Intramolecular Friedel-Crafts Reactions
When the alkylating or acylating group is tethered to the same molecule as the aromatic ring it will react with, the reaction closes a new ring instead of joining two separate molecules — and the same 5-/6-membered-ring preference seen throughout this course decides which ring size actually forms.
Predicting the Ring Size from an Intramolecular Friedel-Crafts Alkylation is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Intramolecular Friedel-Crafts alkylation favours a 6-membered ring, sometimes via a hydride shift
A phenyl group tethered through a four-carbon chain to a leaving group (Ph-CH₂CH₂CH₂CH₂-Cl) can ionise with AlCl₃ and cyclise onto its own ring: counting the new ring formed by connecting the terminal carbon to the ortho position of the phenyl ring gives a 6-membered ring fused to the benzene ring — the tetralin (1,2,3,4-tetrahydronaphthalene) skeleton, favoured for the same ring-strain-plus-entropy reasons that favour 6-membered rings throughout organic chemistry. Because the initially-formed cation from a primary chloride is unstable, a 1,2-hydride shift typically occurs during ionisation/cyclisation to generate a more stable secondary carbocation before ring closure, which is why the actual major product is a METHYL-substituted tetralin (the hydride shift leaves a methyl branch) rather than plain, unbranched tetralin. Read an intramolecular Friedel-Crafts alkylation by first counting the ring size the tether would form (favouring 5 or 6), then checking whether the initial carbocation is stable enough to cyclise directly or would first rearrange via a hydride/alkyl shift.
Using carbonyl and acid tests to confirm which intramolecular Friedel-Crafts reaction occurred
A product that gives a bright orange-red precipitate with 2,4-dinitrophenylhydrazine (2,4-DNP, the classic carbonyl-specific test, positive for both aldehydes and ketones) but does NOT react with saturated NaHCO₃ (which would only fizz/react with a carboxylic acid, releasing CO₂) tells you two things at once: a carbonyl group (aldehyde or ketone) IS present, and a carboxylic acid group is NOT present. When the starting material is set up for an intramolecular Friedel-Crafts-type cyclisation (an acid chloride or similar acyl group tethered to an aromatic ring), this pattern of results is exactly what you would expect from a successful intramolecular Friedel-Crafts ACYLATION: the acyl chloride cyclises onto its own ring to form a cyclic ketone (an indanone- or tetralone-type product, again favouring a 5- or 6-membered new ring), consuming the acid-chloride/acid-equivalent functionality entirely and leaving only the new ring ketone behind — consistent with a carbonyl-positive, acid-negative result.
OlympIQ exam lens
- ✓Intramolecular Friedel-Crafts alkylation of a tethered aryl halide favours forming a 6-membered ring (the tetralin skeleton)
- ✓A primary carbocation formed during cyclisation often undergoes a hydride shift to a more stable secondary cation before ring closure, giving a methyl-branched product
- ✓Positive 2,4-DNP test + negative NaHCO₃ test together confirm a carbonyl group is present and a carboxylic acid is NOT
- ✓This carbonyl-positive, acid-negative pattern is consistent with a successful intramolecular Friedel-Crafts ACYLATION forming a cyclic ketone
Do not assume the carbocation from a tethered primary alkyl halide cyclises directly without rearranging. A primary cation is too unstable to survive — expect a hydride (or alkyl) shift to a more stable secondary cation first, which is exactly why the product often carries an extra methyl branch rather than being the plain, unbranched ring you might first draw.
Ready to try the pattern?
100% coverage across the 2 linked questions.
Practise 2 linked questions