Phosgene is a colorless gas with the chemical formula COCl₂, consisting of a carbon atom double-bonded to oxygen and single-bonded to two chlorine atoms. First synthesized in 1812 by British chemist John Davy through the reaction of carbon monoxide and chlorine in sunlight, phosgene gets its name from the Greek words for “light” and “born,” reflecting its photochemical origins. Notorious as a chemical weapon responsible for 85% of gas fatalities in World War I, this insidiously toxic compound smells deceptively pleasant – like freshly cut hay or grass – at concentrations already dangerous to human health. Despite its dark history, phosgene remains one of the most important industrial chemicals, essential for manufacturing plastics, pesticides, and pharmaceuticals, with over a million tons produced annually under strict safety controls that reflect hard-learned lessons from its deadly past.

Find a review of the 50 most important industrial gases here.

20 Interesting Facts About Phosgene

Beyond the basics above, what else should we know about Phosgene? Check out the 20 interesting facts below!

1. Phosgene is 3.4 times heavier than air, flowing invisibly along the ground and pooling in trenches during WWI attacks. The gas has a delayed effect – soldiers exposed to lethal doses felt fine for 24 hours before drowning in their own fluids. The compound hydrolyzes slowly in lungs to HCl and CO₂, causing delayed pulmonary edema up to 48 hours later.
2. Welders can accidentally create phosgene when UV light from their torches hits chlorinated solvents like trichloroethylene, leading to “welder’s flu” symptoms that mask potentially fatal exposure.
3. Burning chlorinated plastics like PVC produces phosgene, making house fires more deadly than the flames themselves.
4. The molecule is perfectly planar with all four atoms in one plane, rotating 10¹² times per second.
5. Industrial phosgene production uses activated carbon catalysts at 150°C, converting CO and Cl₂ with 99% efficiency.
6. Workers in phosgene plants wear paper badges containing a yellow dye that turns orange at 0.02 ppm exposure – 50 times below the danger threshold – providing early warning of leaks.
7. Fire investigators test for phosgene using detector tubes that turn from yellow to red at just 0.1 ppm exposure.
8. Phosgene’s C=O bond vibrates at 1,827 cm⁻¹, creating a unique infrared signature for remote leak detection.
9. The Bhopal disaster involved methyl isocyanate made from phosgene, killing thousands in history’s worst chemical accident.
10. UV light decomposes phosgene back into CO and Cl₂, which is why it’s stored in amber bottles away from sunlight.
11. The gas liquefies at 8.3°C (47°F), meaning it can be liquid on cool days but gas at room temperature.
12. Dupont produces 500,000 tons annually for polycarbonate plastics used in bulletproof glass and compact discs.
13. Phosgene detectors use strips of paper soaked in base and phenolphthalein that turn pink from liberated HCl.
14. The compound reacts violently with water but slowly with moisture, allowing it to penetrate deep into lungs.
15. Phosgene’s molecular weight of 98.92 g/mol makes it exactly 100 times heavier than a hydrogen atom, a coincidence used in early chemistry teaching to demonstrate molecular calculations.
16. Modern plants use “no phosgene” routes when possible, reacting CO₂ directly instead of the toxic intermediate.
17. The pleasant hay smell comes from trace impurities – pure phosgene is nearly odorless at dangerous concentrations.
18. Emergency treatment requires immediate positive-pressure oxygen; conventional respirators offer no protection once exposed.
19. Phosgene forms when refrigerants like Freon contact hot surfaces, creating toxic fumes from car air conditioners.
20. The molecule’s 112° Cl-C-Cl bond angle results from oxygen’s double bond repelling the chlorine atoms.

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