Phosgene
Properties
| State | Gas (colorless with smell of freshly cut hay) |
| Color | Colorless |
| Solubility | Reacts with water (hydrolysis to HCl and CO2); soluble in organic solvents |
| Melting Point | -118°C |
| Boiling Point | 8.3°C |
About Phosgene
Phosgene is a colorless gas at room temperature (boiling point 8.3 °C, so it's barely a gas) with a smell described as freshly mown hay or new-cut grass — one of the few chemical hazards where the warning sign is a pleasant odor. John Davy first made it in 1812 by exposing CO and Cl2 to sunlight, which is where the name comes from (Greek phos = light, genes = born). Its grim place in history is World War I, where it accounted for roughly 80% of the 91,000 chemical-warfare deaths. Despite that legacy, phosgene is one of the most important industrial chemicals manufactured today: about 8.5 million tonnes per year flow through enclosed reactors, almost all of it consumed on-site so phosgene essentially never travels by truck or rail. The dominant outlet is methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) for polyurethane foam — the cushions in your couch, the insulation in your refrigerator, the soles of your running shoes. Polycarbonate (Bayer's Makrolon, SABIC's Lexan) is the second-biggest end use, made by reacting phosgene with bisphenol A in the interfacial process. The molecule is trigonal planar at carbon with one C=O and two C-Cl bonds, and its lethality comes from acylating the lysine and serine residues in alveolar surfactant proteins, which destroys the air-blood barrier integrity over 24-48 hours.
Where you'll encounter it
If you've ever used a heat gun on dichloromethane, chloroform, or any chlorinated solvent in a poorly ventilated lab, you've potentially generated trace phosgene — the thermal decomposition of CHCl3 to phosgene and HCl is the reason every undergraduate organic chemistry safety lecture warns against open flames near chlorinated waste. It's also why old-style halon fire extinguishers were phased out: hitting a hot surface with halocarbons regenerates phosgene at percent levels. Industrial chemists building MDI plants design reactors with continuous online IR monitoring at 1810 cm⁻¹ (the C=O stretch) at 0.1 ppm sensitivity, and the entire facility runs at slight negative pressure so any leak draws inward into a scrubber rather than outward. The cruelest property of phosgene exposure is the latent period — soldiers in 1917 trenches sometimes felt fine for hours, walked back to the rear, and then died of pulmonary edema overnight as the alveolar membranes finally gave way.
Common Uses
- MDI and TDI isocyanate manufacture for polyurethane foam, coatings, and elastomers
- Polycarbonate plastic production via interfacial reaction with bisphenol A
- Chloroformate and dialkyl carbonate synthesis for pharmaceutical and pesticide intermediates
- Boc and Cbz amine-protecting-group reagent synthesis in peptide chemistry
- Conversion of carboxylic acids to acyl chlorides in fine-chemical manufacturing
Safety Information
Acutely lethal inhalation hazard. Occupational exposure limits are among the lowest set for any industrial gas, and the immediately-dangerous concentration is only a small multiple above them. LC50 (rat, 4 h) is 5 ppm. GHS H330 (fatal if inhaled), H314 (causes severe skin burns), H335 (may cause respiratory irritation). Chemical Weapons Convention Schedule 3 controlled substance. Symptoms onset is delayed 6-48 hours, so any suspected exposure mandates 24-hour clinical observation regardless of how the patient looks initially. Modern industrial use requires continuous online IR or electrochemical monitoring, double-walled piping, and slight negative-pressure containment with caustic scrubbers on all vents.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.