T H E J O U R N A L O F I N D U S T R I A L A N D ~ E N G I N E E R I N GC H E M I S T R Y
246
voi.
12,
xo.
-1
4
curves extended t o these two points by dotted lines (Fig. I). 26 24
F
22 20 18
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14 12
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l;r
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2 6 9 4 2
t h a t the treatment of organic nitro compounds with alkalies is bad practice, a clause in the Ordnance Department specifications for tetryl stated t h a t the compound must be stabilized by means of water washing alone. Up t o this time i t had been the custom of the du Pont Company t o remove the last traces of acidity and other impurities from tetryl by means of a dilute sodium carbonate boil. This method had been found very satisfactory and reduced materially the time required for stabilization. On account of the objections of the Ordnance Department t o the use of alkali, however, during the year 1 9 1 7 the du Pont Company carried out a t their Eastern Laboratory a thorough investigation of the stability of tetryl including the causes of instability in crude material and the effect of alkalies on these impurities and upon the tetryl itself. GENERAL METHODS O F PREPARATION
2 0
0
40
20
80
TEMPERATURE
100
'C.
MOST COMMON I M P U R I T I E S
FIG. I
The aim in the tests just described has not been t o attain the accuracy t h a t is necessary in proper and exact equilibrium determinations since neither time nor apparatus were available, but rather t o obtain results t h a t could be used in practice. It is safe t o assume, however, t h a t the figures given for the equilibrium concentration of potassium sulfate are very near the true value. The results obtained, therefore, indicate that the value given by D'Ans for 83' C. is high, or else t h a t the equilibrium curve has a maximum between 8 3 O and I O O O C. SUMMARY
Tests made t o determine the equilibrium concentration of potassium sulfate in solution in contact with solid gypsum and potassium penta-calcium sulfate on the one hand and solid syngenite and potassium penta-calcium sulfate on the other a t 100' C. indicate t h a t these concentrations are: I . 05 moles of KzS04 per 1003 moles H2O for the former and 9 . 2 6 moles of K2S04 per 1000 moles HzO for t h e latter equilibrium. The corresponding values for calcium sulfate are: 0.24 and 0. I O mole, respectively, per 1000 moles HzO.
THE STABILITY OF TETRYL' By C. L. Knowles EASTERN LABORATORY, E. I.
DU PONT
DE NBMOURS & CO.,
Tetryl may be prepared by a large number of methods, several of which are of commercial importance. The only one used t o any considerable extent, however, is t h a t depending on the nitration of dimethylaniline by means of sulfuric and nitric acids. The tetryl studied throughout our investigation was prepared in this manner.
CHBSTER, PA.
During the war, when tetryl was being manufactured in great quantities for use in high explosive shell, the question of its stability was almost continually under discussion. Due t o the general belief 1 Presented a t the 58th Meeting of the American Chemical Society, Philadelphia, Pa., September 2 t o 6, 1919.
The impurities most frequently found in tetryl may be divided into the following classes: first, lower or higher nitrated dimethylanilines, and second, impurities in the dimethylanilines used or compounds formed from the nitration of these impurities. The possibility of the presence of sodium picrate and picric acid should also be considered, since i t has long been known t h a t tetryl is hydrolyzed t o a slight extent t o picric acid by means of water alone. When sodium carbonate is used for stabilization, an appreciable quantity of sodium picrate may be formed. From a search of the literature we concluded t h a t some penta-nitrated material might be present in our tetryl, brought about by the substitution of a nitro group in the m-position on the benzene ring. I n most cases in which an aniline or phenol has the m-position substituted by a nitro group, this group is very mobile and the molecule consequently unstable. It was also found t h a t in exceptional cases dimethylaniline might be nitrated t o the penta stage in such a way t h a t the m-hydrogen would be replaced by a n NOz group. This tetranitrophenylmethylnitramine gives on hydrolysis with water trinitromethylnitramidophenol, the m-nitro group being replaced by the hydroxyl radical. On boiling samples of our tetryl with water in t h e laboratory for stabilization purposes, i t was noticed t h a t in most cases the wash water became deeply colored and often showed some acidity even after the acid had previously been completely removed from the tetryl sample. On evaporating these liquors nearly to dryness, a colorless crystalline compound was obtained which was easily identified as trinitromethylnitramidophenol, the hydroxyl group being in
Mar., 1920
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
the m-position t o the nitramido radical, m. p. 187' C. This indicated a t once the presence of tetranitrophenylmethylnitramine in our tetryl. Because of the relatively small quantities of material which could be obtained in this way, i t was thought advisable t o prepare a sample of the pure tetranitrophenylmethylnitramine-hereafter called m-nitrotetryl---and t o carefully determine its properties, stabiljty, etc. A few hundred grams of the material were prepared by nitrating dimethylaniline t o the mono-stage, separating the m- and $-isomers, and nitrating t h e former t o the penta-nitro derivative. The pure compound melts a t 146" C. On boiling with water, m-nitro-tetryl was hydrolyzed t o soluble trinitromethylnitramidophenol and this compound was found t o be identical with t h a t recovered from our tetryl wash waters on evaporation. I n a similar way sodium carbonate reacts with m-nitro-tetryl, and in this case not only is the m-nitro group replaced, but the entire methylnitramido radical is removed with the substitution of a second ONa group, giving the di-sodium salt of styphnic acid which is very soluble in aqueous solutions. I n the presence of moisture nitrous acid gas is evolved which is very objectionable for a high explosive compound. It is possible t h a t some m-nitro-tetryl in tetryl may be formed from nitration of the dimethylaniline, t h e group meta t o the side chain being substituted during the early stages of nitration. It was thought more probable, however, t h a t the chief source of m-nitrotetryl was the monomethylaniline present in the dimethylaniline used. I n order t o definitely determine this point, mixtures of methyl and dimethylanilines were nitrated t o tetryl. I t was found t h a t the per cent of m-nitro-tetryl formed varied directly as the monomethylaniline content of the mixture nitrated. Moreover, i t was found t h a t by our method of nitration, tetryl prepared from pure dimethylaniline contained no m-nitro-tetryl. C A U S E S OF INSTABILITY IN TETRYL
From the reactions of m-nitro-tetryl, we felt sure t h a t the instability of our crude tetryl was largely due t o the presence of this compound. Its mobile m-nitro group, and the ease of formation of nitrous acid would tend t o decrease the stability materially. Moreover, with water, it forms a soluble compound, developing a t once traces of acidity. However, in order t o determine accurately the effect of traces of m-nitrotetryl on the stability and sensitiveness of tetryl, mixtures were made up and tests for stability and sensitiveness made. I n making tests on a great many samples of tetryl, it was noticed t h a t the stability of the material, as shown by the ordinary stability tests depending on the coloration of a test paper, bore no relation t o the physical properties of the tetryl. The advisability of using tests of this sort for nitro-aromatic compounds has been questioned for some time. Especially in the case of the Abel heat test, which has attained a much wider use than i t deserves, the sensitiveness is so great t h a t the slightest trace of foreign material, moisture, or acidity serves t o discolor the test paper.
247
In order t o determine the relative Stability of various samples of tetryl and mixtures it was imperative t h a t a quantitative test be developed. For this purpose, the Obermuller or gas evolution test i n vucuo seemed t o offer some possibilities. It was carefully investigated and with slight alterations proved t o be of great value in determining quantitatively the gas evolved per hour by explosives a t various temperatures. A few runs with this modified apparatus were sufficient t o demonstrate t h a t the purity of tetryl is indicated by its melting point and bears no relation t o the heat test. 5 g. samples of tetryl in our Obermuller test apparatus gave from 14.0 t o 153 mm. increase in pressure per hour depending on the purity of the product. These samples showed melting points of 129.2' C. t o 121.8' C. I n order to determine the sensitiveness of tetryl and its mixtures, an ordinary small drop-test machine was employed. Pure tetryl shows a drop test of 4 oz. a t 14 in. The addition of I O per cent m-nitro-tetryl t o tetryl melting a t 1 2 8 . 6 ~was found t o lower the melting point t o 1 2 4 . 6 ~C. Smaller quantities caused corresponding depressions. Since it has already been shown t h a t the stability of tetryl is indicated by its melting point, the effect of m-nitro-tetryl on the stability is a t once shown. Moreover, the Abel heat test, although unreliable, indicates t h a t the presence of m-nitro-tetryl greatly lowers the stability of tetryl. When one per cent of h-nitro-tetryl is added t o a sample of tetryl having a heat test of 2 5 min., this is lowered t o 1 5 min., while the addition of I O per cent gives a mixture with a 3 min. heat test. Pure m-nitro-tetryl has a heat test of but one minute. The addition of small quantities of m-nitro-tetryl t o tetryl greatly increases the rate of decomposition under vacuum. m-Nitro-tetryl itself ilz vata4o gives a n increase in pressure of 588 mm. per hr., while ordinary commercial tetryl gives but 20-30 mm., showing t h a t a m-nitro-tetryl decomposes approximately 2 5 times as fast as pure tetryl. Since it has been found by previous practice t h a t tetryl could be easily purified by means of sodium carbonate, i t was thought advisable t o determine the effect of this reagent on m-nitro-tetryl and on tetryl itself. A small sample of pure m-nitro-tetryl went into solution a t once when boiled with dilute sodium carbonate. On evaporating and acidifying the liquor, colorless needles of styphnic acid, melting a t 1 7 4 ' ~ were obtained. The hydroxyl groups are in the I - and g-position, both the m-nitro and the methylnitramido groups having been hydrolyzed off. On account of the rapidity of this reaction and the solubility of the disodium salt of styphnic acid, this method of removing m-nitro-tetryl appears t o be almost ideal. As mentioned above, m-nitro-tetryl is converted by water alone into soluble trinitromethylnitramidophenol, but the reaction proceeds much more slowly than when sodium carbonate is used and is therefore less satisfactory for purifying tetryl.
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Having proved t h a t m-nitro-tetryl may be satisfactorily removed from tetryl by means of sodium carbonate, it appeared advisable to determine the effect of this alkali on tetryl itself. After a thorough laboratory investigation it was shown conclusively t h a t sodium picrate was the sole reaction product when sodium carbonate acted on tetryl. During the reaction the whole nitromethylamine radical is replaced by the ONa group, nitrous acid being evolved. As might be expected, this reaction takes place to a greater extent on boiling tetryl with caustic alkalies, sodium or potassium picrate being formed with the liberation of methylamine. With sodium carbonate solutions the rate of hydrolysis of tetryl is relatively slow. However, sufficient sodium picrate was prepared in this manner t o carefully study its properties and prove its identity. The picric acid after acidification and recrystallization melted a t 1 2 2 . 5' C. and showed a molecular weight of 1 2 9 , the theory for picric acid. D E T E C T I O N OF S O D I U M P I C R A T E I N T E T R Y L
Since sodium picrate is very soluble in water, i t appears t h a t any of this compound formed would be easily removed by the subsequent water washes after neutralization. To settle this point, a sensitive test for the determination of small quantit:es of sodium picrate in tetryl was developed and i t was found t h a t none:of this product could be detected in our finished tetryl nor in our second wash water after the alkali boil. This indicated t h a t any sodium picrate formed was removed by the first water wash. E F F E C T OF SODIUM P I C R A T E O N S T A B I L I T Y O F T E T R Y L
Since sodium picrate is a relatively stable explosive, i t is difficult t o conceive of its lowering the stability of tetryl and its complete removal seems of little importance. Varying mixtures of tetryl and sodium picrate have been carefully made up and the stability and sensitiveness determined. Even large quantities of sodium picrate were found t o affect the tetryl but slightly. The addition of I t o I O per cent sodium picrate t o tetryl has no effect on the drop test, the mixture giving a detonation in each case with a 4 oz. weight a t 14 in., the drop test for pure tetryl. Pure sodium picrate cannot be detonated with an 8 oz. weight a t 24 in. The addition of I O per cent of picrate to tetryl was found t o lower the melting point by only 0.6' C., while the presence of smaller amounts could hardly be detected from t h c drop in melting point, indicating t h a t the stability of the material was not impaired. ILloreover, the stability of mixtures of sodium picrate and tetryl gave approximately the same gas evolution test as pure tetryl. The decomposition in vacuo of a mixture containing I O per cent sodium picrate was less than t h a t for ordinary crude tetryl. CONCLUSIONS
It is hoped t h a t this discussion of the stability of tetryl has brought out the following points, most of
Vol.
12,
No. 3
which have been under more or less discussion since the outbreak of the war. I-As a n indication of t h e stability of tetryl, ordinary qualitative heat tests are of little value. 11-The Obermuller or gas evolution test in vacuo shows the true stability of tetryl t o be indicated by the melting point, t o which it is directly proportional. 111-The instability of ordinary tetryl is due t o a large extent t o the presence of tetranitrophenylmethylnitramine (m-nitro-tetryl). This impurity is formed by nitration of the monomethylaniline present in the dimethylaniline used. IV-The nitration of pure dimethylaniline under ordinary conditions gives no m-nitro-tetryl. V-m-Nitro-tetryl is unstable and its presence greatly decreases the stability of tetryl. VI-m-Nitro-tetryl may be removed from tetryl by boiling with water, by which i t is converted into soluble trinitromethylnitramidophenol, and more readily by a sodium carbonate solution, whereby the soluble sodium salt of styphnic acid (trinitroresorcinol) is formed. VII-Tetryl is slightly hydrolyzed by a dilute sodium carbonate solution t o sodium picrate. VIII-Sodium picrate is insensitive and stable and its presence does not lower the stability of tetryl t o any appreciable degree. IX-Sodium carbonate is a very satisfactory stabilizing agent for tetryl. It removes acidity and such impurities as m-nitro-tetryl. Any sodium picrate formed and not removed by washing is non-injurious t o the tetryl. THE MANUFACTURE OF TRINITROXYLENE FOR USE AS A SUBSTITUTE FOR TNT IN BURSTING CHARGES FOR HIGH EXPLOSIVE SHELL' By John Marshall EASTSRN LABORATORY, E. I . DU FONT DE
NEMOURS & CO.,
CHESTER,
PA.
From its physical properties trinitroxylene, or T N X , would appear t o be most unfavorable for use as the bursting charge for high explosive shell. The melting point of the predominating isomer is very high, 182' C., making i t impossible t o load by direct casting. Moreover, this compound is almost insoluble in other lower melting nitro bodies a t low temperatures such as are preferable for casting, seemingly precluding the casting of such a mixture. It has not more t h a n 80 per cent the explosive strength of T N T , and it is much less sensitive t o detonation. The acute situation which developed in the T N T supply in the summer of 1917, however, made i t necessary t o consider the use even of materials which appeared unfavorable. About this time i t was suggested by a member of the du Pont Company's chemical department t h a t T N X might be used to advantage as a partial substitute for T N T b y casting a mixture of T N X suspended in molten T N T . Experiments were immediately started a t the Eastern Laboratory for the preparation of a few pounds of T N X for experimental purposes. The nitration process 1 Presented a t the 58th &Meetingo f the American Chemical Society, Philadelphia, Pa., September 2 to 6, 1919.
