US2669584A

US2669584A - Selective reduction of polynitro aromatics

Info

Publication number: US2669584A
Application number: US156929A
Authority: US
Inventors: Edmund W Lowe
Original assignee: Purdue Research Foundation
Current assignee: Purdue Research Foundation
Priority date: 1950-04-19
Filing date: 1950-04-19
Publication date: 1954-02-16
Anticipated expiration: 1971-02-16

Description

Patented Feb. 16, 1954 SELECTIVE REDUCTION OF POLYNITRO AROMATICS Edmund W. Lowe, Lafayette,

Ind., assignor to Purdue Research Foundation, Lafayette, 11111.,

a corporation of Indiana No Drawing. Application April 19, 1950, Serial No. 156,929

16 Claims. (01. 260-580) 1 This invention relates principally to the reduction of polynitro aromatic compounds. More particularly, this invention relates to the selective reduction of certain polyintro aromatic compounds, at least one but not all of the nitro groups present being completely reduced to an amino group while at least one nitro group remains unaiiected.

The selective reduction of aromatic dinitro com pounds to form nitroamines has long been known in the art. Such reductions are almost invariably conducted in strongly alkaline solutions and reducing agents such as the hydrosulfides, sulfides and polysulfides of sodium are commonly employed. During the course of the reaction, the sulfide ion of the selected reducing agent is oxidized to thiosulfate. By such methods m-dinitrobenzene has been reduced to m-nitroaniline, 2,6-dinitrotoluene to Z-amino G-nitrotoluene, 2,4-dinitrotoluene to Z-nitro 4-aminotoluene, 2,4- dinitroanilin to 1,2-diamino l-nitrobenzene, 2,4- dinitrodiphenylamine to 2 amino 4-nitrodiphenylamine, and the like.

Similar methods have been applied to the selective reduction of various trinitro aromatics with varying success. Thus, picric acid (in the form of the sodium salt) is reduced to picramic acid (4,6-dinitro-2-aminophenol) by sodium sulfide, the yield being 75%. However, the application of this and similar reducing methods to 2,4,6- tr'initrotoluene (TNT), certain derivatives of TNT and the like result in yields of reduction products ranging from zero to poor.

I have discovered an improved method for the selective reduction of certain trinitro aromatic compounds whereby the correspondingdinitroamino and/or diaminonitro compounds are obtained in yields higher than hitherto believed possible.

One object of my invention is to provide an improved process for the selective reduction of certain trinitro aromatic compounds.

Another object of my invention is to provide an improved process for the reduction of certain trinitro aromatic compounds to the corresponding dinitroamino and/or diaminonitro compounds.

An additional object of my invention is to provide an improved process for the selectiv reduction of 2,4,6-trinitrotoluene.

A further object of my invention is to provide an improved process for the reduction of 2,4,6- trinitrotoluene to 2,6 dinitro 4-aminotoluene and/or 2,4-diamino G-nitrotoluene.

Another object of my invention is to provide an 2 improved process for the selective reduction of condensation derivatives of 2,4,6-trinitrotoluene.

Additional objects of my invention will become apparent as the description thereof proceeds.

In accordance with my improved process, the sodium hydroculfide, sodium sulfide, sodium polysulfide, etcetera, reducing agents of the prior art are employed bu the reaction proceeds through a difierent mechanism. As has been mentioned previously, in prior art reduction processes with these reducing agents, the sulfide ion of the selected reducing agent is oxidized to thiosulfate. In my improved process, the sulfide ion of these reducing agents is oxidized to free sulfur.

This new reaction mechanism is achieved by conducting my improved process under conditions differing radically from those employed in the prior art. By the practice of my invention, the selective reduction of certain trinitro aromatics proceeds with the production of the desired reduction products in high yields in sharp contrast to the processes of the prior art which result in zero yields or, at best, poor yields of the reduction products when applied to the same starting materials.

For the better understanding of my invention, it will be explained in detail in the following example in connection with the reduction of TNT to 2,6-dim'tro 4-aminoto1uene (hereinafter referred to as DNAT) and to 2,4-diamino 6-nitrotoluene (hereinafter referred to as DANI). It should be emphasized that the following example is illustrative only and is capable of wide variations with respect to both starting materials and operating conditions.

E'wample 1 Two gram moles of TNT are dissolved in 1000 cc. ethyl acetate and to the resulting solution is added a strong aqueous solution containing two gram moles ammonium acetate to act as a buffer in subsequent operation. Obviously, if desired, the buffer salt may be formed in situ by adding the appropriate amounts of acetic acid (e. g., g. glacial acetic acid) and ammonium hydroxide (e. g., cc. of 28-29% ammonia solution) to the ethyl acetate solution of TNT.

The above reaction mixture is vigorously stirred and to it is added, portionwise, over the course of 25-35 minutes, a solution made by dissolving 720 g. of 70% sodium hydrosulfide (9 moles) in 1200 cc. water.

During the reduction reaction, the hydroxyl ion concentration of the reaction mixture would ordinarily tend to increase rapidly in accordance with the reaction:

I have discovered it is absolutely essential that that increase in alkalinity be prevented and, in fact, that the whole reduction occur at an alkalinity distinctly lower than employed in the processes of the prior art. To this end, a strong acid is added as required and as described in detail subsequently, to maintain the alkalinity of the reaction mixture at the desired level. As a result of extensive experimentation, I have determined that the pH of the reaction mixture should be maintained the approximate range 7.5 to 8.0 (as determined by Hydrion test paper) during the first additions of reducing solution, for example, during the addition of the first two thirds or thereabouts of the reducing solution and in the approximate range of 8.0 to 8.5 thereafter.

Most conveniently, the desired alkalinity is maintained by the portionwise addition of a strong acid, for example, hydrochloric acid, as required, said portionwise additions being interspersed as necessary between the additions of successive portions of the reducing agent. In the present example, a total of about 400 cc. of B. hydrochloric acid is required to maintain the reaction mixture at the proper alkalinity level and this is added preferably in ten or twelve approximately equal portions as conditions require.

As will be evident to those skilled in the art, the presence of ammonium acetate in the reaction mixture aids materially in maintaining the pH within the required range. This buffer salt is highly ionized with the formation of cations and anions which unite respectively with hydroxyl or hydrogen ions to form acids or bases that are weakly ionized. Accordingly, during the reduction reaction, as highly ionized sodium hydroxide forms in accordance with the equation previously given, the resulting hydroxyl ions unite, in large measure, with the ammonium ions from the buffer salt to form the weakly ionized ammonium hydroxide. Thus, due to the presence of the buffer salt, the alkalinity of the reaction mixture increases comparatively slowly. On the other hand, on adding a portion of strong acid to the reaction mixture, the hydrogen ions so introduced unite, in large measure, with the acetate ions from the buffer salt to form the weakly ionized acetic acid. As a result, the addition of a portion of strong acid which would, under ordinary circumstances,

throw the reaction mixture far on the acid side, has, in the presence of the buffer salt, relatively little efiect on the pH of the mixture.

While the above explanation has been based on the use of ammonium acetate as the buffer salt, any ammonium salt will be of benefit in the control of pH as will be salts of weak acids. Salts formed by the union of a weak base and a weak acid are especially useful. For convenience in operation, the buffer salt selected should preferably not form insoluble hydrolytic products on reaction with water and of course should not react with the materials employed in the synthesis.

It is obvious that the pH of the reaction mixture can be maintained Within the desired range without the use of buffers if an extremely large number of very small additions of reducing agent are made, these being interspersed with an extremely large number of very small additions of 4 acid. Such operations are tedious and difiicult to conduct on the laboratory scale but in commercial operations, where large volumes are involved, the alternate and frequent additions of small portions of reducing agent and acid to the reaction mixture imposes no special problems and can be easily accomplished automatically, for example, by the use of suitable proportioning pumps.

On adding the initial portion of the reducing agent to the TNT solution, the temperature rises rapidly. Below 50 C., the reaction proceeds slowly, reduction does not proceed to the extent desired and accordingly low yields of the products sought are obtained. On the other hand, at 70 C. or above, low yields are likewise obtained due to the formation of large amounts of tar, traces of which also contaminate the desired reduction compounds resulting in the production of lowered yields of dark colored products. Accordingly, the spontaneous temperature rise is allowed to proceed until a temperature in the neighborhood of 60 C. is reached following which the temperature is maintained at this level plus or minus 5 C.) by the application of appropriate cooling means as required. After some two thirds of the reducing agent has been added, further cooling is unnecessary and should be discontinued. After all the reducing agent has been added, the temperature will gradually increase to about 70 C. and should be maintained at from 70 C. to the boiling point of the solution (about 73 C.) for a period of one hour following the last addition of reducing agent, heat being applied to the reaction mixture as necessary and stirring being continued.

After reduction is complete, ethyl acetate is removed by distillation. Due to the presence of large quantities of separated solids in the reaction mixture, it is preferable to agitate during the distillation operation, the reaction mixture being thinned if desired by addition of up to say an equal volume of water to facilitate proper stirring.

When removal of organic solvent is complete, the distillation residue is cooled to about room temperature and filtered, the resulting reddishbrown filter cake being washed with three or four times its volume of water, the filtrate and washings being discarded.

The washed filter cake is slurried into 2000 cc. of 10% hydrochloric acid and the whole is brought to a boil and filtered hot through a preheated filter. The insoluble material is again extracted by being boiled with a second 2000 cc. portion of 10% hydrochloric acid followed by filtration as before. The combined filtrates are cooled to room temperature and the separated DNAT hydrochloride is removed by filtration. The DNAT hydrochloride is slurried with three times its volume of water which results in hydrolysis of the hydrochloride with separation of the free base which is filtered, washed thoroughly with water and dried. Yellowish brown crystals, M. P. 1'71-1'71.5 C.

The dilute hydrochloric acid extracts from which the initial crop of DNAT has been removed are brought to pH 3 to 5 by addition of concentrated ammonia solution. A second crop of DNAT separates as the free base which is removed by filtration and thoroughly washed with water and dried. Amorphous yellow solid, M. P. above 0., usually l68-1'70 C. Total yield (both crops), 45-50% of theory.

The filtrate from which DNAT has been removed is treated with additional amounts of ammonia solution until the pI-I is 8. An orange red precipitate of DANT forms. The slurry is cooled to 20 C. or below with stirring and the DANT separated by filtration and washed with water. The DANT is frequently contaminated with a small amount of fiocculent green insoluble matter. This is easily separated by slurrying the mixture in water, allowing the comparatively massive DANT crystals to settle and decanting the still suspended impurity. The deep orange red DANT melts at 131-133" 0.; yield, 35-40%. On recrystallization from water, the melting point is increased to 134.0-1345" C.

DNAT is tasteless, odorless and burns rapidly with a bright yellow smoky flame. The material is insoluble in petroleum ether, practically insoluble in water, fairly soluble in alcohol, chloroform and benzene and very soluble in ethyl acetate and dioxane. This amine is soluble to the extent of 47 g. per liter in glacial acetic acid at 25 C. and 9.1 g. per liter in 10% sulfuric acid at the same temperature. Ten percent hydrochloric acid dissolves 47 g. per liter DNAT at 100 C. and 17 g. per liter at 40 C.

DANT melts in a flame without burning but on further heating ignites and burns with a luminous smoky flame. In the cold, the base is insoluble in heptane, slightl soluble in benzene and ethylene dichloride, fairly soluble in ethanol and very soluble in ethyl acetate. One liter of water dissolves about 8 g. at 20 C- DANT forms a monohydrochloride and a dihydrochloride; the former can be recrystallized unchanged from water but the dihydrochloride is partially converted to the monohydrochloride by this procedure. The dihydrochloride is very soluble in water containing a small amount of hydrochloric acid.

By proceeding in accordance with Example 1, the total amine yield is about 80-85% of theory and DNAT and DANT are produced in approximately equal quantities although slightly more of the monoamine forms. The ratio in which the two. compounds are formed can be varied extensively by changing the reducing agent-TNT ratio. As would be expected, increasing the proportion of the reducing agent enhances the production of the diamino compound. It has also been observed that when very low or very high ratios are employed to produce predominantly DNAT or DANT respectively, the total yield of reduction products is somewhat lower than when an intermediate ratio is used and the two amines formed in approximately equal amounts. All these facts can be deduced from the data presented in the table (Examples 26).

NOTE.The ratio employed in Example 5 is the same as that of Example 1.

To attain the objects of my invention it is essentia1 that the polynitro compound be highly dispersed or, preferably, partially or completely dissolved in the reaction medium. This is con..-

veniently accomplished through use of a solvent for the polynitro compound as set forth in Example 1. A large number of solvents are suitable for the purpose. The use or ethyl acetate has already been described. Ethanol is also suitable although recovery thereof after reaction requires a more accurate fractionation column than is necessary with ethyl acetate. Dioxane is reasonably satisfactory as a solvent but its use apparently promotes tar formation, especially if the reduction is-conducted at a slightly higher temperature than recommended.

While by the process of my invention, TNT can be reduced with the formation or DNAT and/or DANT in high yields, as has been mentioned previously, the sulfide reduction of TNT by classical procedures gives rise to no amines or, at best, very poor yields of amines. This is shown in Examples 7 to 11, wherein TNT was reduced with a variety of sulfide reducing agents in accordance with classical procedures.

Example 7 TNT, dissolved in a mixture of ethanol and ethyl acetate, was treated with an aqueous solution of sodium hydrosulfide. The pH of the reaction mixture was not controlled. On working up the reaction product, only a trace of acid soluble material was obtained. Acidification of the reaction liquor gave rise to brown nitrogen dioxide fumes indicating that degradation of nitro groups had occurred with formation of nitrite ions.

Example 8 A solution of TNT in alcohol or ethyl acetate was treated with an aqueous solution of sodium sulfide at 65 C. At the conclusion of the reaction, 40% of the TNT charge was recovered unchanged together with 47% by weight of a high melting, acid insoluble material of unknown structure. No DNAT or DANT could be isolated and the reaction liquor produced brown fumes on acidification.

Example 9 Attempts to reduce TNT in organic solution by addition of an aqueous solution of sodium disulfide were unsuccessful. No acid soluble reaction products were produced.

Example 10 rise to brown nitrogen dioxide fumes.

Example 11 Hydrogen sulfide was passed through a solution of two moles TNT in 1000 cc. dioxane containing 12 drops concentrated (28-29%) ammonia solution, the temperature being kept below 40 C. After the rate of absorption of the gas had become low, the above quantity of ammonia solution was again added following which hydrogen sulfide was passed through the reaction mixture as before. The sulfur formed was removed by filtration and, after evaporation of the solvent, a yield of an orange colored crude was obtained of which less than 50% was soluble in boiling 10% hydrochloric acid. A yield of DNAT of about 25% was obtained from the acid soluble material. On repeating the reaction at 50 60 0., the DNAT formed was contaminated with a black tar which could amine.

not be separated from the,

As the foregoing examples show, by the practice of my invention, TNT can be reduced with the formation of DNAT and/or DANT in high yields and the relative proportion of the two amines in the product can be varied at will over a wide range. On the other hand, the reduction of TNT in accordance with the various procedures of the prior art for the reduction of TNT or similar polynitro aromatic compounds usually results in the formation of no acid soluble product of any kind. In a few instances, prior art procedures result in the formation of amine reduction products but the yield is invariably low, only DNAT is formed and frequently this is contaminated with tars so as to render the amine unsuitable for use as a starting material for further synthesis.

While my invention has been described largely in connection with the selective partial reduction of TNT, it may be applied to a large variety of polynitro aromatic compounds among which may be mentioned symmetrical trinitrobenzene, 2,4,6- trinitrobenzoic acid, 2,4,6-trinitrophenylethyl alcohol, materials formed by the condensation of 2,4,S-trinitrophenylethyl alcohol with secondary amines such as diethanol amine, morpholine, etcetera, 2,4,6 trinitrostilbene, 2,4,6 trinitrostyrene, 2(2,4=,6'-trinitrophenyl) furan and the like. The process of my invention can also be applied to the selective reduction of dinitro aromatics such as m-dinitrobenzene, 2,6-dinitrotoluene, 2,4-dinitrotoluene, 2,4-dinitroaniline, 2,4- dinitrodiphenylamine and the like but fair to excellent methods for the selective reduction of these compounds are described in the prior art.

The amines produced in accordance with my invention are widely useful, for example, as intermediates in the preparation of sulfur colors and of azo colors of the pigment type, the insoluble dye type and the soluble dye type. DNAT,

for example, has great utility as the diazotizable component in the manufacture of azo colors. Due to the presence of two nitro groups on the ring, each in meta position to the amino group, colors made by reacting diazotized DNAT with a suitable coupling component are usually stronger I and deeper in shade than those obtained from diazotized p-toluidine or diazotized mononitro p-toluidines. Furthermore, pigment type or insoluble dye type azo colors made by reacting diazotized DNAT with a suitable coupling component are usually less soluble in oils and lacquer solvents than the corresponding colors obtained with diazotized p-toluidine or diazotized mononitro p-toluidines.

DANT is useful either as the diazotizable' or tetrazotiaable component or as the coupling component in the preparation of azo colors. Thus, DANT may be tetrazotized and coupled with two molecular equivalents of a single coupling component or, usually successively, with one molecular equivalent of each of two coupling. components. When a single coupling component is employed, both coupling reactions may occur at the same position on the coupling component or, with some coupling components, it is possible to so control reaction conditions that the two couplings proceed separately and successively and occur at different positions on the coupling component. Azo colors produced by reactin tetrazotized DANT with appropriate coupling agents are usually stronger and deeper in shade than the corresponding colors produced from tetrazotized m-phenylene diamine or tetrazotized 2,4-toluylene diamine. Also, the colors from tetrazotized DANT, if of the pigment or insoluble dye.type,

8 are usually less soluble in oils and lacquer solvents than corresponding colors obtained from the other diamines just mentioned. These benefical effects are attributed to a nitro group in DANT meta to both amino groups.

DANT is also an excellent coupling component, giving azo colors usually surpassing the corresponding colors obtained when using m-phenylene diamine or 2,4-toluylene diamine as the coupling component in strength, shade and (with pigment type or insoluble dye type colors) insolubility in oils and lacquer solvents. Obviously, DANT may be used both as the diazotizable or tetrazotizable component and the coupling component in forming an azo color. Thus, tetrazotized DANT may be coupled with one molecular equivalent of DANT following (or preceding) which the tetrazotized DANT is coupled with a molecular equivalent of a second coupling component. Or, if desired, the tetrazotized DANT may be coupled with two molecular equivalents of DANT, this obviously resulting in a color analogous in structure to Bismark brown.

DNAT and DANT, especially the latter, are of utility in producing sulfur colors of the immedial yellow type. Thus, DANT may be fused with sulfur to produce an orange yellow color. If desired, this color may be produced by fusing the crude reaction mixture (after removal of organic solvent and filtration but prior to extraction of amines therefrom) produced by reducing TNT with about six molecular equivalents of a sulfide reducing agent in accordance with the teachings of this invention (see Example 6).

Be it remembered, that while my invention has been described in connection with various specific details thereof, these are illustrative only and in no way limit the scope of my invention except as these may be included in the accompanying claims.

I claim:

1. A process for the selective reduction of polynitro monocyclic aromatic compounds carrying not more than three nitro groups on the benzene nucleus comprising adding an alkali metal sulfide to a solution of such a polynitro aromatic compound and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and belorv approximately pH 8.5.

2. A process for the selective reduction of polynitro monocyclic aromatic compounds carrying not more than three nitro groups on the benzene nucleus comprising adding an alkali metal sulfide to a solution of such a polynitro aromatic compound in the presence of an ammonium salt of a weak acid and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

3. A process for the selective reduction of polynitro monocyclic aromatic compounds carrying not more than three nitro groups on the benzene nucleus comprising the portionwise addition of an alkali metal sulfide to a solution of such a polynitro aromatic compound while maintaining the reaction mixture alkaline and below approximately pH 8.5 by additions of a strong acid thereto interspersedly with respect to said additions of reducing agent.

4. A process for the selective reduction of polynitro monocyclic aromatic compounds carrying not more than three nitro groups on the benzene nucleus comprising the portionwise addition. of an alkali metal sulfide to a solution of' such a polynitro aromatic compound in the presence of an ammonium salt of a weak acid while maintaining the reaction mixture alkaline and below approximately pI-I 8.5 by additions of a strong acid thereto interspersedly with respect to said additions of reducing agent.

5. A process for the selective reduction of 2,4,6-trinitrotoluene comprising adding an alkaline metal sulfide to a solution of 2,4,6-trinitrotoluene and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

6. A process for the selective reduction of 2,4,6-trinitrotoluene comprising adding an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene in the presence of an ammonium salt of a weak acid and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

'7. A process for the selective reduction of 2,4,6-trinitrotoluene comprising the portionwise addition of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene while maintaining the reaction mixture alkaline and below approximately pH 8.5 by addition of a strong acid thereto interspersedly with respect to said additions of reducing agent.

8. A process for the selective reduction of 2,4,6-trinitrotoluene comprising the portionwise addition of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene in the presence of an ammonium salt of a Weak acid while maintaining the reaction mixture alkaline and below approximately pH 8.5 by additions of a strong acid thereto interspersedly with respect to said additions of reducing agent.

9. A process for the preparation of 2,6-dinitro 4-amino toluene comprising adding approximately three molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

10. A process for the preparation of 2,6-dinitro 4-amino toluene comprising adding approximately three molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene in the presence of an ammonium salt of a weak acid and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

11. A process for the preparation of 2,6-dinitro 4-amino toluene comprising the portionwise addition of approximately three molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene while maintaining the reaction mixture alkaline and below approxi mately pH 8.5 by additions of a strong acid thereto interspersedly with respect to said additions of reducing agent.

12. A process for the preparation of 2,6-dinitro e-amino toluene comprising the portionwise addition of approximately three molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene in the presence of an ammonium salt of a weak acid while maintaining the reaction mixture alkaline and below approximately pH 8.5 by additions of a strong acid thereto interspersedly with respect to said additions of reducing agent.

13. A process for the preparation of 2,4-diamino B-nitrotoluene comprising adding approximately six molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitroluene and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

14. A process for the preparation of 2,4-diamino S-nitrotoluene comprising adding approximately six molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene in the presence of an ammonium salt of a weak acid and removing alkali produced by said reduction at the rate necessary to maintain the reaction mixture alkaline and below approximately pH 8.5.

15. A process for the preparation of 2,4-diamino 6-nitrotoluene comprising the portionwise addition of approximately six molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene while maintaining the reaction mixture alkaline and below approximately pH 8.5 by addition of a strong acid thereto interspersedly with respect to said addition of reducing agent.

16. A process for the preparation of 2,4-diamino G-nitrotoluene comprising the portionwise addition of approximately six molecular equivalents of an alkali metal sulfide to a solution of 2,4,6-trinitrotoluene in the presence of an ammonium salt of a weak acid while maintaining the reaction mixture alkaline and below approximately pH 8.5 by additions of a strong acid thereto interspersedly with respect to said additions of reducing agent.

EDMUND W. LOWE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,878,950 Lyford Sept. 20, 1932 2,464,194 Zimmerman Mar. 8, 1949 OTHER REFERENCES Tiemann, Berichte, vol. 3, p, 217 (1870).

Claims

1. A PROCESS FOR THE SELECTIVE REDUCTION OF POLYNITRO MONOCYCLIC AROMATIC COMPOUNDS CARRYING NOT MORE THAN THREE NITRO GROUPS ON THE BENZENE NUCLEUS COMPRISING ADDING AN ALKALI METAL SULFIDE TO A SOLUTION OF SUCH A POLYNITRO AROMATIC COMPOUND AND REMOVING ALKALI PRODUCED BY SAID REDUCTION AT THE RATE NECESSARY TO MAINTAIN THE REACTION MIXTURE ALKALINE AND BELOW APPROXIMATELY PH 8.5.