Urea nitrate

Urea nitrate, or more correctly, uronium nitrate, is an intriguing organic compound consisting of a protonated urea cation coordinated to a nitrate anion. It's perhaps most well known for its use as a high explosive, for which it has gathered an unpleasant reputation. I should probably point out that I'm not interested in using urea nitrate as an explosive, but rather to make nitrourea and eventually semicarbazide.

Urea nitrate is usually synthesized by the direct combination of urea and nitric acid. However I decided to use another method that avoids the use of expensive (for me anyway) nitric acid.

To a 200ml conical flask, I added 27g of potassium nitrate and 16ml of water. With a bit of warming, most of the potassium nitrate dissolved, giving a milky solution. I then added in a solution of 15.85g of recrystallized urea in 20ml of water. The resulting solution was cloudy, probably due to small amounts of undissolved urea and potassium nitrate in suspension. I stirred the mixture well, then added 30ml of 33% hydrochloric acid in portions with frequent stirring. This took a few minutes. With the first addition, a little bit of fizzing occurred, but after this, the mixture became clear and the later additions had no visible effect. Next I heated the mixture up until it began to steam but not bubble, then let it cool to room temperature.

I then chilled the mixture to 0 C and a large amount of crystalline urea nitrate precipitated. I filtered off the urea nitrate crystals, pressed them on the filter to expel as much liquid as possible, then dried them. I was left with 22.8g of urea nitrate as colourless-white crystals. This is a 70% yield.

(NH2)2CO + HCl ==> (NH2)2CO.HCl    /    (NH2)2CO.HCl + KNO3 <==> (NH2)2CO.HNO3 + KCl