Safe handling of diphosgene, triphosgene

Suresh B. Damle
Discovery Research, PPG Industries Inc., Monroeville, Pa.

Chemical & Engineering News (8 Feb 1993) Vol. 71, No. 6, pp. 4.

This is in response to the letters from Mark D. Hollingsworth and Kenneth J. TerBeek (C&EN, July 13, 1992, page 4) regarding the triphosgene warning.

Diphosgene (trichloromethylchloroformate) is a liquid boiling at 128-130 C, and triphosgene [bis(trichloromethyl) carbonate], is a solid (melting point, 80 C; boiling point, 206 C). Both have been introduced as convenient-to-handle, safer substitutes for phosgene.

Both diphosgene and triphosgene can be used as a substitute for phosgene in all the reactions such as the formation of chloroformates, carbonates, ureas, and isocyanates and for chlorination, carboxylation, and dehydration. In fact, in all these reactions, one mole of diphosgene behaves like two moles of phosgene, and one mole of triphosgene behaves like three moles of phosgene. Unfortunately, the chemists who developed the use of these reagents and the suppliers of these reagents have referred to them in various publications as safer and more convenient to handle than highly toxic phosgene gas in various publications. A publication on triphosgene refers to it as an example of the "defusing" of dangerous and risky chemicals.

The toxicity of both diphosgene and triphosgene is exactly the same as phosgene since both decompose to phosgene on heating and upon reaction with any nucleophile. Even a trace of moisture leads to formation of phosgene. We have observed with an open ampule of diphosgene that one can detect phosgene in vapor space with a phosgene badge. In the case of triphosgene, either obtained commercially or freshly prepared from our laboratory, we still found phosgene in the vapor space upon opening the bottle, even when the compound had been stored in a nitrogen blanket in a tightly sealed bottle in a refrigerator.

One may argue that the phosgene formation is either due to self-decomposition or due to reaction with trace amounts of moisture in the air. Therefore, both diphosgene and triphosgene should be handled with exactly the same precautions taken with phosgene. Of course, diphosgene and triphosgene offer the user the convenience of weighing an exact amount to be used in a reaction. And the compounds offer some safety owing to their lower vapor pressures being in liquid and solid states. These should be used as added safety only.

Both of these reagents can be safely handled if one takes all the precautions as for phosgene. Anything less may result in a dangerous situation.

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