Friedel-Crafts acylation: benzene + acetyl chloride → acetophenone
Ready to scaleElectrophilic aromatic substitution
Textbook electrophilic aromatic substitution via the AlCl3-generated acylium ion. The product ketone (acetophenone) deactivates the ring toward further acylation, so unlike F-C alkylation, monoacylation is clean — no polyacylation problem. AlCl3 is used at 1.1 equiv (not catalytic) because it complexes the product 1:1; aqueous acidic workup decomposes the complex to free the ketone.
Conditions
Suspend AlCl3 (1.1 equiv) in dry CH2Cl2 (~3 mL per mmol acetyl chloride) under N2. Cool to 0 °C; add acetyl chloride (1.0 equiv, slowly via syringe — exotherm!). The mixture turns yellow-brown as the AlCl3-AcCl complex forms. Add benzene (1.05 equiv, slight excess) dropwise over 10 min keeping T < 5 °C. Warm to RT; stir 1–4 h until HCl evolution ceases.
Workup
Cautiously pour onto crushed ice; separate organic; wash with sat. NaHCO3 (decomposes residual AlCl3 complex), water, brine; dry MgSO4; concentrate. Distil at reduced pressure (bp 202 °C at 760 mmHg, or ~80 °C at 10 mmHg). Pure acetophenone is a colourless liquid with a sweet almond-like odour.
| Compound | Role | Equiv | MW (g/mol) | ρ (g/mL) | Safety |
|---|---|---|---|---|---|
| Acetyl chloride75-36-5 · C2H3ClOAcetyl chloride — limiting reagent. Moisture-sensitive (hydrolyses to acetic acid + HCl); transfer by syringe under N2. Stench is characteristic — work in a hood. | reactant | 1.0 | 78.50 | 1.110 | DangerSDS |
| Benzene71-43-2 · C6H6Benzene — slight excess; suppresses polyacylation (already kinetically disfavoured by ring deactivation, but excess locks this in). Carcinogen — strict hood discipline. | reactant | 1.05 | 78.11 | 0.880 | DangerSDS |
| Aluminium chloride7446-70-0 · AlCl3Aluminum chloride (anhydrous) — Lewis acid. Listed at 1.1 equiv rather than catalytic because the ketone product complexes the AlCl3 1:1; stoichiometric load is needed to keep free AlCl3 available for the next turnover. Aqueous workup decomposes the complex. | reagent | 1.1 | 133.33 | — | Danger |
| Dichloromethane75-09-2 · CH2Cl2Dichloromethane — anhydrous. Chlorobenzene (108-90-7) and carbon disulfide (also classic) are alternatives but DCM is the modern bench standard. Excludes water — AlCl3 reacts violently with H2O. | solvent | — | 84.93 | 1.330 | WarningSDS |
| Acetophenone98-86-2 · C8H8OAcetophenone — colourless liquid, bp 202 °C; sweet almond/hawthorn-like odour. Used industrially as a fragrance ingredient and pharmaceutical intermediate (e.g. ephedrine synthesis). | product | — | 120.15 | 1.030 | WarningSDS |