-C reactions do not proceed on strongly deactivated rings.
Q: Why does Friedel-Crafts acylation not suffer from the carbocation rearrangements that plague Friedel-Crafts alkylation?
A: The acylium ion (R-C≡O+) is stabilised by resonance between the carbon and the oxygen. This resonance stabilisation means the cation has no driving force to rearrange to a different structure, unlike simple carbocations in alkylation.
Q: Draw or describe the product when a phenyl ring is connected via a gem-dimethyl carbon to a -CH2COCl group, and the whole molecule undergoes intramolecular F-C acylation with AlCl3.
A: The product is 3,3-dimethyl-1-indanone. The acylium carbon attacks the ortho position of the phenyl ring, closing a five-membered ring. The ketone carbonyl is at C-1, and the two methyl groups sit on C-3 of the indanone framework.
This material connects directly to intermolecular Friedel-Crafts acylation (same mechanism, just without the ring closure). It also builds on the broader topic of electrophilic aromatic substitution: the same directing-effect logic (ortho/para vs. meta directors, activating vs. deactivating groups) applies here.
The ring-size preferences (5- and 6-membered rings favoured) echo Baldwin's rules for ring closure, which you will encounter more broadly in intramolecular reactions throughout organic chemistry. Understanding why -NO2 shuts down F-C reactions ties into the electronic effects of substituents, which is central to predicting reactivity across many EAS reactions.
Friedel-Crafts, acylation, intramolecular cyclisation, electrophilic aromatic substitution, EAS, acylium ion, acyl chloride, Lewis acid, AlCl3, cyclic ketone, indanone, tetralone, ring closure, directing effects, activating groups, deactivating groups, nitro group, aromatic substitution, carbocation rearrangement, Baldwin's rules