Preparation of Sodium Azide

[Anders Hoveland]

Anhydrous hydrazoic acid (formula HN3 ) boils at 35.7degC, and has a freezing point of (minus) -80degC. This liquid gives of extremely poisonous fumes with a sharp corrossive unbearable smell, somewhat comparable to that of NO2. However, concentrations of fumes below the odor threshold are easily sufficient to cause poisoning. The liquid gives off vapors which can initiate explosion of the liquid if there is any nearby flame.

The actual creation of azides is difficult in the laboratory, but it can be carried out. Azides are as poisonous as cyanide, even sodium azide can give of some HN3 fumes when dissolved in water. Acids should not be added to basic azides, highly poisonous HN3 gas will result, and this gas has the potential for explosion. There are basically two routes, from sodium amide, or from hydrazine. The chain of reactions either way is long and complex.

Preparation of Sodium Amide

by heating sodium with ammonia gas
Sodium and dry ammonia, when heated to 320-350degC, react to form sodium amide and hydrogen. Bubbling anhydrous NH3 gas into molten sodium (heated between 300-350degC) for two hours produces sodium amide NaNH2. Below 250 degC, an unknown substance with a lower proportion of nitrogen is obtained. After complete reaction of the sodium, the unknown product did not produce hydrogen gas on reaction with water. With the reaction conducted at 200degC, the product contained 71.2% sodium by weight and 23.4% nitrogen. This suggests formation of some Na2NH. However, this observation was published in the early part of the century (without modern analytical equipment) and there does not seem to be any other published detailed investigations into the unknown product of this reaction. The advantage of using molten sodium is that the reaction proceeds about five times faster and the sodium can be completely reacted.

by reactiving sodium with liquid ammonia
Metallic sodium dissolves in cold anhydrous liquid ammonia to form a deep blue colored solution. If Sodium Ferrate (which acts as a catalyst) is added to a blue solution of sodium dissolved in ammonia, the solution will slowly turn to a bronze color as all the sodium reacts.

Sodium and anhydrous liquid ammonia react, rather than simply dissolve into the typical blue solution, with the use of a black catalyst prepared originally by the reaction of a ferric salt with oxides of sodium in liquid ammonia. Liquid ammonia (500ml) was stirred at -32C and treated with 0.3g Fe(NO3)3 hexahydrate and then with 1g of metallic sodium. Air was bubbled through the mixture to discharge the initial blue color and produce a black precipitate of catalyst. Additional sodium 24g, added portionwise was then consumed in about 20 minutes with dissappearance the blue colour and formation of a grey suspension of sodium amide. The solubility of sodium amide thus prepared is very low (about 1mol per liter of liquid ammonia).


Sodium Azide from Sodamide

Sodium azide has been prepared by passing nitrous oxide over sodamide heated to 230degC.

2 NaNH2 + N2O --> NaN3 + NaOH + NH3

Nitrous oxide can be generated by the gentle heating of small ammounts of ammonium nitrate until decomposition, traces of nitric oxide and nitrogen dioxide are also produced, these can be eliminated by bubbling the gases into a solution of sodium carbonate, leaving only nitrogen and nitrous oxide. Sodium nitrite will be produced in the solution. Depending on the ratio of NO2 to NO in the decomposition products, sodium nitrate may be the main byproduct forming, or some of the nitric oxide may not be absorbed by the basic solution, where it will either react with air to form nitrogen dioxide, or in the absence of air the unstable gas will decompose into nitrous oxide and nitrogen gas within several minutes.

Sodium nitrate can also be reacted with sodamide, both dissolved in anhydrous liquid ammonia under pressure at a temperature of 100degC to produce the dissolved azide. This is dangerous since the pressurized container is likely to burst since anhydrous ammonia has a boiling point of only (minus) -33degC. The reaction can take place at 80degC, while heating to 135degC allows a shorter reaction time and gives higher yields. First the two reactants react to form nitrous oxide, and then the intermediate nitrous oxide reacts with more amide.

KNO3 + 3KNH2 --> KN3 + 3KOH + NH3


Another method is to react a dispersion of sodium amide in a hydrocarbon solvent with isopropyl nitrite dissolved in a hydrocarbon solvent. Excess amide should be used, and the solution of nitrite should be gradually added to the amide dispersion, with rapid stirring.

The below might be a possible reaction:

(13)NaNH2 + (6)PrNO2 --> (2)NaNO2 + (3)NaN3 + (6)NaOPr + (2)NaOH + (8)NH3

where NaNH2 is sodamide, and PrNO2 is the nitrite ester of isopropanol. NaN3 is sodium azide and NaOPr is sodium isopropoxide, which is analogous to sodium methoxide.

Ispropyl Nitrite can easily be made by adding concentrated HCl solution cooled to 0degC to a cold solution of sodium nitrite and isopropyl alcohol.


Sodium Azide from Hydrazine

Hydrazine hydrate can be converted to anhydrous hydrazoic acid by reaction with a nitrous acid ester in ether at 0°C with some sodium ethoxide present. This pure hydrazoic acid may be useful for the synthesis of Diaminotetrazole.

Hydrazoic acid HN3 can also be obtained by the action of nitrous acid on hydrazine sulphate.
(W. Wislicenus, Berichte, 1892, 25, p. 2084)

Hydrazine sulfate reacts with a solution of hydrogen peroxide and sulfuric acid to produce hydrogen azide gas in up to a 28% yield. It is important that the reactants be free from any traces of dissolved copper ions.

[Anders Hoveland]

Procedure for preparing a solution of sodium azide
This procedure does not require either sodamide (NaNH2) or hydrazine, which makes this procedure more attractive.

WARNING: Sodium azide gives off highly posionous hydrazoic acid (HN3) fumes when mixed with water. Keeping the pH alkaline helps prevent the fumes, but solutions should be kept in a well sealed container, and stored in an area with plenty of ventilation. Acids should not be added to sodium azide, since this will give off the poison gas.

Nitrourea can be made by a cold nitration with mixed concentrated nitric and sulfuric acids on urea, similar to the procedure for nitroguanidine. The difference is that nitrourea should be kept away from water, since it can hydrolyze and be degraded.

Preparation of Nitrourea
9 grams of urea nitrate, was added in small portions to 32 mLof ice cold (-3 C) concentrated sulfuric acid at such a rate that the temperature did not exceed 5 C. Total time for addition was approximately one-half hour, after which the mixture was poured on 75 grams of ice. The white precipitate was filtered, washed with ice cold water, just sufficient to cover it, and air dried. Material obtained weighed 5.3 grams and melted with decomposition at 157-158 C.

20g of nitrourea, 65g of zinc dust and 10 mL pure methanol (without water) are mixed in mortar and ground into a thick paste. A glass beaker is filled with 12 ml of glacial acetic acid and 15 mL pure methanol, then placed into an ice water bath. The cold nitrourea and zinc paste is added in small portions with gentle stirring. The reaction will give off much heat, so the initial portions should be smaller, and added over longer intervals of time, so the mixture does not overheat. The rate of addition should be adjusted to keep the temperature from rising too much, ideally around 10 degC. Do not allow the temperature at any time to rise to 35C. If the mixture becomes too thick or the temperature rises too rapidly, cold methanol should be added (no more than 50g). The additions to the paste should be done over a time of around 3 hours.

After completion, the beaker containing the reaction mixture should be allowed to stand in ice water bath for another hour. The beaker is then removed from bath and allowed to warm slowly to normal temperature. The final volume of the mixture should be around 130 mL. After one hour at room temperature, the mixture is placed on water bath, heated to 25 degC, and stirred for 30 minutes, then heated to 32-35C, and stirred for 30 more minutes, finally being heated to 40 degC and stirred at this temperature for another 15 minutes.

When the reduction reaction is complete, the solution is immediately filtered using a vacuum funnel, and filtering the solid product dry. The solid is washed with 90 mL of water, then filtered. The solids left over are discarded. The filtrates (solution that dissolved out the semicarbazide and passed through the filter paper) are combined. The solution will contain around 10g of dissolved semicarbazide. The yield for the reaction of nitrourea to semicarbazide at this point is 60%)

Dilute sulfuric acid (5% solution) is added to the solution until the pH just reaches 6.5. It is important that the resulting solution not be too acidic or contain excess acid. It may be helpful to pre-cool the acid solution.

9.2g of Sodium Nitrite is separately dissolved in 125 mL water, and this solution is reacted with the semicarbazide sulfate solution (which contains the equivalent of 10g semicarbazide). The addition should be slowly done, over the period of roughly ten minutes, the temperature should be kept below 20 degC, but ideally 10 degC is good. The reaction may give off some poisonous vapors so do it outside or in a fumehood. After this reaction, immediately proceed to the final step, which is to gradually add a pre-chilled concentrated solution of sodium hydroxide.

The resulting solution contains dissolved sodium azide, along with byproducts of sodium carbamate, and sodium sulfate (much of it will precipitate out as solid crystals on the bottom below 15 degC).

[Anders Hoveland]

See the post at the bottom of the page on:

www.sciencemadness.org/talk/viewthread.php?tid=7553

I am wondering whether azides could perhaps actually be prepared by
heating finely powdered sodium nitrate and urea at 160degC. If it is so easy, why has it never been mentioned before?

it could be quite possible since "hydrazine was obtained by oxidizing urea with sodium hypochlorite in the presence of benzaldehyde, which, by combining with the hydrazine, protected it from oxidation."
(P. J. Sohestakov, J. Russ. Phys. Chem. Soc., 1905, 37, p. 1)
Without the benzaldehyde, the hydrazine only transiently forms as an intermediate, quickly being destroyed.

It is known that hot sodium nitrate can oxidize sodium amide to sodium azide. Both reactants are dissolved in anhydrous liquid ammonia under pressure at a temperature of 100degC to produce the dissolved azide. The reaction can take place at 80degC, while heating to 135degC allows a shorter reaction time and gives higher yields.

The two usual methods of azide preparation involved either sodium amide and nitrous acid, or hydrazine and sodium nitrite. Since heated nitrate can replace warmed nitrite as the oxidizer of hydrazine, and since hydrazine is known to be an intermediate in the oxidation of urea, and since acidified nitrite is known to oxidize urea, one would think that heated nitrate could potentially oxidize urea, and oxidize the intermediate hydrazine formed to form small ammounts of azide. So the alleged procedure involving heating urea with NaNO3 is not without some precedent.