In chemistry, azide (, ) is a linear, polyatomic anion with the Chemical formula and structure . It is the conjugate base of hydrazoic acid . are with the formula , containing the azide functional group.[ The dominant application of azides is as a propellant in .]
Preparation
Sodium azide is made industrially by the reaction of nitrous oxide, with sodium amide in liquid ammonia as solvent:
Many inorganic azides can be prepared directly or indirectly from sodium azide. For example, lead azide, used in , 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.
Bonding
Azide is isoelectronic with carbon dioxide , cyanate , nitrous oxide , nitronium ion , molecular beryllium fluoride and cyanogen fluoride FCN. Per valence bond theory, azide can be described by several resonance structures; an important one being .
Reactions
Azide salts can decompose with release of nitrogen gas. The decomposition temperatures of the alkali metal azides are: Sodium azide (275 °C), Potassium azide (355 °C), Rubidium azide (395 °C), and Caesium azide (390 °C). This method is used to produce ultrapure alkali metals:
Protonation of azide salts gives toxic and explosive hydrazoic acid in the presence of :
Azide as a ligand forms numerous transition metal azide complexes. Some such compounds are Explosive.
Many inorganic covalent azides (e.g., fluorine azide, chlorine azide, bromine azide, iodine azide, silicon tetraazide) have been described.
The azide anion behaves as a nucleophile; it undergoes nucleophilic substitution for both aliphatic and aromatic systems. It reacts with , causing a ring-opening; it undergoes Michael-like conjugate addition to 1,4-unsaturated carbonyl compounds.[
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Azides can be used as precursors of the metal nitrido complexes by being induced to release nitrogen, generating a metal complex in unusual oxidation states (see high-valent iron).
Redox behaviour and trend to disproportionation
Azides have an ambivalent redox behavior: they are both Redox and Redox, as they are easily subject to disproportionation, as illustrated by the Frost diagram of nitrogen. This diagram shows the significant energetic instability of the hydrazoic acid (or the azide ion) surrounded by two much more stable species, the ammonium ion on the left and the molecular nitrogen on the right. As seen on the Frost diagram the disproportionation reaction lowers ∆G, the Gibbs free energy of the system , where F is the Faraday constant, z the number of exchanged in the redox reaction, and E the standard electrode potential). By minimizing the energy in the system, the disproportionation reaction increases its Thermodynamics stability.
Destruction by oxidation by nitrite
Azides decompose with nitrite compounds such as sodium nitrite. Each elementary redox reaction is also a comproportionation reaction because two different N-species () converge to a same one (respectively ) and is favored when the solution is acidified. This is a method of destroying residual azides, prior to disposal.
Azide (−⅓) (the Reducing agent, electron donor) is Redox in (0), nitrous oxide () (+1), or nitric oxide (NO) (+2) while nitrite (+3) (the Oxidizing agent, electron acceptor) is simultaneously Redox to the same corresponding species in each elementary redox reaction considered here above. The respective stability of the reaction products of these three comproportionation redox reactions is in the following order: , as can be verified in the Frost diagram for nitrogen.
Applications
In 2005, about 251 tons of azide-containing compounds were annually produced in the world, the main product being sodium azide.
Primary explosives and propellants
Sodium azide is the propellant in automobile . It decomposes on heating to give nitrogen gas, which is used to quickly expand the air bag:[
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Heavy metal azides, such as lead azide, , are shock-sensitive which violently decompose to the corresponding metal and nitrogen, for example:
Silver azide and barium azide are used similarly.
Some organic azides are potential rocket propellants, an example being 2-dimethylaminoethylazide (DMAZ) .
Microbial inhibitor and undesirable side effects
Sodium azide is commonly used in the laboratory as a bacteriostatic agent to avoid microbial proliferation in abiotic control experiments in which it is important to avoid microbial activity. However, it has the disadvantage to be prone to trigger unexpected and undesirable side reactions that can jeopardize the experimental results. Indeed, the azide anion is a nucleophile and a Redox species. Being prone to disproportionation, it can behave both as an oxidizing agent and as a reducing agent. Therefore, it is susceptible to interfere in an unpredictable way with many substances.
Purification of molten sodium
Sodium azide is used to purify metallic sodium in laboratories handling molten sodium used as a coolant for fast-neutron reactors.
As hydrazoic acid, the Protonation form of the azide anion, has a very low reduction potential ( E°red = −3.09 V), and is even a stronger Reducing agent than lithium ( E°red = −3.04 V), dry solid sodium azide can be added to molten metallic sodium ( E°red = −2.71 V) under strict anoxic conditions ( e.g., in a special anaerobic glovebox with very low residual to reduce impurities still present into the sodium bath. The reaction residue is only gaseous .
As E°ox = − E°red, it gives the following series of oxidation reactions when the redox couples are presented as reductants:
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( E°ox = +3.09 V)
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( E°ox = +3.04 V)
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( E°ox = +2.71 V)
Click chemistry
The azide functional group is commonly utilized in click chemistry through copper(I)-Catalysis azide-alkyne cycloaddition (CuAAC) reactions, where copper(I) catalyzes the cycloaddition of an organoazide to a terminal alkyne, forming a triazole.
Other uses
A very damaging and illegal usage of sodium azide is its diversion by Poaching as a substitute of sodium cyanide to poison some animal species by blocking the electron transport chain in the cellular respiration process.
Safety
Azides are
Heavy metals azides, such as lead azide () are primary detonation when heated or shaken. Heavy-metal azides are formed when solutions of sodium azide or vapors come into contact with heavy metals (Pb, Hg…) 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, freeze drying equipment, cooling traps, water baths, waste pipes), and thus lead to violent explosions.
See also
External links
