The principal source of the azide moiety is sodium azide. As a pseudohalogen compound, sodium azide generally displaces an appropriate leaving group (e.g., Br, I, OTs) to give the azido compound. Aryl azides may be prepared by displacement of the appropriate diazonium salt with sodium azide, or trimethylsilyl azide; nucleophilic aromatic substitution is also possible, even with chlorides. Anilines and aromatic hydrazines undergo diazotization, as do alkyl amines and hydrazines.
Appropriately functionalized aliphatic compounds undergo nucleophilic substitution with sodium azide. Aliphatic alcohols give azides via a variant of the Mitsunobu reaction, with the use of hydrazoic acid. Hydrazines may also form azides by reaction with sodium nitrite:
Azide salts can decompose with release of nitrogen gas. The decomposition temperatures of the alkali metal azides are: NaN3 (275 °C), KN3 (355 °C), RbN3 (395 °C), and CsN3 (390 °C). This method is used to produce ultrapure alkali metals.
Protonation of azide salts gives toxic hydrazoic acid in the presence of strong acids:
H+ + N- 3 -> HN3
Azide salts may react with heavy metals or heavy metal compounds to give the corresponding azides, which are more shock sensitive than sodium azide alone. They decompose with sodium nitrite when acidified. This is a method of destroying residual azides, prior to disposal.
2 NaN3 + 2 HNO2 -> 3 N2 + 2 NO + 2 NaOH
Many inorganic covalent azides (e.g., chlorine, bromine, and iodine azides) have been described.
The azide anion behaves as a nucleophile; it undergoes nucleophilic substitution for both aliphatic and aromatic systems. It reacts with epoxides, causing a ring-opening; it undergoes Michael-like conjugate addition to 1,4-unsaturated carbonyl compounds.
About 250 tons of azide-containing compounds are produced annually, the main product being sodium azide.
Detonators and propellants
Sodium azide is the propellant in automobile airbags. It decomposes on heating to give nitrogen gas, which is used to quickly expand the air bag:
2 NaN3 -> 2 Na + 3 N2
Heavy metal salts, such as lead azide, Pb(N3)2, are shock-sensitive detonators which decompose to the corresponding metal and nitrogen, for example:
Pb(N3)2 -> Pb + 3 N2
Silver and barium salts are used similarly. Some organic azides are potential rocket propellants, an example being 2-dimethylaminoethylazide (DMAZ).
Because of the hazards associated with their use, few azides are used commercially although they exhibit interesting reactivity for researchers. Low molecular weight azides are considered especially hazardous and are avoided. In the research laboratory, azides are precursors to amines. They are also popular for their participation in the "click reaction" and in Staudinger ligation. These two reactions are generally quite reliable, lending themselves to combinatorial chemistry.
Heavy metal azides, such as lead azide are primaryhigh explosivesdetonable when heated or shaken. Heavy-metal azides are formed when solutions of sodium azide or HN3 vapors come into contact with heavy metals or their salts. Heavy-metal azides can accumulate under certain circumstances, for example, in metal pipelines and on the metal components of diverse equipment (rotary evaporators, freezedrying equipment, cooling traps, water baths, waste pipes), and thus lead to violent explosions.
Some organic and other covalent azides are classified as highly explosive and toxic: inorganic azides as neurotoxins; azide ions as cytochrome c oxidase inhibitors.
It has been reported that sodium azide and polymer-bound azide reagents react with di- and trihalomethanes to form di- and triazidomethane respectively, which are both unstable without being handled in solutions. Various explosions have been reported during the concentration of reaction mixtures in rotary evaporators. The hazards of diazidomethane (and triazidomethane) have been well documented.
Solid halogen azides are very explosive and should not be prepared in the absence of solvent.
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