Diphosgene (Trichloromethyl chloroformate, ClCO2CCl3) is a chemical originally developed for chemical warfare, a few months after the first use of phosgene. At room temperature it is a stable colourless liquid with a vapour pressure of 10 mmHg @ 20 °C. It decomposes to phosgene around 300 °C.
The vapour is a lung irritant and has an LC100 for rabbits at a concentration of 0.9 mg/l of air.
Diphosgene was used as a poisonous gas in artillery shells by Germany during World War I. The first recorded battlefield use was in May of 1916. Diphosgene was developed because the vapours could destroy the filters in gas masks in use at the time.
Being a liquid, diphosgene is safer to handle and easier to measure than phosgene gas, which makes it a popular substitute for phosgene in the laboratory.
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