<center><font size="8">Nitroguanidine</font>
<font size="6">from Sulphamic Acid and Urea</font>
<b>By Axt</b></center>
Nitroguanidine is an explosive of some industrial and military significance, where its primary use is as an additive for propellants. The temperature
of explosion for nitroguanidine is low, and when added to nitrocellulose-nitroglycerine powders it has the effect of both reducing muzzle flash and
lowering erosion when compared to propellants of comparable power[1]. These are known as triple based propellants.
When used as a high explosive nitroguanidine detonates at a rate of 7650m/s at 1.55g/cm3 and its explosive power is relatively high, comparable to
TNT[1]. Nitroguanidine is insensitive, failing to detonate from a 3.20m drop of a 2.5kg weight where TNT will detonate with a drop of 1.48m.[2]. It
also has a low critical diametre of between 1.27-1.43mm[2].
While the toxicity of nitroguanidine is stated as being “high“ by Dobratz et al.[2], a study on its acute oral toxicity in mice by Hyatt et al.[3]
placed it into the category of being practically non-toxic with a dose of 5000mg/kg producing less then 50% mortality .
The preparation of guanidine by fusing sulphamic acid and urea was patented by Mackay[4] and the mechanism was later studied by Boivin[5,6]. While no
definite conclusions were obtained, the probable mechanism is shown in figure 1. The urea and sulphamic acid first fuse to form ammonium urea
sulphonate, which dissociates into cyanamide and ammonium bisulphate which reacts to form guanidine bisulphate.
<center><img src="http://www.sciencemadness.org/scipics/axt/guanidine-sulphate-mech.jpg">
<i>Figure 1: Preparation of guanidine bisulphate by fusing urea with sulphamic acid.</i></center>
Nitroguanidine is prepared by dissolving guanidine nitrate in cold concentrated sulphuric acid (figure 2), then precipitating by pouring into ice
water[7,8]. Though the yield obtained in this study was low, its an easy preparation and the precursors are both cheap and readily available.
<center><img src="http://www.sciencemadness.org/scipics/axt/nitroguanidine-mech.jpg">
<i>Figure 2: Preparation of nitroguanidine from guanidine nitrate.</i></center>
<font size="4"><b>Precursors:</b></font>
99% Sulphamic acid was sold in a hardware store as a descaler for removing calcium and rust deposits. Urea was also purchased from the hardware store
in a 15kg bag for use as a fertiliser. Potassium hydroxide is available from hydroponics suppliers for raising pH.
<font size="4"><b>Synthesis:</b></font>
200g sulphamic acid and 124g urea were mixed in a shallow Pyrex bowl and placed into a fan forced kitchen oven. The mixture was left in the oven at
220°C for 2 hours, where it gave off some ammonia and melted into a clear liquid gradually thickening and forming a white semi-solid due to ammonium
and guanidine sulphate separated of from the melt. This was then cooled and broken up to leave 302g of crude residue (figure 3).
The crude residue was then blended with 240g of potassium hydroxide in 1L of methylated spirits for 30 minutes (figure 3). This freed the guanidine
and ammonia bases and precipitated potassium sulphate, 750ml of this free based alcohol solution was recovered by filtering the blended mixture.
<center><img src="http://www.sciencemadness.org/scipics/axt/gn-prep.jpg">
<i>Figure 3: Crude fusion product (left); Blending with alcoholic KOH solution (centre); Crude ammonium and guanidine nitrate salts
(right).</i></center>
The alcoholic solution of guanidine and ammonia was then neutralised with 70% nitric acid, this precipitated residual potassium hydroxide as its
nitrate which was filtered out. On boiling down the alcohol to a concentrated solution, then evaporating dry, 84g of crude ammonium and guanidine
nitrates was obtained (figure 3).
50g of the crude mixture of nitrates was directly converted to nitroguanidine by slowly dissolving small portions into 75ml of 98% sulphuric acid, the
temperature was maintained from -5 to +10°C throughout the addition which took approximately 1.5 hours. The milky solution (figure 4) was left for 10
minutes at 10°C after the last addition then poured into 500ml of iced water. On standing the nitroguanidine precipitated as small crystals (figure
4) which were filtered and dried. Yield was 18.5g.
<center><img src="http://www.sciencemadness.org/scipics/axt/nq-nitration.jpg">
<i>Figure 4: Guanidine nitrate dissolved in sulphuric acid (left); precipitation of nitroguanidine from 500ml water
(right).</i></center>
The nitroguanidine was recrystallised from boiling water to remove any nitrourea that may have been present, on cooling a mass of fine needles
precipitated characteristic of nitroguanidine (figure 5).
<center><img src="http://www.sciencemadness.org/scipics/axt/nitroguanidine-crystals.jpg">
<i>Figure 5: 30x magnification of crystals of nitroguanidine from this study when crystallised from water (left); Nitroguanidine at 25x from the
literature[7] obtained from rapid cooling of a water solution.</i></center>
<font size="4"><b>Explosive Properties:</b></font>
The small fine needles obtained were pressed into a drinking straw, taped to a metal disk and initiated with 0.5g PETN under sand. The dent (figure 6)
shows that the nitroguanidine did detonate however with low power. This low observed power can be attributed to the very low density at which the
needles were pressed.
The nitroguanidine only deflagrated with difficulty when heated on a spoon over a gas flame, where it blackened, melted, emitted fumes then
deflagrated leaving little residue. It would not deflagrate when held in the flame of a match.
<center><img src="http://www.sciencemadness.org/scipics/axt/nq-plate.jpg">
<i>Figure 6: detonation of low density nitroguanidine against metal plate.</i></center>
<b>References:</b>
1] B.T. Fedoroff and O. E. Sheffield “Encyclopedia of Explosives and Related Items“. vol. 6; pg. G154. Picatinny Arsenal; New Jersey; USA (1960)
2] Dobratz, B and Crawford, P. "LLNL Explosives Handbook - Properties of Chemical Explosives and Explosive Simulants" Lawrence Livermore National
Laboratory. California. (1985)
3] G. F Hyatt, S. K Sano, C. R Wheeler and D. W Korte “Acute Oral Toxicity of Nitroguanidine in Mice”; Letterman Army Institute of research
Presidio of San Francisco, CA. (1984)
4] J. S. Mackay “Preparation of Guanidine Sulfates” US patent #2464247 (1949)
5] J. L. Boivin and A.L. Lovecy “Mechanism for the Formation of Guanidine from Urea and Ammonium Sulphamate” Canadian Journal of Chemistry; 33[7];
pg. 1222-1225; (1955)
6] J. L. Boivin and M. Tremblay “Synthesis of Guanidine from Urea, Ammonium Benzenesulphonate, and Ammonium Sulphamate” Canadian Journal of
Chemistry; 36[2]; pg. 378-382; (1958)
7] T. L. Davis “The Chemistry of Powder and Explosives” Pg. 380; New York, John Wiley & Sons; London, Chapman & Hall. (1941)
8] G. B. L. Smith, V. J. Sabetta, and 0. F. Steinbach “Quantitative Study of the Preparations of Guanidine Nitrate and Nitroguanidine”; Industrial
and Engineering Chemistry; 23[10]; pg. 1124-1129; (1931)
[Edited on 4-8-2007 by Axt] |
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