GB736465A - Preparation of aromatic isocyanates - Google Patents

Abstract

Aromatic isocyanates are prepared by introducing into an inert liquid reaction medium substantially simultaneously an aromatic amine, in an inert solvent therefor, and phosgene in at least 50 per cent excess of the stoichiometric equivalent of the amine, maintaining the temperature during reaction at about 130-300 DEG C., separating the unreacted phosgene and hydrogen chloride formed, and thereafter isolating the pure aromatic isocyanate. In one form of the invention, the aromatic amine is mixed with non-reactive liquid which has a boiling point above that of the aromatic isocyanate to be formed and the reaction with the phosgene is carried out at a temperature above the boiling point of the isocyanate but below the boiling point of the non-reactive liquid so that the isocyanate is distilled out as it is formed. The isocyanate is preferably collected in a receiver which contains a solvent for the isocyanate held at a temperature sufficiently high to prevent the formation of the corresponding carbamyl chloride. In a second form of the invention, the amine is dissolved in a non-reactive solvent having a boiling point lower than that of the isocyanate to be formed and phosgene is passed in while refluxing the liquid, the hydrogen chloride and unreacted phosgene being removed through the reflux condenser. Inert liquids or solvents specified include hydrocarbon fractions of petroleum or coal tar origin, and particularly chlorinated hydrocarbons such as the chlorinated benzenes and chlorinated diphenyls. Examples are given of the preparation by the first method of 2,4-tolylene diisocyanate, phenyl isocyanate, p-chlorophenyl isocyanate and m-phenylene diisocyanate. Further examples describe the preparation by the second method of 2,4-tolylene diisocyanate, methylene-bis-(4-phenyl isocyanate), 2,6-tolylene diisocyanate, phenyl isocyanate, m-phenylene diisocyanate, and 1,5-tetrahydronaphthalene diisocyanate. The second method may be carried out continuously by continuously withdrawing and working up liquid from the reaction vessel as the amine and phosgene are introduced. 1,5-Diaminotetrahydronaphthalene is prepared by adding sodium to a boiling solution of 1,5-diaminonaphthalene in amyl alcohol, treating with water, acidifying the alcohol layer, evaporating until crystals form, dissolving in ether, precipitating as the carbonate by introducing carbon dioxide, dissolving the carbonate in acetic acid, and treating with potassium hydroxide solution.