Process for preparing isocyanic acid derivatives

A process for preparing isocyanic acid derivative having a isocyanatomethyl group and/or isocyanurate ring structure, which consists essentially of reacting at 70.degree. to 200.degree. C (A) an organic halogen compound having a monohalogenomethyl group or monohalogenomethylene group, or mixtures thereof, with (B) an alkali cyanate selected from the group consisting of lithium cyanate, sodium cyanate, potassium cyanate, rubidium cyanate, cesium cyanate, ammonium cyanate and mixtures thereof, in (C) an organic solvent selected from the group consisting of N,N-disubstituted organic acid amines, N,N'-disubstituted organic sulfonic acid amides, dialkyl sulfoxides, dialkyl sulfones, polyalkylene sulfoxide, polyalkylene sulfone, macrocyclic polyethers and mixtures thereof, in the presence of (D) at least one polyhalogen compound selected from the group consisting of compounds having dihalogenomethyl group, dihalogenomethylene group or trihalogenomethyl group and tetrahalogenomethanes in an amount in the range of from 1 to 20 weight percent, based on the weight of said organic halogen compound (A), until isocyanic acid derivative of (A) is formed, said polyhalogen compound (D) being compatible with organic compounds or organic halogen compounds as reaction solvent and being present at the start or midway of the reaction and being effective to prevent side reactions and being unchanged by the reaction, and recovering from the reaction mixture said isocyanic acid derivative.