Benzoyl Chloride
Properties
| State | Liquid (colorless to slightly yellow with pungent, irritating odor) |
| Color | Colorless to pale yellow |
| Solubility | Reacts with water (hydrolysis to benzoic acid and HCl); miscible with ether, chloroform, benzene |
| Melting Point | -1°C |
| Boiling Point | 197°C |
About Benzoyl Chloride
Benzoyl chloride is the workhorse aromatic acyl chloride of organic synthesis — formula C6H5COCl, molar mass 140.567 g/mol — and the standard reagent whenever you need to install a benzoyl (PhCO-) group onto an amine, alcohol, or other nucleophile. The reactivity comes from the carbonyl carbon's electrophilicity (the chloride leaving group plus the aromatic ring stabilizing the acylium-like transition state), and the aromatic ring is what slows hydrolysis enough to make the Schotten-Baumann procedure practical: stir the substrate with aqueous NaOH, add benzoyl chloride dropwise, and the amine or alcohol gets benzoylated faster than the reagent gets hydrolyzed. That selectivity is the whole point of the Hinsberg-style classification of amines and the basis for using benzoylation as a hydroxyl protecting group in carbohydrate and nucleoside chemistry. The same compound is the upstream precursor for benzoyl peroxide (BPO), the radical initiator used to start polystyrene and polyacrylate polymerizations and the active ingredient in 2.5-10% acne creams. Benzoyl chloride also smells like trouble — it's a vicious lachrymator, opening the bottle in open air will have a whole bench tearing up. Industrially it's made from benzaldehyde plus Cl2, or from benzoic acid plus SOCl2.
Where you'll encounter it
If you've ever protected a sugar hydroxyl with a benzoate group during a multi-step total synthesis, you reached for benzoyl chloride and pyridine in dichloromethane — it's standard operating procedure in carbohydrate chemistry because the benzoate group is robust under most conditions but cleaves cleanly with sodium methoxide in methanol. In a process-chemistry plant, benzoyl chloride is the feedstock for benzoyl peroxide manufacture: a tank of NaOH, a stream of H2O2, and a controlled addition of benzoyl chloride produces the BPO crystals that go into your acne wash and your polystyrene reactor.
Common Uses
- Benzoylation reagent for amines, alcohols, and amino acid side chains in synthesis
- Industrial precursor to benzoyl peroxide (BPO) for polymerization and acne treatments
- Schotten-Baumann reagent and Hinsberg amine-classification test
- Benzoate hydroxyl protecting group in carbohydrate and nucleoside chemistry
- Intermediate for benzophenone, benzyl benzoate, and pharmaceutical building blocks
Safety Information
GHS: Skin Corrosion 1B, Eye Damage 1, Acute Tox. 4 (inhalation). Powerful lachrymator — vapor causes severe tearing within seconds at low ppm levels. ACGIH ceiling TLV is 0.5 ppm (no time average — never exceed). Reacts violently with water, alcohols, and amines, releasing HCl and significant heat. Always handle in a fume hood with face shield, nitrile/butyl gloves, and a lab coat; quench spills with dry sand, never water. Store under nitrogen if long-term, and keep away from DMSO (exothermic decomposition).
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.