Phosgene, or carbonyl chloride, is a very toxic gas with a formula of COCl2.
Phosgene is an easily liquefied colorless gas with a smell that is often described as either freshly mown or stale hay.
Phosgene is not very stable. It reacts with water, forming carbon dioxide and hydrochloric acid. It is also quite reactive with various organic substances, which makes phosgene an important industrial chemical and precursor to things like plastics. Heat causes phosgene to decompose into carbon monoxide and chlorine.
Phosgene is sold in cylinders. However, given its extreme toxicity, organizations do not sell it to home chemists.
Seriously, if you want to prepare phosgene, reconsider. It is an extremely potent choking agent. Death from phosgene is slow, messy and not pretty at all.
- Do something else!
Phosgene is an extremely powerful choking agent, powerful enough to be used as a chemical weapon. The lethal dose is 0.01-0.03 g/l. There is no known antidote.