Azide is the anion with the formula N3 −. It is the conjugate base of hydrazoic
acid. 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. Preparation
= Inorganic azides= Sodium azide is made industrially by the
reaction of nitrous oxide, N2O with sodium amide in liquid ammonia as
solvent: N2O + 2 NaNH2 → NaN3 + NaOH + NH3
Many inorganic azides can be 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. Some azides are produced by treating the carbonate salts with
hydazoic acid. = Organic azides=
The principal source of the azide moiety is sodium azide. As a pseudohalogen
compound, sodium azide generally displaces an appropriate leaving group
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
Alkyl or aryl acyl chlorides react with sodium azide in aqueous solution to give
acyl azides, which give isocyanates in the Curtius rearrangement.
The azo-transfer compounds, trifluoromethanesulfonyl azide and
imidazole-1-sulfonyl azide, are prepared from sodium azide as well. They react
with amines to give the corresponding azides:
RNH2 → RN3 Dutt–Wormall reaction
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. Reactions
= Inorganic azides= Azide salts can decompose with release
of nitrogen gas as discussed under Applications. The decomposition
temperatures of the alkali metal azides are: NaN3, KN3, RbN3, and CsN3. This
method is used to produce ultrapure alkali metals.
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.
Azides can be used as precursors of the metal nitrido complexes. Azide complexes
thus is induced to release N2, generating a metal complex in unusual
oxidation states. = Organic azides=
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
In the azide alkyne Huisgen cycloaddition, organic azides react as
1,3-dipoles, reacting with alkynes to give substituted 1,2,3-triazoles. This
reaction is very popular in click chemistry.
Another azide regular is tosyl azide here in reaction with norbornadiene in a
nitrogen insertion reaction: Applications
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 air bags. 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).
= Other= 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.
The antiviral drug zidovudine contains an azido group. Some azides are valuable
as bioorthogonal chemical reporters. Safety
Azides are explosophores and toxins. Sodium azide is toxic = 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, 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 have been well documented.
Solid iodoazide is explosive and should not be prepared in the absence of
solvent. See also
Pentazenium References
External links Synthesis of organic azides, recent
methods Synthesizing, Purifying, and Handling
Organic Azides