Diphosgene

Diphosgene is a chemical compound with the formula ClCO2CCl3. This colorless liquid is a valuable reagent in the synthesis of organic compounds. Diphosgene is related to phosgene but is more conveniently handled because it is a liquid, whereas phosgene is a gas.

Production and uses
Diphosgene is prepared by radical chlorination of methyl chloroformate under UV light.

Diphosgene converts to phosgene upon heating or upon catalysis with charcoal. It is thus useful for reactions traditionally relying on phosgene. For example, it convert amines into isocyanates, secondary amines into carbamoyl chlorides, carboxylic acids into acid chlorides, and formamides into isocyanides. Diphosgene serves as a source of two equivalents of phosgene:
 * 2 RNH2 +  ClCO2CCl3  &rarr;  2 RNCO  +  4 HCl

With &alpha;-amino acids diphosgene gives the acid chloride-isocyanates, OCNCHRCOCl, or N-carboxy-amino acid anhydrides depending on the conditions.

It hydrolyzes to release HCl in humid air.

Diphosgene has supplanted phosgene in some large scale industrial reactions such as the production of (di-)isocyanates from of amines because it is safer to handle than phosgene.

Role in warfare
Diphosgene was originally developed for chemical warfare, a few months after the first use of phosgene. It was used as a poison 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 vapors could destroy the filters in gas masks in use at the time.

Safety
Diphosgene has a relatively high vapor pressure of 10 mmHg (1.3 kPa) at 20 °C and decomposes to phosgene around 300 °C. Exposure to diphosgene is similar in hazard to phosgene and the MSDS should be consulted.