Nitroform is a valuable compound for producing propellants and explosive compo- nents by its high oxygen content and an unstable hydrogen atom, which facilitates its forming of derivatives. Nitroform is used, for example, for preparing hydrazinium nitroformat (HNF) which is a salt between hydrazine and nitroform and is used as oxidiser in, inter alia, rocket propellants. The oxidiser is chlorine-free, which is increasingly desired in propellant compositions. Nitroform is also used as a starting material for producing energetic plasticisers.

The most well-known method for preparing nitroform is nitration of acetylene with nitric acid, using a mercury catalyst. An industrial process for preparing nitroform according to this method is described by A. Wetterholm : Tetrahedron, 1963, Vol. 19, pp 155-163. The use of acetylene gas as starting material requires specific measures of precaution in the process, and the use of a mercury catalyst has obvious drawbacks from the environmental point of view.

An object of the present invention is to provide a simple and safe method for pre- paring nitroform. According to the invention, a starting material dissolved or sus- pended in sulphuric acid is nitrated. The starting material is selected from a group consisting of 1) a gem-dinitroacetyl urea with the general formula wherein X is H or a group , wherein Y is an alkoxy or amino group, and salts thereof;

2) gem-dinitroacetyl guanidine, and 3) 4, 6-hydroxypyrimidine.

In nitration, a nitrating agent consisting of nitric acid, nitrate salts or nitrogen pentoxide is added to the sulphuric acid solution/suspension. It will thus be easy to control the nitration. Further nitro groups are added to the starting material in nitration, forming a gem-trinitro compound which by hydrolysis splits off nitroform.

The nitration is carried out at a temperature from-10°C to +80°C, preferably from +10°C to +60°C. The sulphuric acid may have a concentration of 70-100%, prefer- ably about 95%. The nitric acid, which is the preferred nitrating agent, is added as a concentrated acid, especially with a concentration of 85-100%. The molar ratio between nitrating agent and substrate may be 2.0-6. 0: 1, and is preferably 3.0-4. 0: 1.

Good yields have been obtained with such a low molar ratio as 3.0 : 1, which in com- bination with reuse of the sulphuric acid, as will be mentioned below, makes the method economically attractive. The nitration can be carried out in an ordinary stain- less steel reactor without any special arrangements.

After completion of the nitration, the product (the gem-trinitro compound) is hydro- lysed for splitting off nitroform by mixing the reaction mixture with an aqueous medium, for instance pouring it onto crushed ice, diluting it with water or the like. The nitroform prepared can be extracted from the reaction mixture using a polar extracting agent which is stable in the environment at issue, for instance methylene chloride or diethyl ether. A neutralising agent can then be added to precipitate the corresponding salt of nitroform. It is also possible to extract directly from the nitration mixture the primarily prepared gem-trinitro compound and subsequently hydrolyse the same for splitting off nitroform, for instance by adding a base. The sulphuric acid may then be used several times.

The starting material according to a) above can be produced by nitration of barbi- turic acid to gem-5,5-dinitrobarbituric acid which is then hydrolyse with water to form gem-dinitroacetyl urea or is treated with a ring-opening nucleophile, for instance ammonia, methanol, ethanol, isopropanol or an amine, to form substitut- ed gem-dinitroacetyl urea. This is described in Langlet et al : Synthesis and reactions of 5,5-dinitrobarbituric acid, Conference Proceedings, XIX : th European Colloquium on Heterocyclic Chemistry, Aveiro Portugal, July 19-22,2000, p. A-70. The starting material according to b) above can correspondingly be produced by nitration of 2-amino-4,6-dihydroxypyrimidine followed by hydrolysis with water, as described

in Latypov et al : A new convenient route to gem-dinitroalifatic compounds, Confe- rence Proceedings, 31st International Conference of ICT : 2000. The starting material according to c) is a commercially available product.

The invention will be described below by way of Examples.

Example 1 Potassium dinitroacetyl urea (6.8 g) was dissolved in sulphuric acid (20 ml, 95% concentration). Concentrated nitric acid (1.4 ml) was added by drops. The temperature was increased to about 40°C for one hour. The reaction mixture was poured onto 60 g crushed ice and then extracted with 2x50 ml diethyl ether. The ether phase was dried with sodium sulphate. Potassium hydroxide dissolved in ethanol was added. A yellow precipitate of potassium nitroform (4.6 g; 68% yield) was obtained.

Example 2 4,6-dihydroxypyrimidine (4 g) was dissolved in sulphuric acid (20 ml, 95% concen- tration). Fuming nitric acid was added (6 ml) during cooling with ice and was then kept at room temperature for 12 hours. The reaction mixture was poured onto crushed ice and extracted with methylene chloride. Yield of nitroform 60%, measured by UV spectroscopy in methylene chloride.

Example 3 Dinitroacetyl urea (3 g) was dissolved in sulphuric acid (20 ml, 95% concentration).

Concentrated nitric acid (0.57 ml) was added. The reaction mixture was stirred for one hour at 20°C. Then it was poured onto 77 g crushed ice, heated to 50°C for 10 minutes and extracted with 2 x 125 ml methylene chloride. Yield of nitroform 62%.

Example 4 Dinitroacetyl urea (3 g) was dissolved in sulphuric acid (20 ml, 95% concentration).

Concentrated nitric acid (0.57 ml) was added. The reaction mixture was stirred for one hour at 20°C with a cover (covering layer) consisting of 30 mi methylene chlo- ride. The methylene chloride was separated from the reaction mixture and mixed with 30 ml n-heptane. The mixture was evaporated to 20 mi and then trinitroacetyl urea precipitated. The trinitroactyl urea could now easily be hydrolyse with potassium hydroxide for splitting off potassium nitroform.