Friedel-Crafts Acylation Revision Notes | A Level AQA Chemistry

1. Why Benzene Reacts Differently

Benzene has a ring of six carbon atoms with a delocalised system of $\pi$ electrons spread equally above and below the plane of the ring. This delocalisation gives benzene exceptional stability — around 150 kJ mol$^{-1}$ more stable than a hypothetical cyclohexatriene with three isolated double bonds.

Because of this stability, benzene strongly resists reactions that would destroy the delocalised system. Alkenes undergo electrophilic addition, but benzene undergoes electrophilic substitution instead. The ring acts as an electron-rich site that attracts electrophiles, yet the driving force of the reaction is always to restore aromaticity at the end.

This preference for substitution over addition is a central AQA exam point. The thermodynamic stability gained by restoring aromaticity provides the driving force for proton loss from the Wheland intermediate. Keep that principle in mind — it underpins every mechanism you will draw for benzene chemistry.

2. Generating the Electrophile: The Role of AlCl3

Benzene's $\pi$ system is electron-rich, but it will only react with a sufficiently powerful electrophile. An acyl chloride alone (for example, ethanoyl chloride, $\text{CH}_3\text{COCl}$) is not electrophilic enough to attack the ring. This is where aluminium chloride ($\text{AlCl}_3$) comes in as a catalyst.

Aluminium chloride is a strong Lewis acid — it accepts chloride as the C–Cl bond breaks heterolytically, generating the acylium ion and AlCl4-, generating the acylium ion ($\text{RCO}^+$), which is the actual electrophile that attacks benzene.

The equation for electrophile generation is:$\text{CH}_3\text{COCl} + \text{AlCl}_3 \rightarrow \text{CH}_3\text{CO}^+ + \text{AlCl}_4^-$The $\text{AlCl}_4^-$ ion is regenerated as $\text{AlCl}_3$ at the end of the mechanism when it reacts with the proton lost from the Wheland intermediate, which is why $\text{AlCl}_3$ is described as a catalyst.

3. The Full Mechanism: Electrophilic Substitution Step by Step

With the acylium ion generated, the mechanism proceeds in two clear stages: attack by the $\pi$ system on the electrophile, followed by loss of a proton to restore aromaticity. Both stages require careful curly arrow drawing — this is where most marks are lost in exams. The first stage produces the Wheland intermediate — a positively charged, non-aromatic arenium ion where one carbon is sp$^3$ hybridised (bonded to H and the acyl group) and the positive charge is delocalised over the remaining ring carbons.

Worked example: Drawing the Friedel-Crafts Acylation Mechanism

Outline the mechanism for the reaction of benzene with ethanoyl chloride (CH3COCl) in the presence of AlCl3. Name the organic product.

1. $\text{CH}_3\text{COCl} + \text{AlCl}_3 \rightarrow \text{CH}_3\text{CO}^+ + \text{AlCl}_4^-$ — Draw a curly arrow from the C–Cl bond of ethanoyl chloride pointing towards the Al atom of AlCl3. This shows the electron pair moving to Al, breaking the C–Cl bond heterolytically and forming the acylium ion. The arrow must start from the bond, not from the Cl atom label.
2. Curly arrow: from the $\pi$ system of benzene $\rightarrow$ to the $\text{C}^+$ of $\text{CH}_3\text{CO}^+$ — The electron-rich pi system of benzene donates a pair of electrons to the electrophilic carbonyl carbon of the acylium ion. Draw the Wheland intermediate: a six-membered ring with a positive charge, where one carbon is now sp3 (bonded to H and to the acyl group). The delocalisation is broken at this stage — show this clearly.
3. Curly arrow: from the C–H bond on the $\text{sp}^3$ carbon $\rightarrow$ into the ring; $\text{AlCl}_4^-$ acts as base, accepting $\text{H}^+$ — The C–H bond breaks, the electron pair re-enters the ring to restore the delocalised pi system. AlCl4- accepts the proton, regenerating AlCl3 and producing HCl. Aromaticity is fully restored in the product.
4. $\text{C}_6\text{H}_5\text{COCH}_3$ (phenylethanone / methyl phenyl ketone) — The product is phenylethanone, also called acetophenone. It is an aryl ketone — the acyl group (CH3CO–) has replaced one hydrogen on the benzene ring. The overall equation is: $\text{C}_6\text{H}_6 + \text{CH}_3\text{COCl} \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COCH}_3 + \text{HCl}$

Answer: Organic product: phenylethanone, $\text{C}_6\text{H}_5\text{COCH}_3$. Overall equation: $\text{C}_6\text{H}_6(\text{l}) + \text{CH}_3\text{COCl}(\text{l}) \xrightarrow{\text{AlCl}_3} \text{C}_6\text{H}_5\text{COCH}_3(\text{l}) + \text{HCl}(\text{g})$

Notice that the overall reaction is a substitution: one H atom on benzene is replaced by the acyl group $\text{–COCH}_3$. Under AQA conditions, Friedel-Crafts acylation is limited to monosubstitution — only one acyl group replaces one hydrogen. The acyl group that has been introduced is electron-withdrawing, which deactivates the ring towards further electrophilic attack, making polysubstitution far less likely than in alkylation.

4. Monosubstitution and Why It Matters

A key feature of Friedel-Crafts acylation — and one that makes it synthetically superior to Friedel-Crafts alkylation — is that it reliably gives monosubstituted products. The acyl group (–COR) withdraws electron density from the ring through both inductive and mesomeric effects, making the ring less reactive towards a second electrophilic attack.

This selectivity is extremely valuable in synthesis. When chemists need to introduce exactly one substituent at a controlled position, acylation is the preferred route. The product (an aryl ketone) can then be further modified — for example, by reduction of the carbonyl group — to give a product that would have been difficult to obtain cleanly by alkylation alone.

5. Friedel-Crafts Acylation in Synthesis

Friedel-Crafts acylation is not an isolated reaction — it is a key step in multi-stage organic synthesis. The aryl ketone produced can serve as a platform for further transformations, making acylation one of the most strategically important reactions in aromatic chemistry.

Consider a synthesis route that requires an aryl alkyl group (an alkylbenzene) without the problems of polysubstitution. The approach is: first carry out Friedel-Crafts acylation to introduce the acyl group cleanly, then reduce the carbonyl group to a $\text{CH}_2$ group using a Clemmensen reduction (zinc amalgam and concentrated HCl) or a Wolff-Kishner reduction. This two-step sequence gives a monosubstituted alkylbenzene far more reliably than direct Friedel-Crafts alkylation.

• Acylation followed by reduction: converts $\text{C}_6\text{H}_5\text{COCH}_3$ to $\text{C}_6\text{H}_5\text{CH}_2\text{CH}_3$ (ethylbenzene) — a reliable route to monoalkylbenzenes
• Acylation followed by nucleophilic addition: the carbonyl group of the aryl ketone can react with reducing agents (e.g. $\text{NaBH}_4$) to give a secondary alcohol
• Acylation as a directing step: the carbonyl group directs subsequent electrophilic substitution to the meta position, allowing controlled synthesis of meta-disubstituted benzenes

6. Exam Technique and Common Mistakes

Mechanism questions on Friedel-Crafts acylation are a reliable source of marks — and a reliable source of dropped marks. The errors students make are predictable, which means they are entirely avoidable with the right preparation.

Friedel-Crafts acylation brings together three core themes of A-level organic chemistry: the unique reactivity of benzene, the generation of a reactive electrophile using a Lewis acid catalyst, and the strategic value of the reaction in synthesis. Mastering the mechanism — with correct curly arrows, an explicit Wheland intermediate, and a clear account of the catalytic cycle — is the foundation for scoring full marks on any question in this area.