The reaction of an organic substrate with phosgene is called phosgenation. Phosgenation of diols give carbonates (R = H, alkyl, aryl), which can be either linear or cyclic:
An example is the reaction of phosgene with bisphenol A to form polycarbonates. Phosgenation of diamines gives di-isocyanates, like toluene diisocyanate (TDI), methylene diphenyl diisocyanate (MDI), hexamethylene diisocyanate (HDI), and isophorone diisocyanate (IPDI). In these conversions, phosgene is used in excess to increase yield and minimize side reactions. The phosgene excess is separated during the work-up of resulting end products and recycled into the process, with any remaining phosgene decomposed in water using activated carbon as the catalyst. Diisocyanates are precursors to polyurethanes. More than 90% of the phosgene is used in these processes, with the biggest production units located in the United States (Texas and Louisiana), Germany, Shanghai, Japan, and South Korea. The most important producers are Dow Chemical, Covestro, and BASF. Phosgene is also used to produce monoisocyanates, used as pesticide precursors (e.g. methyl isocyanate (MIC).
Aside from the widely used reactions described above, phosgene is also used to produce acyl chlorides from carboxylic acids:
For this application, thionyl chloride is commonly used instead of phosgene.