Inchemistry,azide (/ˈeɪzaɪd/,AY-zyd) is alinear,polyatomic anion with theformulaN−3 andstructure−N=N+=N−. It is theconjugate base ofhydrazoic acidHN3.Organic azides areorganic compounds with the formulaRN3, containing the azidefunctional group.[1] The dominant application of azides is as apropellant inair bags.[1]
Sodium azide is made industrially by the reaction ofnitrous oxide,N2O withsodium amideNaNH2 inliquid ammonia assolvent:[2]
Many inorganic azides can be prepared directly or indirectly from sodium azide. For example,lead azide, used indetonators, may be prepared from themetathesis reaction betweenlead nitrate and sodium azide. An alternative route is direct reaction of the metal withsilver azide dissolved inliquid ammonia.[3] Some azides are produced by treating thecarbonatesalts withhydrazoic acid.
Azide has a linear structure and isisoelectronic withcarbon dioxideCO2,cyanateOCN−,nitrous oxideN2O,nitronium ionNO+2, molecularberyllium fluorideBeF2 andcyanogen fluoride FCN. Pervalence bond theory, azide can be described by severalresonance structures; an important one beingN−=N+=N−. The analogous neutraltrinitrogen molecule can also have a linear structure, but also a cyclicisomer is known.
Azide salts can decompose with release ofnitrogen gas. The decomposition temperatures of thealkali metal azides are:NaN3 (275 °C),KN3 (355 °C),RbN3 (395 °C), andCsN3 (390 °C). This method is used to produce ultrapure alkali metals:[4]
Protonation of azide salts gives toxic and explosivehydrazoic acid in the presence ofstrong acids:
Azide as aligand forms numeroustransition metal azide complexes. Some such compounds areshock sensitive.
Manyinorganiccovalent azides (e.g.,fluorine azide,chlorine azide,bromine azide,iodine azide,silicon tetraazide) have been described.[5]
The azide anion behaves as anucleophile; it undergoesnucleophilic substitution for bothaliphatic andaromatic systems. It reacts withepoxides, causing a ring-opening; it undergoesMichael-likeconjugate addition to 1,4-unsaturatedcarbonyl compounds.[1]
Azides can be used as precursors of themetal nitrido complexes by being induced to releaseN2, generating ametal complex in unusualoxidation states (seehigh-valent iron).
Azides have an ambivalentredox behavior: they are bothoxidizing andreducing, as they are easily subject todisproportionation, as illustrated by theFrost diagram of nitrogen. This diagram shows the significant energetic instability of thehydrazoic acidHN3 (or the azide ion) surrounded by two much more stable species, theammoniumionNH+4 on the left and the molecularnitrogenN2 on the right. As seen on the Frost diagram the disproportionation reaction lowers ∆G, theGibbs free energy of the system(−∆G/F = zE, where F is theFaraday constant, z the number ofelectrons exchanged in the redox reaction, and E thestandard electrode potential). By minimizing the energy in the system, the disproportionation reaction increases itsthermodynamical stability.
Azides decompose with nitrite compounds such assodium nitrite. Each elementaryredox reaction is also acomproportionation reaction because two different N-species (N−3 and NO−2) converge to a same one (respectivelyN2, N2O and NO) and is favored when the solution is acidified. This is a method of destroying residual azides, prior to disposal.[6] In the process, nitrogen gas (N2) and nitrogen oxides (N2O and NO) are formed:
(The parenthetical notation below marks theoxidation state of the nitrogen in each species.)
Azide (−1/3) (thereductant,electron donor) isoxidized inN2 (0),nitrous oxide (N2O) (+1), ornitric oxide (NO) (+2) whilenitrite (+3) (theoxidant,electron acceptor) is simultaneouslyreduced to the same corresponding species in each elementary redox reaction considered here above. The respective stability of the reaction products of these threecomproportionation redox reactions is in the following order:N2 > N2O > NO, as can be verified in the Frost diagram for nitrogen.
In 2005, about 251 tons of azide-containing compounds were annually produced in the world, the main product being sodium azide.[7]
Sodium azideNaN3 is the propellant in automobileairbags. It decomposes on heating to give nitrogen gas, which is used to quickly expand the air bag:[7]
Heavy metal azides, such aslead azide,Pb(N3)2, are shock-sensitivedetonators which violently decompose to the corresponding metal and nitrogen, for example:[8]
Silver azideAgN3 andbarium azideBa(N3)2 are used similarly.
Some organic azides are potentialrocket propellants, an example being2-dimethylaminoethylazide (DMAZ)(CH3)2NCH2CH2N3.
Sodium azide is commonly used in the laboratory as abacteriostatic agent to avoid microbial proliferation inabiotic 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 anucleophile and aredox-active species. Being prone todisproportionation, it can behave both as an oxidizing and as areducing agent. Therefore, it is susceptible to interfere in an unpredictable way with many substances.[9][10][11] For example, the azide anion canoxidizepyrite (FeS2) with the formation ofthiosulfate (S2O2−3), orreducequinone intohydroquinone.[12] It can also reducenitriteNO−2 intonitrous oxideN2O, andFe2+ intoFe0 (zerovalent iron, ZVI).[12] Azide can also enhance theN2O emission in soil. A proposed explanation is the stimulation of the denitrification processes because of the azide’s role in the synthesis of denitrifying enzymes.[13] Moreover, azide also affects theabsorbance andfluorescence optical properties of thedissolved organic matter (DOM) fromsoils.[14] Many other interferences are reported in the literature forbiochemical andbiological analyses and they should be systematically identified and first rigorously tested in the laboratory before to use azide asmicrobial inhibitor for a given application.
Sodium azideNaN3 is used to purify metallic sodium in laboratories handling molten sodium used as a coolant forfast-neutron reactors.[15]
As hydrazoic acid, theprotonated form of the azide anion, has a very low reduction potential (E°red = −3.09 V), and is even a strongerreductant than lithium (E°red = −3.04 V), dry solidsodium 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 residualO2(< 1 ppm vol.) to reduceNa+ impurities still present into the sodium bath. The reaction residue is only gaseousN2.
AsE°ox = −E°red, it gives the following series of oxidation reactions when the redox couples are presented as reductants:
The azidefunctional group is commonly utilized inclick chemistry throughcopper(I)-catalyzed azide-alkynecycloaddition (CuAAC) reactions, where copper(I) catalyzes the cycloaddition of an organoazide to a terminal alkyne, forming atriazole.[16][17][18]
A very damaging and illegal usage of sodium azide is its diversion bypoachers as a substitute ofsodium cyanide to poison some animal species by blocking theelectron transport chain in thecellular respiration process.
Azides areexplosophores[9][19][20] and respiratory poisons.[9][21]Sodium azide (NaN3) is nearly as toxic assodium cyanide (NaCN) (with an oralLD50 of 27 mg/kg in rats) and can be absorbed through the skin. When sodium azide enters in contact with an acid, it produces volatilehydrazoic acid (HN3), as toxic and volatile ashydrogen cyanide (HCN). When accidentally present in the air of a laboratory at low concentration, it can cause irritations such as nasal stuffiness, orsuffocation and death at elevated concentrations.[22]
Heavy metal azides, such aslead azide (Pb(N3)2) areprimaryhigh explosivesdetonable when heated or shaken. Heavy-metal azides are formed when solutions of sodium azide orHN3 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,freezedrying equipment, cooling traps, water baths, waste pipes), and thus lead to violent explosions.[9]
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