Preparation of nitrourea

N-nitrourea, or N-nitrocarbamide, is an interesting compound with a variety of uses in organic synthesis. It is also a powerful secondary explosive and thus must be handled with caution. However, I'm not so interested in nitrourea as an explosive, but rather as a precursor to semicarbazide.

Nitrourea can be made by the dehydration of urea nitrate. The reaction is a bit finicky, and straying too far from the ideal temperature range can result in a significant loss of product.

To 150ml beaker, I added 17ml of 98% sulphuric acid. I placed the beaker in an ice bath (an arbitrary amount of sodium chloride was added to the ice bath to get it even colder). Once the temperature of the sulphuric acid had dropped to -2 C, I began adding 11g of urea nitrate in very small portions with constant stirring, at such a rate that the temperature never rose above 0 C. Complete addition of the urea nitrate took about 30 minutes. The viscosity of the mixture gradually increased and fine white particulate began to form. I continued stirring for a few minutes after all the urea nitrate was added.

Then I began adding pieces of ice (one at a time) to the mixture. This generates a fair amount of heat, so its best to start with small pieces as the nitrourea product is sensitive to warm-hot water. As more and more ice was added, lots of white nitrourea precipitated. Once the volume of the mixture had reached 125ml, I filtered off the nitrourea and washed it on the filter with 70ml of ice cold water to help remove sulphuric acid.

The product turned out to be very difficult to dry. I let it sit in a desiccator bag over anhydrous calcium chloride for over a week, after which it was still sticky from moisture. I decided this was as dry as I was going to get it, so I weighed the product and calculated the yield anyway. I got 4.33g of slightly wet nitrourea as a sticky white powder. Yield: 46%.

The nitrourea should be sufficiently pure and usable for most reactions. For maximum purity, the nitrourea can be recrystallized from ethanol. However I decided not to do this.

(NH2)2CO.HNO3 == -H2O ==> NH2CONHNO2

Nitric acid

Nitric acid, or hydrogen nitrate, is a very common laboratory acid with a wide array of uses. One of the main uses is for nitrating various compounds. There are many explosives based around nitro groups, which makes nitric acid incredibly useful as an explosives precursor. I plan to use nitric acid to make 4-nitrotoluene which in turn I will eventually use to synthesize a local anesthetic called benzocaine.

Nitric acid is usually supplied at an azeotropic concentration of 68%. However the nitric acid produced in this procedure is almost pure.

To a 1000ml round-bottom flask, I added 101g of powdered potassium nitrate and 54ml of 98% sulphuric acid. I then quickly set the flask up for simple distillation with the receiving flask in an ice bath. I then began distilling the mixture at a medium heat, gradually increasing the temperature whenever the yellow nitric acid stopped coming over. At the start of distillation, the flask was full of red nitrogen dioxide gas. This quickly faded to a yellow once the nitric acid started coming over. Gradually increasing the temperature throughout distillation is needed to free the nitric acid from all the sulphate salts formed. Pure nitric acid initially comes over at about 70-80 C then later on, azeotropic nitric comes over at over 100 C. However the vapour temperature was never allowed to rise above 100 C, so when nothing came over below this, the distillation was ended.

I collected the yellow nitric acid in the receiver and transferred it to a glass bottle for storage. I got 38ml of practically pure nitric acid which corresponds to a 91% yield.

Such concentrated acid is very dangerous, and despite what one might expect, its best to wear no gloves at all when handling it since anything above 87% nitric acid can ignite latex and nitrile.
The yellow colour of the nitric acid is due to a small amount of decomposition to nitrogen dioxide. This really isn't a problem though, as the contamination is very minimal.

KNO3 + H2SO4 ==> KHSO4 + HNO3

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