Phosgene, from pyrolysis

Chlorobenzenes are stable compounds and decompose slowly only under excess heating at high temperatures to release some HCl gas and traces of phosgene. It is possible, under certain limited conditions of incomplete combustion or pyrolysis, to form polychlorinated dibenzo-/)-dioxins (PCDDs) and dibenzofurans (PCDFs) from chlorobenzenes (Cm OROCARBONS and cm OROHYDROCARBONS, toxic aromatics). 

Pyrolysis of amidino dichlorides (from urea and phosgene) to carbodiimide [67],... 

Preparation of cyanides.1 An attractive route to cyanides involves as the first step dehydration of alkylformamides with phosgene-tricthylamine (1,857) or phosphoryl chloride-diisopropylamine (13,249) to form isocyanidcs followed by isocyanidc-cyanide rearrangement. This rearrangement traditionally was conducted in the gas phase, but proceeds in almost quantitative yield when carried out by flash pyrolysis at 600°. This route to cyanides is attractive because allyl isocyanides rearrange without allylic rearrangement. Moreover, optically active carboxylic acids can be obtained from optically active amines without raccmization. 

For larger amounts we recommend the gas-phase pyrolysis of 4,5-dicyano-l,3-dithiol-2-one (8) [Eq. (3)], which is readily available from sodium cyanide, carbon disulfide, and phosgene [16]. Both the starting materials and most of the pyrolysis products are extremely toxic, and the price to be paid for large-scale production is hence high. 

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