Sodium azide

Sodium azide is the inorganic compound with the formula NaN3. This colorless salt is the gas-forming component in legacy car airbag systems. It is used for the preparation of other azide compounds. It is an ionic substance, is highly soluble in water and is very acutely poisonous.
Structure.
Sodium azide is an ionic solid. Two crystalline forms are known, rhombohedral and hexagonal. Both adopt layered structures. The azide anion is very similar in each form, being centrosymmetric with N–N distances of 1.18 Å. The Na+ ion has an octahedral geometry. Each azide is linked to six Na+ centers, with three Na-N bonds to each terminal nitrogen center.
Preparation.
The common synthesis method is the "Wislicenus process", which proceeds in two steps in liquid ammonia. In the first step, ammonia is converted to sodium amide by metallic sodium:
It is a redox reaction in which metallic sodium gives an electron to a proton of ammonia which is reduced in hydrogen gas. Sodium easily dissolves in liquid ammonia to produce hydrated electrons responsible of the blue color of the resulting liquid. The Na+ and NH2– ions are produced by this reaction. 
The sodium amide is subsequently combined with nitrous oxide:
These reactions are the basis of the industrial route, which produced about 250 tons per year in 2004, with production increasing owing to the popularization of airbags.
Laboratory methods.
Curtius and Thiele developed another production process, where a nitrite ester is converted to sodium azide using hydrazine. This method is suited for laboratory preparation of sodium azide:
Alternatively the salt can be obtained by the reaction of sodium nitrate with sodium amide.
Chemical reactions.
Treatment of sodium azide with strong acids gives hydrazoic acid, which is also extremely toxic:
Aqueous solutions contain minute amounts of hydrogen azide, the formation of which is described by the following equilibrium:
Sodium azide can be destroyed by treatment with nitrous acid solution:
Applications.
Automobile airbags and airplane escape chutes.
Older airbag formulations contained mixtures of oxidizers and sodium azide and other agents including ignitors and accelerants. An electronic controller detonates this mixture during an automobile crash:
The same reaction occurs upon heating the salt to approximately 300 °C. The sodium that is formed is a potential hazard alone 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. Newer-generation air bags contain nitroguanidine or similar less sensitive explosives such as guanidine nitrate.
Organic and inorganic 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 SnCl2 in ethanol or lithium aluminium hydride or a tertiary phosphine, such as triphenylphosphine in the Staudinger reaction, with Raney nickel or with hydrogen sulfide in pyridine.
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 and a 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; however, some gram-positive bacteria (streptococci, pneumococci, lactobacilli) are intrinsically resistant.
Agricultural uses.
It is used in agriculture for pest control of soil-borne pathogens such as "Meloidogyne incognita" or "Helicotylenchus dihystera".
It is also used as a mutagen for crop selection of plants such as rice, barley or oats.
Safety considerations.
Sodium azide has caused deaths for decades. It is a severe poison. It possesses the NFPA 704's highest rating of 4 on the health scale. It may be fatal in contact with skin or if swallowed. Even minute amounts can cause symptoms. The toxicity of this compound is comparable to that of soluble alkali cyanides. No toxicity has been reported from spent airbags.
It produces extrapyramidal symptoms with necrosis of the cerebral cortex, cerebellum, and basal ganglia. Toxicity may also include hypotension, blindness and hepatic necrosis. Sodium azide increases cyclic GMP levels in brain and liver by activation of guanylate cyclase.
Sodium azide solutions react with metallic ions to precipitate metal azides, which can be shock sensitive and explosive. This should be considered for choosing a non-metallic transport container for sodium azide solutions in the laboratory. This can also create potentially dangerous situations if azide solutions should be directly disposed down the drain into a sanitary sewer system. Metal in the plumbing system could react, forming highly sensitive metal azide crystals which could accumulate over years. Adequate precautions are necessary for the safe and environmentally responsible disposal of azide solution residues.