Chemistry
Mechanistic Insights into Friedel–Crafts Acylation via Carbocation Intermediates
Quick fact
Friedel–Crafts acylation produces a ketone product that is less reactive towards further acylation, so the reaction typically stops after a single acyl group is added, unlike alkylation which can over-substitute.
Why this is interesting
You've probably seen benzene as a stable, unreactive ring. Yet chemists can attach an acyl group to it with surprising ease—how does that happen without disturbing the aromaticity?
Read the full explanation
Understanding Mechanistic Insights into Friedel–Crafts Acylation via Carbocation Intermediates
Friedel–Crafts acylation is a reaction that adds an acyl group (R-C=O) to an aromatic ring. The classic procedure uses an acid chloride (R-COCl) and a Lewis acid catalyst like AlCl₃. The Lewis acid coordinates to the chlorine, making the carbonyl carbon even more electrophilic. The acid chloride then loses a chloride ion, forming an acylium ion (R-C≡O⁺), which is a resonance-stabilized carbocation. This acylium ion is the actual electrophile that attacks the aromatic ring. The ring donates two of its π electrons to form a new C-C bond, creating a sigma complex (arenium ion). A base (often the AlCl₄⁻ complex) then removes a proton from the ring, restoring aromaticity and yielding the acylated product. This mechanism is a classic example of electrophilic aromatic substitution.
A deeper explanation
The key to understanding Friedel–Crafts acylation is the acylium ion. This cation is stabilized by resonance: the positive charge is delocalized onto the oxygen atom (R-C≡O⁺ ↔ R-C⁺=O). This stability is much greater than that of a typical alkyl carbocation, and it prevents rearrangements. In Friedel–Crafts alkylation, the carbocation formed can undergo hydride or alkyl shifts to a more stable form, leading to rearranged products. Acylium ions, however, do not rearrange, so acylation gives a single product without skeletal changes. Additionally, the acyl group is electron-withdrawing, so after the first acylation, the ring becomes deactivated towards further electrophilic attack, preventing polyacylation (a common problem in alkylation). This mechanistic insight is crucial for predicting the outcome of acylation reactions and for designing synthetic routes that require reliable introduction of acyl groups.