Process for the preparation of aliphatic isocyanates
The present invention relates to a process for preparing aliphatic or cycloaliphatic isocyanates by phosgenation of symmetric ureas. 
     Certain symmetric ureas are byproducts of the reaction when chemical compounds usable as active substances in plant health compositions are prepared. This is the case in particular of the NN&#39; dialkylureas, in particular NN&#39; diisopropyl urea obtained when hydantoin 1-(3,5-dichlorophenyl)-3-isopropyl carbamoyl is manufactured. 
     It is known that aromatic isocyanates, in particular phenyl isocyanates, can be prepared by phosgenation of an aromatic symmetric urea beginning at a temperature of 150° C. (see Chem. Rev. 43-203, 1948). 
     It is also known (see U.S. Pat No. 3,275,669) that isocyanates can be prepared in two stages by cold phosgenation (avoiding too great an excess of phosgene) of symmetric urea, giving allophanyl chloride, followed by thermal decomposition of this compound, preferably dissolved in an inert solvent, into isocyanate whereby hydrochloric acid is given off. Although this process is described as theoretically applicable to aliphatic symmetric ureas, the isocyanate yield differs considerably according to the starting urea. Although yields can be excellent for ureas substituted by linear alkyl radicals, they are poor and even as low as a few percent in cases where the nitrogen atoms are substituted by branched, especially secondary, alkyl radicals such as isopropyl or cyclohexyl. 
     An explanation for this phenomenon has been proposed according to which, in the case of substituted alkyl radicals, another reaction occurs: ##STR1## which considerably restricts or even completely prevents isocyanate formation. 
     Thus, it has not been possible hitherto to obtain aliphatic isocyanates in a single stage with satisfactory yields by phosgenation or the corresponding aliphatic symmetric ureas. 
     The present invention relates to a process which does not have the aforementioned disadvantages and which consists of preparing aliphatic isocyanates by phosgenation of symmetric ureas, characterized by causing phosgene to react with a disubstituted NN&#39; symmetric urea in a single stage as follows: 
     
         RNHCO--NHR+COCl.sub.2 →2RNCO+2HCl 
    
     In the urea formula, R is an alkyl radical with one to six carbon atoms. This reaction is carried out in solution in a high-boiling inert organic solvent at a temperature of 150° to 350° C. 
     The R radical is preferably an alkyl radical with one to four carbon atoms or the cyclohexyl radical, the best results being obtained when R is one of the propyl, isopropyl, or butyl radicals. 
     The phosgene can be introduced into the reaction medium by bubbling in the gaseous state or in the solution form in an organic solvent. In general the isocyanate yield is improved if there is excess phosgene with respect to the substituted urea; the reaction is preferably carried out with a molar phosgene/urea ratio of 1/1 to 3/1. 
     The solvent usable in the process according to the invention must have a high boiling point, over 150° C., the minimum temperature necessary for the reaction. Examples of suitable organic solvents which can be cited are quinoline, 1-chloronaphthalene, N-methyl caprolactam, and, preferably, sulfolane and paraffin oil. However, this list is not exhaustive and other equivalent solvents can also be used. 
     According to another essential characteristic of the process, the temperature of the reaction medium must be between 150° and 350° C., approximately, and preferably between 200° and 250° C. The temperature enables the reaction to be started and facilitates it by favoring elimination of the resulting hydrochloric acid as well as distillation of isocyanate. If the reaction is carried out below 150° C., the isocyanate yield is very low, since the starting products combine according to the prior art, principally into either allophanyl chloride or into chloroformamidinium chloride. Above 350° C., on the other hand, the reaction is theoretically possible, but the risk of the urea decomposing by heat increases and the choice of a solvent with a sufficiently high boiling point becomes increasingly narrower. 
     The heating time is a function of the nature of the urea, that of the solvent, and the phosgene/urea molar ratio. In general, times between 0.5 and 10 hours, preferably between 1.5 and 7 hours, will be used. 
    
    
     The examples hereinbelow illustrate the process according to the invention. 
     EXAMPLE 1 
     A 250 ml three-neck flask, equipped with a thermometer, an agitator, a distiller, and a down-coming tube for bubbling in phosgene is charged with 100 ml sulfolane (tetramethylene sulfone with a boiling point of 285° C.) and 14.4 g (i.e., 0.1 mole) diisopropyl urea. The apparatus is then flushed with nitrogen for 10 min. It is heated to 225° C. with the aid of a metal bath, while stirring. The reaction mixture is held at this temperature for 6 hours, while the phosgene is bubbled in at a slow rate regulated by cooling the reservoir. The isocyanate distills slowly. When all the phosgene has passed in a little nitrogen is bubbled through and heating is stopped. The reaction times vary from 4 hours to 5 hours 45 minutes. The hydrochloric acid and excess phosgene given off are trapped in soda and xylene bubblers respectively. 
     The distillate is assayed to determine the quantity of isocyanate obtained. 
     The table below gives the yields obtained as a function of the phosgene/urea molar ratio. 
     
                       Table 1                                                     
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Molar ratio                                                               
          Phosgene passage                                                
                         % isocyanate/urea                                
COCl.sub.2                                                                
          time           yield                                            
______________________________________                                    
1         5 h 25 m       42.5                                             
1.8       2 h 45 m       58.0                                             
2.2       4 h            68.4                                             
3         4 h 40 m       65.9                                             
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     EXAMPLE 2 
     One operates as in Example 1 with different solvents and different phosgene/urea ratios. 
     The supplementary conditions and results are set down in the table hereinbelow. 
     
                       Table 2                                                     
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Molar                       Phosgene                                      
ratio Type       Boiling    passage                                       
                                   % isocyanate/                          
COCl.sub.2                                                                
      of solvent point      time   urea yield                             
______________________________________                                    
2     paraffin oil                                                        
                 190°-                                             
                            5 h    84.7                                   
                 230° C./                                          
                 0.1 mmHg                                                 
1.15  quinoline  237° C.                                           
                            3 h    50                                     
1.1   N-methyl   321° C.                                           
                            2 h 35 m                                      
                                   48                                     
      caprolactam                                                         
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     EXAMPLE 3 
     One operates as in Example 1 establishing the phosgene/urea molar ratio at 2 and the temperature at 225° C., the solvent being paraffin oil and the time taken for phosgene is made to vary. 
     This time and the associated yields are given in the table hereinbelow. 
     
                       Table 3                                                     
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                  % isocyanate/urea                                       
Time              yield                                                   
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0 h 35 m          48.2                                                    
1 h 55 m          64.0                                                    
3 h 25 m          70.8                                                    
5 h               84.7                                                    
6 h               84.5                                                    
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     EXAMPLE 4 
     One proceeds as in Example 1, beginning from different NN&#39;-dialkylureas under appropriate conditions. These conditions as well as the associated isocyanate yields are given in the table hereinbelow. 
     
                                           Table 4                                 
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                          Phosgene                                        
Starting                                                                  
        COCl.sub.2        passage                                         
                               Isocyanate                                 
urea R =                                                                  
        *   Solvent                                                       
                   Temperature                                            
                          time yield, %                                   
__________________________________________________________________________
CH.sub.3.sup.-                                                            
        2   chloronaph-                                                   
                   240° C.                                         
                          2 h 25 m                                        
                               44.7                                       
            thalene **                                                    
CH.sub.3 CH.sub.2 CH.sub.2.sup.-                                          
        2.4 chloronaph-                                                   
                   250° C.                                         
                          3 h 10 m                                        
                               88.8                                       
            thalene **                                                    
CH.sub.3 (CH.sub.2).sub.3.sup.-                                           
        2.6 chloronaph-                                                   
                   225° C.                                         
                          1 h 35 m                                        
                               72.5                                       
            thalene **                                                    
(CH.sub.3).sub.3 C.sup.-                                                  
        1.5 paraffin oil                                                  
                   230° C.                                         
                          2 h 15 m                                        
                               43.0                                       
cyclohexyl                                                                
        2   paraffin oil                                                  
                   250° C.                                         
                          2 h  37.0                                       
 ##STR2##                                                                 
         2   paraffin oil                                                 
                    225° C.                                        
                           1 h 35 m                                       
                                64.0                                      
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 * = molar ratio                                                          
 ** boiling point: 263° C.                                         
 
    
     EXAMPLE 5 
     The purpose of this example is to compare the process described in U.S. Pat. No. 3,275,669 (process A) for phosgenating a symmetric urea and the process according to the invention (INV). It should first be noted that the aforementioned process requires two stages for obtaining isocyanate. 
     The tests were carried out according to the method described in the reference for the known process and according to the method of Example 1 for the process according to the invention. The precise conditions characteristic of each process and the yields obtained are shown in the table hereinbelow. With regard to the known process, the yields are also expressed in isocyanate, it being assumed that the second stage of thermal decomposition of allophanyl chloride into isocyanate was practically quantitative. 
     
                                           Table 5                                 
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                       Phosgene                                           
                             % isocya-                                    
Starting          Temper-                                                 
                       passage                                            
                             nate/urea                                    
urea COCl.sub.2 /urea                                                     
            Solvent                                                       
                  ature                                                   
                       time  yield Process                                
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diiso-                                                                    
     1.8    Sulfolane                                                     
                  25° C.                                           
                       4 h 15 m                                           
                             11.1  A                                      
propyl                                                                    
urea 1.8    Sulfolane                                                     
                  225° C.                                          
                       5 h 45 m                                           
                             58.0  INV                                    
dicyclo-                                                                  
     1.1    Dichloro-                                                     
                  2° C.                                            
                       1 h 50 m                                           
                             12.5  A                                      
hexyl       ethylene                                                      
urea 2.0    Paraffin                                                      
                  250° C.                                          
                       2 h   37.0  INV                                    
            oil                                                           
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     With regard to the dicyclohexyl urea, it should be pointed out that the phosgene/urea molar ratio is not critical and a ratio on the order of 2 leads to a similar yield. 
     This table shows clearly that the process according to the invention enables alkylisocyanates to be obtained, particularly those which have a secondary carbon on the nitrogen atom, not only in a single stage instead of two stages with the known process, but with 3 to 5 times the yield. 
     All the above examples illustrate the essential characteristics of the process according to the invention, namely that it is carried out in a single stage, that it enables alkylated aliphatic isocyanates to be prepared by both linear and branched or cyclic radicals to be prepared, and that the yields can be excellent and clearly superior to those of processes known hitherto using the same starting products.