Friedel–Crafts Acylation

Introduction

Friedel–Crafts acylation installs a carbonyl-containing acyl group on an aromatic ring, usually by reacting an acyl chloride with a Lewis acid such as AlCl₃. The product after work-up is an aryl ketone. This resembles Friedel–Crafts alkylation in its ring attack and rearomatisation, but the electrophile is a resonance-stabilised acylium species. It generally avoids the skeletal rearrangement and repeated ring alkylation problems of direct alkylation.

Core explanation

An acyl chloride has the form R–C(=O)–Cl. Coordination to AlCl₃ helps remove chloride and forms an acylium ion , commonly represented by the resonance contributors R–C≡O⁺ and R–C⁺=O. These drawings have the same atom connectivity and differ in electron placement; neither is a separate hydride-shifted skeleton. The carbonyl carbon is electrophilic, so the aromatic pi system bonds to that carbon. The ring enters a nonaromatic positively charged sigma complex while the attacked carbon still bears H.

A base then removes that ring H, and the C–H pair restores aromaticity. After hydrolytic work-up releases any product–Lewis-acid complex, the organic product is Ar–C(=O)–R. For benzene and acetyl chloride, CH₃COCl, this is acetophenone, C₆H₅COCH₃. The bond from the ring goes to the acyl carbonyl carbon , not to oxygen or to the methyl carbon. Chloride leaves the acyl reagent and does not appear in the ketone product.

The acylium ion is stabilised by resonance. It does not commonly undergo the 1,2-hydride or alkyl shifts characteristic of some alkyl carbocations in Friedel–Crafts alkylation. Thus an acyl chloride can transfer a carbon skeleton more faithfully. The acyl group installed on the ring is electron-withdrawing and deactivates further EAS, so repeated acylation of the same ring is generally less troublesome than polyalkylation after an alkyl group is installed. These are important synthetic advantages, not merely differences in names.

Friedel–Crafts acylation still has limitations. A strongly deactivated ring may not attack the acylium electrophile efficiently. A basic amino group can coordinate or react with AlCl₃, disrupting the intended ring substitution. Substituents already on the ring can direct the position of acylation, and mixtures may result if more than one position is available. A Lewis acid may bind strongly to the ketone product, which is why an aqueous work-up is commonly shown even when the electrophilic attack and deprotonation steps are complete.

An aryl ketone can later be reduced at its carbonyl group to an alkylbenzene. For example, benzene → acetophenone by acylation → ethylbenzene after carbonyl-to-methylene reduction. This two-step strategy can install a straight alkyl chain without passing through a rearrangeable primary alkyl electrophile in the ring substitution step. The reduction reagent must be specified for a real synthesis; acylation by itself stops at the ketone.

The aromatic reaction remains EAS: temporary loss of aromaticity is followed by proton removal. Do not confuse acylation with nucleophilic acyl substitution at an acyl chloride, where a nucleophile attacks the carbonyl and chloride departs. Here an aromatic ring acts as the nucleophile, but the distinctive ring sigma complex and rearomatisation are essential parts of the mechanism.

Step-by-step reasoning

Identify the acyl chloride RCOCl and Lewis acid. Draw the acylium ion with two resonance contributors and mark its carbonyl carbon as electrophilic. Choose the aromatic ring position using any existing substituent's directing effect. Draw ring pi attack at the acylium carbon to form a sigma complex, then remove H from that ring carbon. After work-up, draw Ar–CO–R and check that the installed carbonyl remains intact.

Visual explanation

Draw R–C(=O)–Cl beside AlCl₃, then show R–C≡O⁺ ↔ R–C⁺=O with the same carbon skeleton. A curved arrow from a benzene pi bond points to the acylium carbon. The next drawing has a ring carbon bonded to C(=O)R and still bearing H, with positive charge delocalised around the ring. The final product has an aromatic circle restored and a ketone outside the ring.

Real-world analogy

An established circle receives a new package through a coordinator. This package has a rigid labelled core that does not easily rearrange before delivery. Once attached, it makes the circle less welcoming to a second package. The package is an acyl group, its rigid core is the resonance-stabilised acylium ion, and the reduced willingness for another delivery is ring deactivation by the carbonyl.

Real-world example

Acetophenone can be prepared from benzene and acetyl chloride using AlCl₃ followed by work-up. It is both a useful aromatic ketone and a potential intermediate for making ethylbenzene after an appropriate carbonyl reduction. A synthetic chemist may prefer this sequence to direct alkylation if carbocation rearrangement or polyalkylation would complicate the target.

Why?

Why is acylation less prone to repeated ring substitution than alkylation? The installed carbonyl withdraws electron density from the aromatic ring, making formation of another positively charged sigma complex less favourable. An alkyl group usually donates electron density and makes the first product more reactive. The two installed groups therefore change the ring's next-step kinetics in opposite directions.

Common misconception

"Friedel–Crafts acylation directly gives an alkylbenzene." It installs R–C(=O)–, so the immediate product is an aryl ketone. Converting that carbonyl into a methylene and obtaining an alkylbenzene requires a separate reduction step. Do not omit the carbonyl in the acylation product.

Worked example

Question: Predict the main organic product when benzene reacts with CH₃COCl and AlCl₃ followed by aqueous work-up. Will the installed group commonly rearrange before attack?

Reasoning: AlCl₃ helps generate the resonance-stabilised CH₃CO⁺ acylium electrophile. Benzene attacks its carbonyl carbon, then deprotonates and rearomatises. Acylium resonance stabilises the original skeleton against ordinary alkyl-cation shifts.

Answer: Acetophenone, C₆H₅COCH₃, is formed; ordinary hydride or alkyl rearrangement of the acylium group is not expected.

Quick check

1. Which atom of an acylium ion forms the new bond to an aromatic ring? Answer: The electrophilic carbonyl carbon, giving a ring–C(=O)–R connection.

Exam focus

Show acylium generation, its resonance contributors, aromatic sigma-complex formation and deprotonation. Draw the ketone after work-up, not an alkylbenzene. State that acylium rearrangement and repeated acylation are usually less problematic than the corresponding issues in direct Friedel–Crafts alkylation, while checking ring deactivation and amino-group complications.

Advanced insight

An aryl ketone can complex strongly to the Lewis acid through its carbonyl oxygen, so apparent catalyst turnover may be limited until aqueous work-up breaks that association. This explains why laboratory schemes may use substantial AlCl₃ despite describing it as a Lewis-acid catalyst in the mechanistic outline. Product release is a separate practical consideration from C–C bond formation.

Summary

Friedel–Crafts acylation uses an acyl chloride and Lewis acid to generate an acylium electrophile. Aromatic attack, sigma-complex formation and deprotonation produce an aryl ketone after work-up. The resonance-stabilised acylium group usually does not undergo ordinary carbocation rearrangement, and the installed carbonyl deactivates the ring toward further substitution. A separate carbonyl reduction can later convert the ketone to an alkylbenzene.

Practice questions

1. What is the immediate organic product of benzene plus acetyl chloride/AlCl₃ after work-up? Answer: Acetophenone, an aryl ketone with structure C₆H₅COCH₃. 2. What resonance forms are commonly used for an acylium ion? Answer: R–C≡O⁺ and R–C⁺=O, with unchanged atom connectivity. 3. Why is polyacylation less common than polyalkylation? Answer: The installed carbonyl group deactivates the ring toward another electrophilic substitution. 4. What extra step is needed to obtain an alkylbenzene from an aryl ketone? Answer: Reduction of the ketone carbonyl to a methylene group, using an appropriate separate reagent system.