Sodium azide is the inorganic compound with the formula NaN3.
This colorless 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, is highly soluble in water, and is very acutely toxic.
Structure and preparation 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.
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
Curtius and Thiele developed another production process where a nitrite ester is converted
to sodium azide using hydrazine.
This method is well suited for laboratory preparation of sodium azide:
2 NaNO2 + 2 C2H5OH +H2SO4 → 2 C2H5ONO + Na2SO4 + 2 H2O
C2H5ONO + N2H4-H2O + NaOH → NaN3 + C2H5OH + 3 H2O
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 ignitors and accelerants.
An electronic controller detonates this mixture during an automobile crash:
2 NaN3 → 2Na + 3 N2 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.
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, 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
are intrinsically resistant.
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 → HN
3 Aqueous solutions contain minute amounts of
hydrogen azide, as described by the following equilibrium:
N− 3 + H
2O HN 3 + OH−
Sodium azide can be destroyed by treatment with nitrous acid solution:
2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH Safety considerations
Sodium azide is a severe poison.
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 and the
lethal dose for an adult human is about 0.7 grams.
No toxicity has been reported from spent airbags.
References
External links International Chemical Safety Card 0950.
NIOSH Pocket Guide to Chemical Hazards.
Straight Dope on Sodium Azide