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. OSHA PEL 0.1 ppm TWA (8-hour); ACGIH TLV 0.1 ppm TWA with a 0.05 ppm Skin notation; NIOSH IDLH 2 ppm. 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.