GB2053191A - Preparation of 2,2', 4,4', 6,6', hexanitrostilbene - Google Patents

Abstract

2,2' 4,4', 6,6'- Hexanitrostilbene is synthesised by reacting 2,4,6 - trinitrotoluene and sodium hypochlorite in an aqueous organic solvent in the presence of an additive, an alkaline earth metal oxide, hydroxide or carbonate. The preferred additive is calcium oxide, whilst the preferred organic solvents are either methanol alone or a mixture of methanol and tetrahydrofuran. The reaction is allowed to proceed for between 1 and 70 hours at a reaction temperature of between 8 DEG and 30 DEG , both the reaction time and temperature depending on the organic solvent chosen. The product is isolated by standard methods in up to 47% yield. The addition of an alkaline earth metal oxide, hydroxide or carbonate to the reaction mixture removes the need for the critical control of the reaction conditions (eg pH, temperature) that is necessary in the previous syntheses of hexanitrostilbene.

Description

SPECIFICATION Production of hexanitrostilbene The present invention relates to the production of 2, 2', 4, 4', 6, 6' -hexanitrostilbene.

2, 2', 4, 4', 6, 6' - Hexanitrostilbene (HNS) has been used as an explosive, but is especially useful as a crystal-modifying additive in melt-cast trinitrotoluene charges.

U.S. Patent No. 3,505,413 describes a process for the production of HNS which comprised reacting in an organic solvent, 2, 4, 6 - trinitrotoluene (TNT) with an alkali metal or alkaline earth metal hypochlorite.

The yield of HNS obtained by this method was typically between 35 to 40%. It has since been reported that increased yields of HNS (about 50%) may be achieved if, subsequent to the initial mixing of TNT and hypochlorite, either an amine is added to the reaction mixture (U.K.Patent No. 1,513,221) or the pH of the reaction mixture is controlled (U.K.Pa tentAppln.No. 9077S78).

The main problems with the previous processes for the production of HNS are, that in order to achieve moderate to high yields (generally above 30%).

the reaction conditions must be critically controlled, and ii) the reaction solvents must contain a proportion of expensive ethereal solvents, normally tetrahydrofuran (THF), which, despite the use of the best available solvent recovery techniques such as in UK PatentAppin. No. 9909 78, still contributes a major part of the total cost of producing HNS.

It has now been found that the incorporation of alkaline earth metal compounds, especially oxides, hydroxides or carbonates into any suitable solvent, either as a slurry or as a saturated solution, eliminates the need for critical control of the reaction conditions in HNS production.

It has further been found that the formation of HNS can be effected in moderate yield in lower aliphatic alcohols, particularly methanol, in the absence of ethereal solvents, either with or without the addition of alkaline earth metal oxides, hydroxides or carbonates.

According to the present invention a process for the production of 2, 2', 4, 4', 6, 6' - hexanitrostilbene comprises reacting, in an aqueous organic solvent, 2,4,6 - trinitrotoluene and either an alkali metal or alkaline earth metal hypochlorite in the presence of an alkaline earth metal compound which is an oxide, hydroxide or carbonate. The process is normally carried out by adding an aqueous solution of hypochlorite to a solution of TNT in an aqueous organic or organic solvent.

The addition of the alkaline earth metal oxide, hydroxide or carbonate additive to the reaction mixture can be achieved in various ways. For example, a dispersion of the additive in an aqueous solution of hypochlorite can be added to the solution of TNT, alternatively, the additive in powdered form, can be added directly to the TNT containing solution, before or after the addition of the hypochlorite. In another embodiment the additive can be added as a saturated solution in the solution of TNT, however, the low solubility of CaO in either water or organic solvents limits the quantity of CaO than can be introduced in this way.

The alkaline earth metal oxides, hydroxides or carbonates that may be useful as additives include strontium oxide, barium oxide, calcium hydroxide and calcium carbonate. However, in the preferred process according to this invention, calcium oxide is added. The ratio of moles of additive per mole of TNT may vary over a wide range but is preferably between 0.4 and 3.2. For economy, amounts in the lower end of this range will normally be used, and even smaller proportions of 0.2 moles or less may provide useful results. The mode of action of these additives is not fully understood, and the invention is not limited by any specific explanation. However, spectroscopic studies show that the characteristic red-coloured intermediate species formed by the reaction of base with TNT are stabilised in the presence of these additives.Chromatographic analyses of the products formed in the presence of these additives also shows that the number of major by-products is considerably reduced compared to the number produced in the absence of additives.

Consequently, in the presence of additives HNS forms a greater percentage of the total crude product obtained by filtration than it does in their absence.

These facts suggest that the additives complex with and stabilise the key intermediates which lead to the formation of HNS.

The reactants should be mixed at a temperature in the range -5 to 65-C depending on the solvent used and, at least during the initial exothermic reaction the temperature should be controlled, for example by cooling coils, preferably within the range 8" to 30"C. The hypochlorite solution can either be added rapidly or slowly to the TNT solution. Once the reactants have been mixed, the reaction mixture is allowed to stand for a period of generally 1 to 70 hours depending on the solvent used. Temperature control is less critical at this stage.

The aqueous component of the aqueous organic solvent may be entirely provided by an aqueous hypochlorite solution or may be at least partialiy provided by an aqueous organic solution of TNT.

The optimum total water content will depend on the nature of the organic component.

The organic component may be a lower aliphatic alcohol, particularly methanol, either alone or mixed with one or more of THF, dioxan, triglyme, dimethylformamide, acetonitrile or dimethyl sulphoxide. One advantageous organic component comprises a mixture, especially a 2 to 1 (by volume) mixture of THF and methanol. In this case the water content should preferably be at least 35% and not more than 50% by volume and the reaction should preferably be conducted at 10 to 20'C and the standing period should be considerably longer than would be used in the absence of additive, preferably between 10 and 25 hr.

A further advantageous organic component of the solvent is methanol alone. In this case the temperature should be above 8"C, especially 10 to 30"C. The water content should be between 10% and 45% by volume, preferably 13%-20%, and the standing time may be considerably lower, typically 1 to 3 hours.

It is interesting to note that the production of HNS can also proceed in the absence of any additive when methanol is the organic component of the solvent. Under these conditions however HNS is obtained in only low to moderate yields, the major product from such reaction usually being 2,2',4,4',6,6' - hexanitrobibenzyl.

The yield of HNS is generally higher, both in the presence and absence of additive, when the aqueous hypochlorite solution is added dropwise, rather than rapidly, to the methanolic solution of TNT.

In all the above reactions sodium hypochlorite is preferably used as the hypochlorite reagent and the solution preferably contains from 2% weight/volume i.e. (2g.per 100 ml.solution) to 10% of available chlorine.

Once the chosen reaction period has elapsed, the reaction may optionally be quenched by the addition of mineral acid to the reactor. The choice of mineral acid is determined by its ability to form, with the added base, salts that are soluble in the aqueous organic solvent. Generally hydrochloric acid will be suitable.

The suspension obtained is filtered to separate the product which may be washed with, consecutively, water, aqueous acetone and acetone, or, water followed by acetone, or aqueous acetone alone to remove impurities. Yields of up to about 47% of isolated, purified HNS are obtainable by this process when the organic components of the aqueous organic solvent is a 2 to 1 (v/v) mixture of THF and methanol respectively. When the organic component chosen is methanol yields of about 20% of isolated, purified HNS are found. Thus useful yields may be obtained in relatively cheap solvents and without external control over pH. In addition, as mentioned previously, product mixtures obtained in the presence of alkaline earth metal oxides, hydroxides or carbonates appear to contain, in addition to HNS, less and fewer major impurities than product mixtures obtained in the absence of additives.

Further, chromatographic analysis shows that the production of less and fewer major impurities is more marked when THF - methanol (2-1) in the organic component of the solvent than when methanol alone is the organic component.

Specific processes in accordance with the invention will now be further described by way of example, with particular reference to the accompanying drawings in which Figure lisa high pressure liquid chromatographic (HPLC) analysis of the crude product obtained in the presence of calcium oxide in THF/Methanol and Figure 2 is a HPLC anlaysis of the crude product obtained in the absence of any alkaline earth metal oxide, hydroxide or carbonate additive in THF/methanol.

Example 1 5g. ofTNTwasdissolved in a mixture of 50ml.THF and 25ml. methanol. 1 g. of calcium oxide powder was added to this solution which was subsequently cooled to OOC, (these weights are equivalent ot 0.8 mols of CaO per mole of TNT). After about 5 min 50ml. of a "5%" sodium hypochlorite solution (containing 5% w/v of available chlorine), which had been previously treated with conic. HCI in order to bring its pH within the range 11.5 to 12.0 and had been precooled to OOC, was added rapidly to the stirred slurry. Sufficient cooling was maintained during this addition, such that the reaction temperature remained below 15"C. Once the main exotherm was over, the cooling means was removed and the mixture was allowed to warm to ambient temperature.

Stirring was continued for 24 hr. and then the reaction was quenched by the addition of about 500ml. of dilute (0. 5M) hydrochloric acid. The solid product was collected by filtration, washed consecutively by water and acetone, and then dried and weighed. The yield of isolated HNS was 43.9%, m.pt.

= 322"C.

Example 2 A procedure similar to that described in Example 1 was carried out, except that stirring was not continuous. In this case the reaction mixture was stirred for only 5 min. after the addition of sodium hypochlorite and then for two further 15 sec. intervals, at 5hr. and 21 hr. after said addition. The yield of HNS was 46.9%, m.pt = 318-321 "C.

Example 3 A procedure similar to that described in Example 1 was carried out except that the calcium oxide was not added as a powder to the THF/methanol solution of TNT, but rather a saturated solution of calcium oxide in THF/methanol was first prepared (by stirring CaO in the said solvent and then removing any solid by filtration) and then TNT was dissolved in this CaO saturated solution, and reacted with the hypochlorite for 24 hr. The yield of HNS by this method was 40.8%, m.pt. = 316-318"C.

Example 4 A procedure similar to that described in Example 1 was carried out, except that the period between the addition of the sodium hypochlorite and the addition of the hydrochloricacid was 70 hr. The isolated yield of HNS was 46%, m.pt. = 320-322"C.

Example 5 A procedure similar to that described in Example 1 was carried out, except that the period between the addition of the sodium hypochlorite and the addition of the hydrochloric acid was 2 hr. The isolated yield of HNS was 39.4%, m.pt. = 318-324 C.

Example 6 A procedure similar to that described in Example 1 was carried out, except that the period between the addition of the sodium hypochlorite and the addition of the hydrochloric acid was 2 hr. and the mole ratio of CaO to TNT was 0.4. The isolated yield of HNS was 35.8%, m.pt. = 317-319"C.

Example 7 0.5g. of TNTwas dissolved in a mixture of 5ml.THF and 2.5ml. methanol. 0.059. of calcium oxide (0.4 moles per mole of TNT) was added and the solution cooled to 0 C. 5ml. of "5%" sodium hypochlorite solution (which had been treated with conc. HC1 as in Example 1) was cooled to 0 C, and then added rapidly to the TNT solution with rapid stirring.

Sufficient cooling was maintained during this addition, such that the reaction temperature remained below 15"C. Once the main exotherm was over, the cooling means was removed and the mixture was allowed to warm to ambient temperature.

After 2 hr. the reaction was quenched by the addition of 50ml.approx 1 .0M HCI. The crude product was filtered off, washed with water only and then dried under reduced pressure. The crude product was not purified further but rather was analysed by high pressure liquid chromatography (HPLC) to ascertain its percentage content of HNS.

HPLC analysis of the crude product indicated the presence of 65.3% HNS in the solid material. This corresponded to a mole % yield of 54.7% HNS. (A typical HPLC trace for the crude product obtained by this method is shown at Figure 1).

Example 8 (comparative) 0. 5g. TNT was dissolved in a mixture of MeOH (2.5 ml.) and THF (5ml.). The solution was cooled to 0 C.

5ml. of "5%" sodium hypochlorite solution (which had been treated with conc. HCI as in Example 1) was cooled to 0 C and added rapidly, with stirring, to the TNT solution. Sufficient cooling was maintained during this addition, such that the reaction temperature remained below 1 5"C. Once the main exotherm was over, the cooling means was removed and the mixture was allowed to warm to ambient temperature. After 2 hr. the reaction was quenched by pouring the mixture into about 50 ml. of 1 .OM HCI.

The crude product was filtered off, washed with water, dried under reduced pressure and analysed by HPLC to ascertain its percentage content of HNS.

HPLC analysis of the crude product indicated the presence of 25% HNS in the solid material. This corresponded to a mole %yield of 19.9% HNS. (A typical HPLC trace for the crude product obtained by this method is shown at Figure 2).

Example 9 The procedure similar to that described in Example 1 was carried out, except that calcium hydroxide (in the mole ratio of 0.6 moles per mole of TNT) replaced calcium oxide as the additive. The yield of HNS was 45.8%, m.pt. = 322-324"C.

Example 10 A procedure similar to that described in Example 1 was carried out except that calcium carbonate (in the mole ratio of 0.5 moles per mole of TNT) replaced calcium oxide as the additive and the period between the addition of the sodium hypochlorite and the addition of the hydrochloric acid was reduced to 6.5 hr. The yield of HNS was 38.9%, m.pt. = 316-318"C.

Example 11 1.0g. of powdered calcium oxide was dispersed in a solution of 5g. of TNT in 150 ml. of methanol. To this cooled (10"C) stirred solution was added rapidly 35 ml. of a "7%" sodium hypochlorite solution (containing 7% (w/v) of free chlorine), in which the excess alkali had been neutralised to between pH 11.5 and 12.0, and which was also cooled to 100C.

The rate of cooling of the TNT solution and the rate of addition of the hypochlorite solution were such that the temperature of the TNT solution was maintained below 16"C during the initial exothermic reaction. Once the exotherm had subsided, cooling was removed and the solution was allowed to warm to ambient temperature. The reaction was then allowed to proceed; with continuous stirring, for 2hr.

It was then quenched by adding 500ml. of 1.OM hydrochloric acid. After the mixture had been allowed to stand for 18 hr. The crude product was collected by filtration. It was then washed, consecutively with water, aqueous acetone (25% H20 - 75% Acetone) and acetone, and then dried under reduced pressure to afford HNS in 17.4% yield, m.pt. > 335 C.

Example 12 A procedure described in Example 11 was followed except that the reaction was quenched by the addition of 10 ml. of concentrated hydrochloric acid.

The yield of HNS was 18%, m.pt. > 332"C.

Example 13 1 .0g. of powdered calcium oxide was dispersed in a solution of 59. of TNT in 150 ml. of methanol. To this cooled (10 C), stirred solution was added dropwise 100ml. of a sodium hypochlorite solution (containing 2.5% (w/v) of free chlorine), in which the excess alkali had been neutralised to pH 11.5 to 12.0, and which was also cooled to 1 0"C. The rate of addition was maintained so as to keep the temperature of the reaction mixture in the range 10" to 120C.

Once the addition was completed, about 10 mins., the reaction mixture was maintained at 12"C, with continuous stirring for 2hr. The HNS was isolated in 19% yield, m.pt. = 3100C, by the method described in Example 11.

Example 14 59. ofTNTwasdissolved in 150ml. of methanol and cooled to 100C. 1 .0g. of powdered calcium oxide was dispersed in 50ml. of sodium hypochlorite solution (containing 5% (w/v) of free chlorine) which had been previously treated with conc. HCI in order to bring its pH within the range 11.5 to 12.0. This slurry was then cooled to 1 0"C and added dropwise to the stirred solution of TNT. The rate of addition was maintained so as to keep the temperature of the reaction mixture in the range 10 to 12"C. Once the addition was completed, about 10 mins., the reaction mixture was maintained at 10"C,with continuous stirring, for 1.5 hr. The HNS was isolated in 13.8% yield, m.pt. = 330"C, by the method described in Example 11.

Example 15 The procedure described in Example 14 was followed except that the crude product was washed with water only and analysed by High Pressure Liquid Chromatography. HPLC analysis indicated the presence of 41.2% HNS in the crude material which corresponded to a mole % yield of 39.4% HNS.

Example 16 5g. ofTNTwas dissolved in 150 ml. of methanol and the solution cooled to 1 0"C. 50ml. of "5%" sodium hypochlorite (containing 5% (w/v) of free chlorine), which had been previously treated with conc.HC1 as in Example 14, was cooled to 10"C. The hypochlorite solution was added dropwise to the TNT solution whilst maintaining the temperature in the region 10 to 1 20C. Once the addition was compiete (about 14 mins.) the mixture was maintained at 1 00C for 1.5 hr. and then quenched by the addition of about 250ml. of 1 .OM HCI. The crude product was filtered off and washed with, consecutively, water, aqueous acetone (25% H2O: 75% Acetone (v/v)) and acetone. The purified product was dried under reduced pressure to yield 12% HNS m.pt. = 319"C.

Example 17 The procedure described in Example 16 was followed except that the crude material was washed with water only and analysed by High Pressure Liquid Chromatography. HPLC analysis of the crude material indicated the presence of 33.5% HNS which corresponded to a mole % yield of 28.6% HNS.

Claims (27)

CLAIMS.

1. A process for the production of 2,2',4,4',6,6' hexanitrostilbene comprising reacting, in an aqueous organic solvent, 2,4, 6 - trinitrotoluene and either an alkali metal or alkaline earth metal hypoch lorite in the presence of an alkaline earth metal compound which is an oxide, hydroxide or car bonate.

2. A process according to Claim 1 wherein the alkaline earth metal compound is calcium oxide.

3. A process according to Claim 1 wherein the alkaline earth metal compound is strontium oxide, barium oxide, calcium hydroxide or calcium car bonate.

4. A process according to any preceding claim wherein the mole ratio of alkaline earth metal compound to 2,4, 6 - trinitrotoluene is between about 0.2 and 3.2.

5. A process according to Claim 4 wherein the mole ratio is between 0.4 and 3.2.

6. A process according to any preceding Claim wherein an aqueous solution of alkali metal or alkaline earth metal hypochlorite is added to a solution of 2,4, 6 - trinitrotoluene in an organic or aqueous organic solvent.

7. A process according to Claim 6 wherein the alkaline earth metal compound is introduced by dispersing it in the solution of hypochlorite.

8. A process according to Claim 6 wherein the alkaline earth metal compound is introduced by adding it in powdered form to the solution of trinitrotoluene either before or immediately after mixing said solution with said hypochlorite.

9. A process according to Claim 6 wherein the alkaline earth metal compound is introduced as a saturated solution in the solution of trinitrotoluene.

10. A process according to any preceding Claim wherein the organic component of the aqueous organic solvent comprises a lower aliphatic alcohol.

11. A process according to Claim 10 wherein the lower aliphatic alcohol is methanol.

12. A process according to either Claim 10 or 11 wherein the water content of the aqueous organic solvent is between about 10% and 45%, by volume.

13.A process according to Claim 12 wherein the water content is between about 13% and 20%, by volume.

14. A process according to any one of Claims 10 to 13 wherein the temperature of the reaction mixture is kept between about 8" and 30"C.

15. A process according to any one of Claims 10 to 14 wherein the reaction is allowed to proceed for between about 1 and 3 hours.

16. A process according to either Claim 10 or 11 wherein the organic component of the aqueous organic solvent further comprises at least one of tetrahydrofuran, dioxan, triglyme, dimethylformamide, acetonitrile and dimethyl sulphoxide.

17. A process according to Claim 16 wherein the organic component comprises methanol and tetrahydrofuran in a ratio of methanol to tetrahydrofuran of about 1 to 2, by volume.

18. A process according to either Claim 16 or 17 wherein the water content of the aqueous organic solvent is between about 35% and 50% by volume.

19. A process according to any one of Claims 16 to 18 wherein the temperature of the reaction mixture is kept between about 10" and 20"C.

20. A process according to any one of Claims 16 to 19 wherein the reaction is allowed to proceed for between about 10 and 25 hours.

21. A process according to any preceding Claim wherein the alkali metal or alkaline earth metal hypochlorite comprises sodium hypochlorite.

22. A process according to any preceding claim wherein the concentration ofthe alkali or alkaline earth metal hypochlorite is between about 2% and 10% (weight/volume) of available chlorine.

23. A process according to any preceding Claim wherein the reaction is quenched by mineral acid.

24. A process according to Claim 23 wherein the mineral acid is hydrochloric acid.

25. A process substantially as hereinbefore described with particular reference to any one of Examples 1 to 7 and 9 to 15.

26. 2,2', 4,4', 6,6' - Hexanitrostilbene when prepared by the process of any preceding Claim.

27. 2,2', 4,4', 6,6' - Hexanitrostilbene substantially as hereinbefore described with particular referpence to either of the Figures.