Sodium azide is an inorganic compound with the formula . This colorless salt is the gas-forming component in some 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 ion has an octahedral geometry. Each azide is linked to six 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 for the blue color of the resulting liquid. The and 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 due to the increased use 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
Acid formation of hydrazoic acid
Treatment of sodium azide with strong acids gives gaseous hydrazoic acid (hydrogen azide; HN3), which is also extremely toxic:
Hydrazoic acid equilibrium
Aqueous solutions contain minute amounts of hydrazoic acid , the formation of which is described by the following equilibrium:
, K = 10−4.6
Destruction
Sodium azide can be destroyed by treatment with in situ prepared nitrous acid (HNO2; not HNO3). In situ preparation is necessary as HNO2 is unstable and decomposes rapidly in aqueous solutions. This destruction must be done with great caution and within a chemical fume hood as the formed gaseous nitric oxide (NO) is also toxic, and an incorrect order of acid addition for in situ formation of HNO2 will instead produce gaseous highly toxic hydrazoic acid (HN3).
Applications
Automobile airbags and aircraft evacuation slides
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. While sodium azide is still used in evacuation slides on modern aircraft, newer-generation automotive air bags contain less sensitive explosives such as nitroguanidine or 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 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. Oseltamivir, an antiviral medication, is currently produced in commercial scale by a method which utilizes the sodium azide.