CS262295B1 - Process for preparing 1-isocyanato-4-chlorobenzene - Google Patents

Abstract

The preparation of l-isocyanato-4-chlorobenzene by phosgenation of p-chloroaniline in chlorobenzene is carried out by continuous transition from the cold phosgenation phase to the warm phosgenation in the presence of the N,N”-diaza bicyclo/2,2,2/octane catalyst at temperatures of 20 to 120 °C, the decomposition of carbomoyl chloride is completed by introducing liquid recycled phosgene at temperatures of 120 to 130 °C.

Description

The present invention relates to a process for the preparation of 1-isocyanato-4'-chlorobenzene by phosgenation of p-chloroaniline in chlorobenzene.

NH · »

NCO chlorobenzene + COCl2 catalyst

Cl + 2HCl

Cl

Reactions of amines respectively. their hydrochloride with phosgene is the most widespread process for the production of isocyanates on an industrial scale. There are many papers and publications devoted to this issue in literature. The largest extension was achieved by the process of phosgenation of free amines in a suitable solvent and usually consists of two phases - cold and hot phosgenoe. In cold phosgenation, the phosgene solution reacts with the amine solution at 0 to 60 ° C with vigorous stirring, and then the resulting hydrochloride and carbamoyl chloride slurry is gradually heated to 100 to 200 ° C in hot phosgenation cascade reactors to complete the reaction and remove from the reaction mixture of hydrogen chloride and excess phosgene.

In order to obtain a satisfactory yield of isocyanates, a sufficient excess of phosgene must be maintained during hot phosgenation. The necessary amount of phosgene is dosed either at the same time in the first stage of the reaction or gradually during cold and hot phosgenation. It is preferable to feed all the phosgene into the first phase of phosgenation. Under certain conditions, a single-stage phosgenation can be carried out, but a large excess of phosgene, a very dilute solution / amine and vigorous stirring at high temperature must be employed.

The use of many catalysts to accelerate the reaction of amine hydrochlorides with phosgene has been described in the literature, e.g. This is usually obtained by absorption into the solvent used or is obtained by condensation from exhaust gases at very low temperatures. Normally, an excess of phosgene of 100 to 500% is used. The isocyanates can also be produced in a batch process, in which the two phosgene phases are carried out sequentially in a single reactor. The disadvantage of this process is the irregular release of hydrogen chloride and phosgene during hot phosgenation and hence their difficult capture.

From this general overview, it is apparent that the published isocyanate preparation routes have various advantages and disadvantages.

It has now been found that 1-isocyanato-4-chlorobenzene can be advantageously prepared by the process of the invention by phosgenation of p-chloroaniline in chlorobenzene, respectively. xylene by a continuous transition from a cold phosgenation phase to a warm phosgenation phase in the presence of an N, N'-diaza-bicyclo (2,2,2) octane catalyst at 20 to 120 ° C, complete decomposition of carbamoyl chloride by introducing liquid recirculated phosgene at 120 to 130 ° C and the product is used in the form of a chlorobenzene or xylene solution after cleration and partial concentration without further rectification for synthetic applications.

The isocyanate preparation itself is carried out by introducing p-chloroaniline, chlorobenzene and a catalyst into the reactor at normal temperature, stirring the batch and commencing the introduction of phosgene, the flow rate of which is kept constant up to about 75 ° C in the reactor, it will decrease as carbomoyl chloride decomposes, while at the same time the temperature is raised by external heating to 120 to 125 ° C so that the decomposition is continuous. Phosgene is then continued at the original flow rate until liquid phosgene freeze-dried from the waste hydrogen chloride begins to be recirculated to the reactor. Once liquid phosgene recirculation is achieved, the phosgene flow is adjusted to a minimum and the phosgenation with liquid phosgene is continued for 1.5 to 2 hours. Then the phosgene gas supply is completely stopped, the temperature in the reactor is raised to about 130 ° C and dissolved HCl and COCl ₂ are stripped off with nitrogen.

The stripped crude isocyanate solution is then cooled and the by-product flakes filtered off, and the clear solution is concentrated in vacuo by distilling off less solvent to remove the last dissolved hydrogen chloride and phosgene residues. The solution thus prepared is used for immediate synthetic application after analysis.

The advantage of this process is low apparatus intensity, controlled decomposition of carbamoyl chloride, considerably reduced consumption of starting phosgene gas, higher actual phosgene concentration in the reaction mixture, considerably low catalyst consumption (about 3 kg / t product) due to used recirculation of liquid phosgene.

The disadvantage is the use of a freezing station to freeze the waste gases at min.

-20 ° C and more cooling surface of liquid phosgene condensers.

The example below illustrates an embodiment of the invention.

Example

The reactor was charged with 127.6 g (1 mol) of p-chloroaniline (99 L), 1000 ml of chlorobenzene and 0.41 g of N, N-diaza-bicyclo (2.2.2) octane catalyst. The batch is thoroughly mixed and then the introduction of phosgene is started at 20 ° C. The temperature in the reactor rises slowly during phosgenation due to the exothermic reaction and reaches 75 ° C after about 40 minutes of phosgene introduction, forming a thick white suspension of amine hydrochloride and carbamoyl chloride, which decomposes rapidly at this temperature.

In the decomposition phase of carbamoyl chloride, at a moderate phosgene flow, the temperature in the reactor is increased to 120-125 ° C over a period of 1 h, so that the decomposition of carbamoyl chloride is relatively smooth and without spontaneous discharge of hydrogen chloride. Phosgene is then continued to be introduced at an increased flow rate for 1.5 hours until liquid phosgene is freeze-dried from the waste gases at -20 ° C into the reactor. As soon as liquid phosgene recirculation is achieved, the flow rate of fresh phosgene gas is adjusted to a minimum and the phosgenation is continued for about 1.5 hours at about 120 ° C.

Thereafter, the temperature in the reactor was raised to 130 ° C, phosgene gas was stopped and the reaction mixture was sprayed with dried nitrogen for 30 minutes.

All the waste gases from the reactor are discharged to the foot of the absorption packed column, which is sprayed with a 15% aqueous NaOH solution from a 15% NaOH recirculation tank.

The stripped crude 1-isocyanato-4-chlorobenzene solution is then cooled to about 25 ° C and about 30 g of a wet off-white paste (mainly p-chloroaniline hydrochloride) is filtered off.

The clear 1-isocyanato-4-chlorobenzene solution of chlorobenzene is then further concentrated under a vacuum of 14.66 to 10.66 kPa by distilling off 150 ml of chlorobenzene to remove the completely dissolved phosgene and hydrogen chloride. In the resulting 1020 g solution, 12.3 wt% of 1-isocyanato-4-chlorobenzene, i.e. 82.5% of theory, was determined by analysis. and 0.08 wt.% dissolved phosgene. The thus prepared 1-isocyanato-4-chlorobenzene halobenzene solution can be used for synthesis, e.g. difluobenzuron, without further rectification.

Claims (1)

SUBJECT OF THE INVENTION A process for the preparation of 1-isocyanato-4-chlorobenzene of formula C1 characterized in that the phosgenation of p-chloroaniline in chlorobenzene is carried out by a continuous transition from a cold phosgenation phase to a warm phosgenation in the presence of an N, N-diaza-bicyclo (2,2,2) octane catalyst at temperatures of 20 to 120 ° C, carbamoyl chloride is completed by introducing liquid recirculated phosgene at temperatures of 120 to 130 ° C.