Patent GB-656726-A (GB)
Title: Improvements in or relating to processes of preparing carbamyl chlorides and isocyanic acid esters and the compounds resulting from said processes
IPC: C07C271/04
Filed: 1948-02-11
Abstract: Carbamyl chlorides are made by treating phosgene with a primary or secondary amine in the vapour phase at elevated temperatures. Amine hydrochloride by-product may condense in the reactor.  The carbamyl chlorides are dehydrochlorinated to the corresponding isocyanate. The reaction may be continuous, the vaporized amine being reacted with 1.2-2.5 mol. proportions of phosgene and the products are led through a hydrochloride trap to a hot condenser where liquid carbamyl chloride is separated from gaseous phosgene and HCl and passed to the dehydrochlorination stage where it is treated with a solvent and 1.25-2.0 mol. proportions of a tertiary organic base, the hydrochloride of which is continuously drawn from the system and the isocyanate is recovered from the solvent by distillation. Alternatively monosubstituted carbamyl chlorides may be dehydrochlorinated by heating.  Temperatures specified for the amine-phosgene reaction are 240-400 DEG  C., preferably 240-300 DEG C. and broadly are within the range covered by the melting point of the corresponding amine hydrochloride and the decomposition temperature.  Sojourn times within the reactor for the phosgene-methylamine reaction are quoted as 0.67-1.4 secs., preferably about 1 sec.  Condenser temperatures range from 20-70 DEG C., for methyl carbamyl chloride separation the range 30-40 DEG  C. is preferred.  An electrostatic precipitator may be used in place of the condenser.  In the dehydrochlorination reaction if the B.Pt. of the isocyanate is above the decomposition temperature of the mono-substituted carbamyl chloride the conversion may be thermal, otherwise a dehydrochlorinating agent is necessary.  Preferred dehydrochlorinating agents are tertiary organic bases, e.g. pyridine and dimethyl aniline although calcium oxide is also mentioned.  In general a 25-100 per cent molar excess of dehydrochlorinating agent should be used.  For methyl isocyanate pyridine in 25 per cent molar excess is preferred.  Inert solvents specified for this stage are toluene, benzene, xylene, kerosene, cyclohexane, carbon tetrachloride, lignin and pet. ether usually in equal volume to that of the base.  Amines specified as starting materials may be cyclic or acyclic, aromatic, aliphatic, alicyclic or heterocyclic.  In examples: (1) phosgene and methylamine are reacted at 265-275 DEG  C. to give methyl carbamyl chloride which after separation is treated with mixed toluene and pyridine with cooling to give methyl isocyanate on distillation; (2) sec. butylamine and phosgene are reacted at about 245 DEG  C. to give sec. butyl carbamyl chloride which on treatment with dimethyl aniline yields sec. butyl isocyonate; (3) isobutyl amine and phosgene are reacted at about 250 DEG  C. to give isobutyl carbamyl chloride which with dimethyl aniline yields isobutyl isocyanate; (4) allylamine and phosgene are reacted at about 250 DEG  C. to give allyl carbamyl chloride which with dimethyl aniline yields allyl isocyanate, and (5) aniline and phosgene are reacted at about 285 DEG  C. to give phenyl carbamyl chloride which after separation by means of an electrical precipitator is refluxed in toluene to yield phenyl isocyanate. Other carbamyl chlorides and isocyanates are obtained, e.g. those derived from ethyl-, isopropyl, propyl, N-butyl-, allyl-, N-octyl-, dodecyl-, cyclohexyl- and di-n-butylamines, and a table is provided giving the reaction conditions necessary for their production.