Azide is the anion with the formula N3−. It is the conjugate base of hydrazoic acid (HN3). N3− is a linear anion that is isoelectronic with CO2 and N2O. Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−. Azide is also a functional group in organic chemistry, RN3. The dominant application of azides is as a propellant in air bags.
The principal source of the azide moiety is sodium azide. Sodium azide is made industrially by the reaction of nitrous oxide, N2O with sodium amide, NaNH2, in liquid ammonia as solvent. The overall stoichiometry is given by 
- N2O + 2NaNH2 → NaN3 + NaOH + NH3
Most inorganic and organic azides are prepared directly or indirectly from sodium azide. For example, lead azide, used in detonators, may be prepared from the metathesis reaction between lead nitrate and sodium azide. An alternative route is direct reaction of the metal with silver azide dissolved in liquid ammonia.
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:
- PhNHNH2 → PhN3
- RNH2 → RN3
A classic method for the synthesis of azides is the Dutt–Wormall reaction in which a diazonium salt reacts with a sulfonamide first to a diazoaminosulfinate and then on hydrolysis the azide and a sulfinic acid.
Azide salts can decompose with release of large volumes of nitrogen gas as discussed under Applications.
Protonation of azide salts gives toxic hydrazoic acid in the presence of strong acids:
- H+ + N3− → 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.
Organic azides engage in useful organic reactions. The terminal nitrogen is mildly nucleophilic. Azides easily extrude diatomic nitrogen, a tendency that is exploited in many reactions such as the Staudinger ligation or the Curtius rearrangement or for example in the synthesis of γ-imino-β-enamino esters.
Azides may be reduced to amines by hydrogenolysis or with a phosphine, e.g. triphenylphosphine, in the Staudinger reaction. This reaction allows azides to serve as protected -NH2 synthons, as illustrated by the synthesis of 1,1,1-tris(aminomethyl)ethane:
- 3 H2 + CH3C(CH2N3)3 → CH3C(CH2NH2)3 + 3 N2
About 250 tons of azide-containing compounds are produced annually, the main product being sodium azide.
Detonators and propellants
- 2 NaN3 → 2 Na + 3 N2
- 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 especially are considered 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.
- Azides are explosophores and toxins.
- Sodium azide is toxic (LD50 oral (rat) = 27 mg/kg) and can be absorbed through the skin. It decomposes explosively upon heating to above 275 °C and reacts vigorously with CS2, bromine, nitric acid, dimethyl sulfate, and a series of heavy metals, including copper and lead. In reaction with water or Brønsted acids the highly toxic and explosive hydrogen azide is released.
- Heavy metal azides, such as lead azide are primary high explosives detonable 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 dichloromethane and chloroform to form di- and triazidomethane resp., which are both unstable in high concentrations in solution. Various devastating explosions were reported while reaction mixtures were being concentrated on a rotary evaporator. The hazards of diazidomethane (and triazidomethane) have been well documented.
- Solid iodoazide is explosive and should not be prepared in the absence of solvent.
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