A -Nitrourea

There are four important groups of A-nitro compounds which are relevant to energetic materials synthesis. These are primary nitramines, secondary nitramines, secondary nitramides (including A-nitroureas and A, A -dinitroureas) and nitrimines. The synthesis and incorporation of these A-nitro functionalities into organic compounds is the focus of this chapter. 

Although a large number of secondary nitramides have been prepared they have not found wide use as explosives because of their facile hydrolysis to acidic primary nitramines in the presence of water. Research has focused on the synthesis of cyclic and bicyclic A-nitroureas and A, A -dinitroureas because of their high performance. 

Primary or secondary aminopyrazines may be converted directly into ureido- or thioureidopyrazines by treatment with isocyanates or isothiocyanates primary aminopyrazines may also be converted into such products indirectly via the corresponding isocyanato- or isothiocyanatopyrazines piperazines may be converted into 1/4-carbamoyl- or thiocarbamoylpiperazines by treatment with isocyanates, A-nitrourea, or isothiocyanates and aminopyrazines may be converted into guani-dinopyrazines by treatment with. S -methylisothiourcas or cyanamide. These processes (and some subsequent intramolecular cyclizations or other reactions) are illustrated in the following examples ... 

Although by our convention the following preparative method is not a nitration, we include it here for the sake of completeness, since it is used in the preparation of Nitraguanidine and Nitrourea... 

If an aqueous solution of nitrourea is boiled, nitrous oxide escapes and the cyanic acid which is produced escapes in part, polymerizes in part, and in part remains in the aqueous liquid where it may react with various substances which may be introduced. If aniline is added to a saturated... 

A fused heterocyclic compound (146) distantly related to the antiinflammatory agent cintazone (Chapter 12), which itself can be viewed as a cyclized derivative of phenylbutazone, retains the activity of the prototype, in the synthesis of 146, reaction of the nitroaniline 139 with phosgene gives intermediate 140, which is then reacted with ammonia to afford the substituted urea (141). Cyclization of the ortho nitrourea function by means of sodium hydroxide leads to the N-oxide (142) this last reaction represents... 

The nitrile group in 82 has been transformed into other versatile functional groups, and the derivatives so obtained have been used in the synthesis of various naturally occurring C-nucleosides and their analogs. Reduction of 82 with lithium aluminum hydride gave the amine 90 which was, in turn, transformed84 into the ureido and N-ni-troso derivatives (91-93) by treatment with nitrourea, followed by benzylation, and nitrosation.85 The diazo derivative 94, obtained by treatment of 93 with alcoholic potassium hydroxide, was a key intermediate in the synthesis of formycin B and oxoformycin B (see Section III,2,a,b). 

Synthesis of nitramines, nitramides and nitroureas via the nitrodesilylation of A/-trimethyisilyl compounds with dinitrogen pentoxide (ref. 122)... 

Davis studied the dehydration of urea nitrates as a route to iV-nitroureas. The nitrate salt of iV-methylurea undergoes dehydration-rearrangement on treatment with concentrated sulfuric acid to give Af-nitro-A -methylurea in 42 % yield. In this compound the nitro and methyl groups are attached to the same nitrogen and so its hydrolysis can provide a route to methylnitramine. In contrast, the nitrate salts of ethyl, n-propyl, n-butyl and n-amyl ureas, give iV-nitro-A -ethylurea (49 %), A -nitro-A -propylurea (60 %), iV-nitro-iV -butylurea (67 %) and iV-nitro-A -amylurea (67 %), respectively, on treatment with concentrated sulfuric acid. 

Li and co-workers recognised the potential of cyclic IV-nitroureas as energetic materials and reported the synthesis of 2,4,6,8-tetranitro-2,4,6,8-tetraazabicyclo[3.3.0]octane-3-one (109) (K-55) from the nitration of 2,4,6,8-tetraazabicyclo[3.3.0]octane-3-one dihydrochloride (108) with absolute nitric acid in acetic anhydride at room temperature the latter obtained from the condensation of lV,lVL(iiformyl-4,5-dihydroxyimidazolidine (107) with urea in aqueous hydrochloric acid. Pagoria and co-workers " " reported the synthesis of 2,4,6-trinitro-2,4,6,8-tetraazabicyclo[3.3.0]octane-3-one (110) (HK-55) in 72% yield from the nitration of (108) with 90 % nitric acid in acetic anhydride at subambient temperature (Table 5.3). HK-55 has a relatively high density (1.905 g/cm ) coupled with a low sensitivity to shock. 

V-nitramines too were reduced to disubstituted hydrazines. Electrolysis in 10% sulfuric acid over copper or lead cathodes reduced AT-nitrodimethylamine to. /V,JV-dimethylhydrazine (yield 69%), 7V-nitro-JV-methylaniline to N-methyl-JV-phenylhydrazine (yield 54%), W-nitropiperidine to JV,A -penta-methylenehydrazine (yield 52%) [734], and nitrourea to semicarbazide (yield 61-69%) [755]. 

Finally, urea nitrate can be taken one step further chemically. Through the process of dehydration (chemical removal of water), urea nitrate can be transformed into nitrourea (NH2CONH-NO2). Typically UNi is added to concentrated sulfuric acid to facilitate this reaction. In theory, the loss of water should make nitrourea more powerful than UNi. There is not a great deal of experimental data available about this material. It was theorized that the group that bombed the World Trade Center may have planned to conduct this reaction. No conclusive evidence exists to prove this speculation. To date, nitrourea has not been seen in any terrorist bombs. 

Helf (Ref 6) investigated ten hew materials as flash smoke suppressants in proplnts. Guanylurea Nitrate was the most promising compd with regard to physical chemical props, stability, and 0 balance. Guanylnitro Urea was unsuitable for proplnts contg NG. Carbohydrazide-N-carboxyamide Carbohy-drazide-N,N -dicarboxyamide showed desirable promise but depended on a special starting material. Ammonium Nitrourea, Diammonium -methylene-bis-(nitrosohydroxylamine) ... 

Figure A. 124 Nitrourea secondary high explosive, used in first World Trade Center terrorist incident.

In a 1.5-1. beaker (Note 1), 191 ml. (180 g.) of aqueous 25% dimethylamine solution (1.0 mole) is diluted with 64 ml. of water and treated with 116 g. (1.1 moles) of nitrourea.2 The temperature of the resulting brownish liquid rises spontaneously to 35-42°. The solution is warmed to 56-60°, and a reaction sets in vigorously with evolution of nitrous oxide. External cooling with water is applied when required the reaction temperature is maintained below 70° during the first 5-7 minutes and below 85° during the second period of 5-7 minutes. After a total of 10-15 minutes, the effervescence slackens and the reaction mixture is kept at 90-100° until the evolution of gas has completely ceased. This usually requires an additional 15-20 minutes. 

This substance is an intermediate product of the decomposition of the important explosive nitroguanidine. It is also present in an aqueous solution of nitrourea or a sulphuric acid solution of nitrourea (Davis and Blanchard [7]). 

Nitrourea, like nitroguanidine, is prepared by the action of sulphuric acid on urea nitrate. It was recommended as an explosive by Badische Anilin und Soda-Fabrik in 1915 [62] but without success, as it was not sufficiently stable. In the presence of water it decomposes at a little above 60°C with the evolution of nitrous oxide. 

The reaction is reversible since nitramine in aqueous solution combines with cyanic acid to reform nitrourea. Nitrourea is decomposed by gaseous ammonia, a reaction which, according to Watt and Makosky [63] proceeds as follows ... 

Urbanski, Kapuscinski and Wojciechowski [41] showed that nitrourea is a more powerful explosive than nitroguanidine. Its lead block expansion is 310 cm3. 

As a primary nitramine, nitrourea can form salts. The potassium, silver, mercuric (Thiele and Lachman [64]) and ammonium (Hantzsch and Wiegner [65]) salts are described in the literature. 

According to the patent listed as Ref 2, NUr alone or in a compn can be detond by a No 8 (strong) MF cap with great brisance. Typically, a compn consisting of NUr (83) and K nitrate (17%) is suggested which is claimed to be insensitive to ordinary percussion or heat. Davis (Ref 3) reports that NUr is a cool expl which he considers quite suitable for use in an expl or propint compn if it were not for the fact that it tends to decomp sponty in the presence of moisture. According to Urbanski (Ref 6) the Ag and Hg (ic) salts of NUr are more sensitive to impact than Nitrourea itself, however, they have no initiating properties... 

L.A.Burrows et al, USP 2376474(1945) CA 39, 3556(1945)(Nitrourea stabilized by blending small amts of non-volatile acid materials, such as oxalic acid, etc) 21)L.A.Burrows, USP 2387742(1945XExplosive rivets) 22)L.A. 

Solutions of nitrourea and nitroguanidine in concentrated sulfuric acid contain actual nitroamide, and these substances give up their nitro group nitrogen in the nitrometer. Nitroamide has been isolated 2 both from an aqueous solution of nitrourea and from a solution of the same substance in concentrated sulfuric acid. 

In water and in. hydrophilic solvents nitrourea dearranges rapidly into cyanic acid and nitroamide. Alkalis promote the reaction. If an aqueous solution of nitrourea is warmed, bubbles of nitrous oxide begin to come off at about 60°. If it is allowed to stand over night at room temperature, the nitrourea disappears completely and the liquid is found to be a solution of cyanic acid. Indeed, nitrourea is equivalent to cyanic acid for purposes of synthesis. It reacts with alcohols to form carbamic esters (urethanes) and with primary and second amines to form mono-and unsym-di-substituted ureas. 

Semicarbazide (Aminourea, Carbamylhydrazine or Hydrazine carboxamide). NHi.NH.CO.NHj mw 75.07 colorl prisms mp 96° d l,484g/cc at 4°. V sol in w sol in ethanol. Prepn is by electrolytic redn of nitrourea in sulfuric acid. The base Is then obtd by reaction of the semicarbazide sulfate with liq ammonia. CA Registry No [57-56-7]. See also in Vol 2, C45-L, under Carbazide for a definition of semicarbazide Semicarbazide forms salts which are either expl or, as in the case of the hydrochloride, are used in propint compns... 

Semicarbazide Hydrochloride (Amidourea hydrochloride). CHsON3.HC1 mw 111.52 N 37.69% OB to C02 —57.39% colorl prisms or snow-white crysts mp 173°, 175—77° (sep values, decompn). V sol in w sol in dQ ethanol insol in absol ethanol and eth, Prepn is by electrolytic redn of nitrourea with cathodes of Cu, Ni, Pb and Hg in hydrochloric acid soln (Refs 1 7). CA Registry No [56341-7]. The hydrochloride is used in rubber-Amm perchlorate solid proplnts in amounts of from 0.1 to 10% as a burning rate depressant. At 300psi chamber press the hydrochloride is reported to effect a 31% depression in the burning rate (Ref 6)... 

The white, finely divided precipitate of nitrourea is filtered upon a 12-cm. Buchner funnel. A hardened filter paper (Note 2) is used in this operation and the nitrourea is pressed as dry as possible. The product is washed in the funnel with four portions of cold water just sufficient to covfer it and pressed dry each time (Note 3). Then it is dried in the air (Note 4). By strongly cooling the filtrates, a further small amount of nitrourea may be obtained and added to the main portion. The yield of air-dried material is 120-150 g. (70-87 per cent of the theoretical amount). It melts with decomposition at 150-164° and is sufficiently pure for use in the preparation of semicarbazide (Note 5). 

Cracks appear when the nitrourea is filtered hence, care must be taken that the water used for washing does not go through the cracks instead of through the material. If desired, the nitrourea may be removed from the funnel, made into a paste with about 800 cc. of cold water, and again filtered. 

Various samples of nitrourea prepared in this way melt differently and therefore the melting point is not a suitable indication of purity. A uniform product can also be obtained by crystallizing half of the crude material obtained in one run from 1 1. of water at a temperature not over 550, and using the filtrate to crystallize the second half. Even this recrystallized material does not have a definite melting point. 

After the apparatus is assembled, 600 cc. of 20 per cent sulfuric acid (Note 3) is placed in the battery jar, and the lead anode in the porous cup is also covered with acid of the same strength. The cell is surrounded by an ice-salt bath, the stirrer is started, and, while the solution is cooling, 50 g. of nitrourea (Note 4) is added to the catholyte. A thermometer is placed in the catholyte, and when the temperature drops to +5° the current is turned on as follows with the rheostat set for maximum resistance, the current is switched on, then the rheostat is gradu-... 

---