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18.2.4.1.1 Method 1: Carbonylation of Primary Amines with Phosgene

DOI: 10.1055/sos-SD-018-00070

Braverman, S.; Cherkinsky, M.; Birsa, M. L.Science of Synthesis, (20051884.

The direct treatment of primary amines or their salts with phosgene is the oldest[‌223‌] and most important method for the preparation of isocyanates. The primary product of the reaction is the carbamoyl chloride 22, which on heating loses a molecule of hydrogen chloride to give the isocyanate 23 (Scheme 22). The byproduct, the amine hydrochloride, reacts at elevated temperature with excess phosgene and produces the isocyanate as the sole reaction product. Usually, a solution of the amine in an inert solvent is added under stirring to excess phosgene in the same solvent at low temperature, and the resulting slurry of carbamoyl chloride and amine hydrochloride is then heated with excess phosgene above 100°C, usually at the reflux temperature of the solvent, until a clear solution is formed.[‌224‌] The use of solvents is necessary because of the formation of a mixture of solid carbamoyl chloride and amine hydrochloride. The common solvents used for the phosgenation include benzene, toluene, xylene, halobenzenes, and Decalin. Other solvents, such as nitrobenzene, ethyl acetate, dioxane, hexane, heptane, and octane, are used occasionally. In one industrial application a solution of the amine in a common organic solvent is added to a solution of phosgene in an organic isocyanate.[‌225‌]

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References