Sodium azide

Sodium azide is the inorganic compound with the formula NaN₃. This colourless azide salt is the gas-forming component in many car airbag systems. It is used for the preparation of other azide compounds. It is an ionic substance and is highly soluble in water. It is extremely toxic.

Structure

Sodium azide is an ionic solid. Two crystalline forms are known, rhombohedral and hexagonal. The azide anion is very similar in each, being centrosymmetric with N–N distances of 1.18 Å. The Na⁺ ion is pentacoordinated.

Preparation

The common synthesis method is the "Wislicenus process," which proceeds in two steps from ammonia. In the first step, ammonia is converted to sodium amide:

2 Na + 2 NH₃ → 2 NaNH₂ + H₂

The sodium amide is subsequently combined with nitrous oxide:

2 NaNH₂ + N₂O → NaN₃ + NaOH + NH₃

Alternatively the salt can be obtained by the reaction of sodium nitrate with sodium amide.

Applications

Automobile airbags and airplane escape chutes

Older airbag formulations contained mixtures of oxidizers and sodium azide and other agents including igniters and accelerants. An electronic controller detonates this mixture during an automobile crash:

2 NaN₃ → 2 Na + 3 N₂

The same reaction occurs upon heating the salt to approximately 300 °C. The sodium that is formed is a potential hazard itself and, in automobile airbags, it is converted by reaction with other ingredients, such as potassium nitrate and silica. In the latter case, innocuous sodium silicates are generated. Sodium azide is also used in airplane escape chutes. No toxicity has been reported from spent airbags. Newer generation air bags contain nitroguanidine or similar less sensitive explosives.

Organic synthesis

Due to its explosion hazard, sodium azide is of only limited value in industrial scale organic chemistry. In the laboratory, it is used in organic synthesis to introduce the azide functional group by displacement of halides. The azide functional group can thereafter be converted to an amine by reduction with either lithium aluminium hydride or a tertiary phosphine such as triphenylphosphine in the Staudinger reaction, with Raney nickel or with hydrogen sulfide in pyridine.

Inorganic synthesis

Sodium azide is a versatile precursor to other inorganic azide compounds, e.g. lead azide and silver azide, which are used in explosives.

Biochemistry and biomedical uses

Sodium azide is a useful probe reagent, mutagen, and preservative. In hospitals and laboratories, it is a biocide; it is especially important in bulk reagents and stock solutions which may otherwise support bacterial growth where the sodium azide acts as a bacteriostatic by inhibiting cytochrome oxidase in gram-negative bacteria; gram-positive (streptococci, pneumococci, lactobacilli) are resistant, a characteristic similar to antibiotic resistance. It is also used in agriculture for pest control.

Azide inhibits cytochrome oxidase by binding irreversibly to the heme cofactor in a process similar to the action of carbon monoxide. Sodium azide particularly affects organs that undergo high rates of respiration, such as the heart and the brain.

Reactions

Treatment of sodium azide with strong acids gives hydrazoic acid, which is also extremely toxic:

H⁺ + N−
3 → HN₃

Aqueous solutions contain minute amounts of hydrogen azide, as described by the following equilibrium:

N−
3 + H₂O HN₃ + OH⁻ (K = 10−4.6)

Sodium azide can be destroyed by treatment with nitrous acid solution:

2 NaN₃ + 2 HNO₂ → 3 N₂ + 2 NO + 2 NaOH

Safety

Sodium azide is acutely toxic. Symptoms are often compared with those of cyanide. Ingestion of dust or solutions can induce the following symptoms within minutes: rapid breathing, restlessness, dizziness, weakness, headache, nausea and vomiting, rapid heart rate, red eyes (gas or dust exposure), clear drainage from the nose (gas or dust exposure), cough (gas or dust exposure), skin burns and blisters (explosion or direct skin contact). Exposure to a large amount of sodium azide may cause these other health effects as well: convulsions, low blood pressure, low heart rate, loss of consciousness, and lung injury, respiratory failure leading to death.