Reduction of nitro compounds with aqueous solutions of a formate
This invention relates to a process for the reduction in aqueous solution of organic nitro compounds containing solubilising groups. 
     It is known that organic nitro compounds containing water solubilising groups such as sulphonic, carboxylic groups, phenolic groups or amino groups may be reduced for example by treatment with metallic iron and hydrochloric acid in aqueous medium, or with hydrogen gas in presence of a hydrogenation catalyst. However, such processes require specialised equipment and are not always convenient to operate. 
     According to the present invention we provide a process for the reduction of organic nitro compounds containing groups which confer solubility in water comprising treatment of said compounds with an aqueous solution of a formate in presence of a hydrogenation catalyst. 
     The formate used in the process may be formic acid or a water-soluble salt thereof, or a mixture of such salts, or of such salts and free formic acid. As formic acid salts there may be used for example alkali metal formates, and especially sodium formate. Preferably strong solutions of formate are used, for example of at least 10% strength by weight. 
     For complete reduction of each nitro group to an amino group at least three molar proportions of formate should be present. 
     As hydrogenation catalysts there may be used for example those based on platinum, palladium and ruthenium, and especially palladium supported on 5 to 100 times its weight of carbon. To give a useful reaction rate there may be used for example between 0.1 and 15 parts of palladium-on-carbon catalyst per 100 parts of organic nitro compound. After reaction the catalyst may be collected and re-used. 
     To increase the reaction rate, especially at lower catalyst usages, it has been found useful in some cases to treat the solution of organic nitro compound with activated carbon before the addition of the catalyst. As has been found with catalytic hydrogenation reactions it may also be advantageous to add catalytic amounts of other metal compounds and especially ferric hydroxide. 
     The pH at which the reduction process is conducted preferably lies between 2 and 12, and most conveniently between 3 and 10; pH adjustment may be effected if necessary using formic acid or mineral acids or the commonly used alkaline substances such as sodium hydroxide and sodium carbonate. 
     Although the reaction may proceed at lower temperatures it is usually found convenient to conduct the reduction process at the boil in an atmosphere of an inert gas such as nitrogen. 
     The process of the present invention is especially useful for reducing to the corresponding amino compounds aromatic nitro compounds of the benzene and naphthalene series which contain sulphonic acid or carboxylic acid solubilising groups. 
    
    
     The invention is illustrated but not limited by the following examples in which parts are by weight. 
     EXAMPLE 1 
     To a solution of 25.9 parts of the disodium salt of a 1-nitronaphthalene-3,6-8-trisulphonic acid in 125 parts of water are added 20.4 parts of sodium formate and 1.5 parts of damp 3% palladium on carbon catalyst (0.93 parts dry weight). The mixture is covered with a nitrogen atmosphere and stirred and heated under reflux for 26 hours. The mixture is then cooled and the catalyst is collected by filtration. Analysis of the filtrate by ultra violet spectroscopy and by titration of an aliquot with sodium nitrite solution shows it to contain 22.2 parts of 1-aminonaphthalene-3,6,8-trisulphonic acid (98% of the theoretical yield). 
     EXAMPLE 2 
     A solution of 20.6 parts of the magnesium salt of 3-nitronaphthalene-1,5-disulphonic acid and 5.5 parts of sulphuric acid in 160 parts of water is reacted with 20.4 parts of sodium formate and 1.24 parts (dry weight) of 3% palladium-on-carbon catalyst according to the procedure of Example 1. After 22 hours a further 10 parts of sodium formate are added and stirring under reflux is continued for an additional period of 20 hours. The mixture is cooled and filtered and the filter cake is washed with dilute sulphuric acid until only the catalyst remains on the filter. Analysis of the combined filtrates by ultraviolet spectroscopy and by titration of an aliquot with sodium nitrite solution indicates that the theoretical yield (17.6 parts) of 3-aminonaphthalene-1,5-disulphonic acid has been formed. 
     EXAMPLE 3 
     A mixture of 27.6 parts of 3-acetylamino-4-hydroxy-5-nitrobenzene sulphonic acid, 75 parts of sodium formate, and 6 parts of damp 3% palladium on charcoal (3 parts dry weight) catalyst in 300 parts of water is stirred together at room temperature for 10 minutes and is then raised to the boil during 30 minutes. The solution is boiled under reflux during 4 hours until the orange solution becomes clear pale brown and no nitro group can be detected and then the catalyst is filtered off. The filtrates are acidified to Congo Red test papers with concentrated hydrochloric acid and then a further 20 parts of concentrated hydrochloric acid are added followed by 10% w/v of sodium chloride. The suspension is stirred at room temperature for several hours to complete the precipitation and then 3-acetylamino-5-amino-4-hydroxybenzenesulphonic acid (22 parts) is filtered off, washed with small amounts of 10% w/v sodium chloride solution, and dried. 
     EXAMPLES 4 - 30 (Table I) 
     Example 2 is repeated, replacing the 20.6 parts of the magnesium salt of 3-nitronaphthalene-1,5-disulphonic acid by the equivalent number of parts (based on nitro group content) of the nitro compounds listed in Table I. The pH of the mixture is adjusted by addition of sodium hydroxide or sulphuric acid to achieve solubility of the starting material. The corresponding amino compounds are produced in excellent yields and if desired may be isolated from solution by procedures well known to the art. 
     
                       Table I                                                     
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Ex.  Organic Nitro compound                                               
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4    1-nitronaphthalene-5-sulphonic acid                                  
5    1-nitronaphthalene-6-sulphonic acid                                  
6    1-nitronaphthalene-7-sulphonic acid                                  
7    1-nitronaphthalene-8-sulphonic acid                                  
8    1,4-dinitrobenzene-2-sulphonic acid                                  
9    4-nitroacetanilide-3-sulphonic acid                                  
10   2,4-dinitromesitylene-6-sulphonic acid                               
11   2-nitrotoluene-4-sulphonic acid                                      
12   5-nitro-2-methoxy-N-β(β&#39;-methoxyethoxycarbonyl)-           
     ethylaniline                                                         
13   5-acetylamino-2-nitrobenzoic acid                                    
14   1-carboxy-2-hydroxy-3-nitrobenzene-5-sulphonic                       
     acid                                                                 
15   2-nitrophenol-4-sulphonic acid                                       
16   2-nitrobenzene sulphonic acid                                        
17   3-nitrobenzene sulphonic acid                                        
18   1-nitronaphthalene-3,8-disulphonic acid                              
19   1-nitronaphthalene-4,8-disulphonic acid                              
20   1-nitronaphthalene-4,6,8-trisulphonic acid                           
21   4-nitro-4&#39;-aminodiphenylamine-2-sulphonic acid                       
22   N-cyclohexyl-4-nitroaniline-2-sulphonic acid                         
23   4-nitro-2&#39;-methoxydiphenylamine-2-sulphonic acid                     
24   6-nitroindazole                                                      
25   1-(3&#39;-nitro-2&#39;-methyl-5&#39;-sulphophenyl)-3-carboxy-                    
     5-pyrazolone                                                         
26   1-nitroanthraquinone-5-sulphonic acid                                
27   1-nitroantraquinone-8-sulphonic acid                                 
28   4,4&#39;-dinitrostilbene-2,2&#39;-disulphonic acid                           
29   2-methoxy-5-nitrobenzene sulphonic acid                              
30   4-methoxy-3-nitrobenzene sulphonic acid                              
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