Dicyandiamide nitrate & nitrodicyandiamide
I recently stumbled across a reference to cyanoguanidine nitrate in “The Encyclopaedia of Explosives and Related Items PATR 2007” and was so
intrigued by it I tracked down the original patent and decided to try it out. The entry below this one was a paragraph on “nitrocyanoguanidine”
which sounded even more interesting but its text goes on to state that no preparation of this compound could be found in the literature. However,
since sulphuric acid converts guanidine nitrate into nitroguanidine this was the obvious first route to try in spite of the potentially labile nature
of the cyano-group under such conditions. The patent, US2537850, claims that when cyanoguanidine (=dicyandiamide) is stirred into a twofold excess of
strong nitric acid dilute with water the corresponding nitrate salt is formed in spite of it being a rather weak base. Not surprisingly the nitrate
salt can’t be recrystallized as it clearly requires the presence of excess nitric acid to stabilize it. It has no melting point as it decomposes
rather vigorously but it does have, allegedly, a characteristic crystal habit. The following preparation is based on this patent’s procedure.
Experimental
Dicyandiamide nitrate
21.02g of dicyandiamide were suspended in 45ml of water and 33ml of white 68-70% nitric acid slowly added at room temperature. The addition took about
5 minutes and the temperature rose from 14° to 28°C but this is close to the 25°C prescribed in the patent.
As the nitric acid was added to the stirred suspension the dicyandiamide began to dissolve and then after the addition of about 15ml of nitric acid a
new, finer grained compound began to precipitate. When the addition was complete the slurry was stirred for a further 20 minutes then chilled to 0°
and vacuum filtered. The aqueous filtrate was isolated and then the filter cake was washed with 40ml of acetone, sucked as dry as possible and then
air dried to give 24.43g of white crystalline granules (66.5% of theory). According to the original patent this material can’t be recrystallized and
it decomposes vigorously before melting.
Attempt to prepare nitrodicyandiamide (nitrocyanoguanidine)
This experiment was based on the well-known preparation of nitroguanidine from guanidine nitrate keeping the same molar ratios of reactants and
adjusting the quantities to match the 24.43g of nitrate salt.
40ml of 96-98% sulphuric acid were placed in a 100ml beaker and cooled in the freezer chest for 2 hours when the temperature had reached -15°C. At
this temperature the sulphuric acid was rather viscous and so was manually stirred while the nitrate salt was sprinkled into it from a spatula. After
the addition of about 6g with the temperature still well below 0°C the whole lot suddenly decomposed within 2 second emitting a column of thick white
fumes. Surprisingly there was almost no loss of sulphuric acid though the acid was now very hot.
The fact that the acid was not expelled from the beaker suggests that the sudden reaction was largely confined to the surface and may, therefore, have
occurred as a result of localized overheating where too much nitrate salt had accumulated due to the difficulty of stirring it in quickly.
Recovery of the filtrate from the initial nitrate salt preparation
The filtrate contains a considerable amount of nitric acid and residual dicyandiamide but this mixture is not stable enough to allow it to be
evaporated down, hydrolysis of the dicyandiamide to guanylurea and then guanidine will occur. In order to make use of this reaction it was decided to
maximise the yield of guanylurea nitrate since it is quickly prepared and sparingly soluble so crystallises readily but the filtrate is too
concentrated with respect of nitric acid and the ratio of nitric acid to dicyandiamide to high.
To the filtrate from the nitrate preparation 7.5g of fresh dicyandiamide were added and the whole liquor diluted to 140ml with water. The suspension
was heat to boiling over about 5 minutes, simmered for another 5 minutes and then allowed to slowly cool to room temperature overnight. The white
crystalline precipitate was filtered off, washed with a little cold water and dried at 45°C. The yield was 15.09g of crude guanylurea nitrate, about
half that expected (circa 28g).
Conversion of guanylurea nitrate to nitroguanylurea
22ml of 96-98% sulphuric acid were placed in a 50ml beaker and cooled in the freezer chest to -15°C. When this temperature had been reached the
guanylurea nitrate was added slowly to the acid and stirred in rapidly to maintain as homogeneous suspension as possible. After the addition of about
half the salt the temperature had risen to about 0°C so the beaker was returned to the freezer and cooled again to -15° before the remaining salt
was stirred in. Once the addition was complete the beaker was returned to the freezer again and chilled for about 20 minutes and then left to stand
and warm up to room temperature for a further 20 minutes. The white slurry was then poured into 50ml of water and 50g of ice and stirred until the ice
had melted before being vacuum filtered. The filter cake was washed with cold water until the filtrate was no longer acid to Congo Red paper and
dried. The yield of crude nitroguanylurea was 11.94g, about 89% of theory based on guanylurea nitrate. It was not recrystallized as it is very
sparingly soluble in all of the solvents tested.
Tests on the dicyandiamide nitrate
Heat: when heated rapidly it melts and sputters but neither ignites nor deflagrates; leaving a chalky, pale yellowish, infusible residue that does
not dissolve in dilute acids.
A small quantity (circa 0.05g) was dissolved in about 2ml of water and tests carried out on about 0.3ml
1) A drop of 1M copper sulphate solution was added followed by drops of 2M NaOH solution, a pale blue precipitate formed but no colour in the
solution. The precipitate slowly turned darker greenish grey; therefore guanylurea is absent.
2) A drop of saturated, roughly 1%, ammonium picrate; no immediate precipitate but the solution slowly turns very slightly cloudy, dicyandiamide and
its salts give no ppt if pure.
3) A few drops of saturated Na2CO3 solution cause effervescence and slowly a white crystalline ppt. The crystals were separated
and dissolved in a little hot water while to a second test tube was added a similar quantity of commercial dicyandiamide in a similar quantity of hot
water. An equal amount of 1% ammonium picrate was added to each test-tube and they were set aside to cool; no picrate formed in either test,
eventually colourless crystals of dicyandiamide formed in both cases indicating that dicyandiamide can be liberated from the product ie its a salt of
said base.
Discussion
The tests show that the compound acts like an acid liberating CO2 from sodium carbonate, guanylurea nitrate may do this also but the lack
of the characteristic pinkish-purple colour or precipitate of the copper guanylurea complex. Guanylurea and guanidine both form sparingly soluble
picrates but the former precipitates slowly so both would appear to be absent. The fact that the dried nitrate salt weighed more than the
dicyandiamide used does suggest that some form of union has taken place; however, the amount of guanylurea nitrate prepared from the filtrate was
roughly equivalent only to the extra dicyandiamide added to the filtrate after the removal of the nitrate salt. This suggests that the “lost”
dicyandiamide not precipitated as the nitrate salt has already been hydrolysed beyond the guanylurea point. Given that guanylurea nitrate is the least
soluble potential product in this reaction series (others are guanidine and urea, both of which form sparingly soluble nitrates but more soluble than
guanylurea nitrate) this raises the question why this compound is not seen in the initial product. The test reported above show that guanylurea is
absent from the product and neither guanidine or guanylurea could be detected in the filtrate from the initial nitrate preparation.
Incidently, it has been reported that attempts to nitrate dicyandiamide with mixed acid or nitric acid/acetic anhydride yield only nitroguanylurea.
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