Azide

In chemistry, azide is a linear, polyatomic anion with the formula N−3 and structure −N=N+=N−. It is the conjugate base of hydrazoic acid HN3. Organic azides are organic compounds with the formula RN3, containing the azide functional group.[1] The dominant application of azides is as a propellant in air bags.[1]

The azide anion

Preparation

Sodium azide is made industrially by the reaction of nitrous oxide, N2O with sodium amide NaNH2 in liquid ammonia as solvent:[2]

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.[3] Some azides are produced by treating the carbonate salts with hydrazoic acid.

Bonding

Azide is isoelectronic with carbon dioxide CO2, cyanate OCN−, nitrous oxide N2O, nitronium ion NO+2 and cyanogen fluoride NCF. Per valence bond theory, azide can be described by several resonance structures; an important one being N−=N+=N−

Reactions

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:[4]

2 MN3 heat→ 2 M + 3 N2

Protonation of azide salts gives toxic hydrazoic acid in the presence of strong acids:

H+ + N−3 → HN3

Azide as a ligand forms numerous transition metal azide complexes. Some such compound are more shock sensitive.

Many inorganic covalent azides (e.g., chlorine, bromine, and iodine azides) have been described.[5]

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.[1]

Azides can be used as precursors of the metal nitrido complexes by being induced to release N2, generating a metal complex in unusual oxidation states (see high-valent iron).

Disposal

Azides decompose with nitrite compounds such as sodium nitrite when acidified. This is a method of destroying residual azides, prior to disposal.[6] In the process, nitrogen, nitrogen oxides, and hydroxides are formed:

3 N−3 + NO−2 + 2 H2O → 5 N2 + 4 OH−
N−3 + 7 NO−2 + 4 H2O → 10 NO + 8 OH−

Applications

About 251 tons of azide-containing compounds are produced annually, the main product being sodium azide.[7] Sodium azide NaN3 is the propellant in automobile airbags. It decomposes on heating to give nitrogen gas, which is used to quickly expand the air bag:[7]

2 NaN3 → 2 Na + 3 N2

Heavy metal azides, such as lead azide, Pb(N3)2, are shock-sensitive detonators which decompose to the corresponding metal and nitrogen, for example:[8]

Pb(N3)2 → Pb + 3 N2

Silver azide AgN3 and barium azide Ba(N3)2 are used similarly. Some organic azides are potential rocket propellants, an example being 2-dimethylaminoethylazide (DMAZ) (CH3)2NCH2CH2N3.

Safety

Azides are explosophores[9][10] and poisons. Sodium azide is as toxic as sodium cyanide (with an oral LD50 of 27 mg/kg in rats) and can be absorbed through the skin. 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.

See also
  • Pentazenium
  • Pentazolate (cyclo-N5−)

References
  1. S. Bräse; C. Gil; K. Knepper; V. Zimmermann (2005). "Organic Azides: An Exploding Diversity of a Unique Class of Compounds". Angewandte Chemie International Edition. 44 (33): 5188–5240. doi:10.1002/anie.200400657. PMID 16100733.
  2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 433. ISBN 978-0-08-037941-8.
  3. Müller, Thomas G.; Karau, Friedrich; Schnick, Wolfgang; Kraus, Florian (2014). "A New Route to Metal Azides". Angewandte Chemie. 53 (50): 13695–13697. doi:10.1002/anie.201404561. PMID 24924913.
  4. E. Dönges "Alkali Metals" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 475.
  5. I. C. Tornieporth-Oetting & T. M. Klapötke (1995). "Covalent Inorganic Azides". Angewandte Chemie International Edition in English. 34 (5): 511–520. doi:10.1002/anie.199505111.
  6. Committee on Prudent Practices for Handling, Storage, and Disposal of Chemicals in Laboratories, Board on Chemical Sciences and Technology, Commission on Physical Sciences, Mathematics, and Applications, National Research Council (1995). Prudent practices in the laboratory: handling and disposal of chemicals. Washington, D.C.: National Academy Press. ISBN 0-309-05229-7.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. Horst H. Jobelius, Hans-Dieter Scharff "Hydrazoic Acid and Azides" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a13_193
  8. Shriver and Atkins. Inorganic Chemistry (Fifth Edition). W. H. Freeman and Company, New York, pp 382.
  9. Treitler, Daniel S.; Leung, Simon (2 September 2022). "How Dangerous Is Too Dangerous? A Perspective on Azide Chemistry". The Journal of Organic Chemistry. 87 (17): 11293–11295. doi:10.1021/acs.joc.2c01402. ISSN 0022-3263. PMID 36052475. S2CID 252009657. Retrieved 18 September 2022.
  10. Mandler, Michael D.; Degnan, Andrew P.; Zhang, Shasha; Aulakh, Darpandeep; Georges, Ketleine; Sandhu, Bhupinder; Sarjeant, Amy; Zhu, Yeheng; Traeger, Sarah C.; Cheng, Peter T.; Ellsworth, Bruce A.; Regueiro-Ren, Alicia (28 January 2022). "Structural and Thermal Characterization of Halogenated Azidopyridines: Under-Reported Synthons for Medicinal Chemistry". Organic Letters. 24 (3): 799–803. doi:10.1021/acs.orglett.1c03201. PMID 34714083. S2CID 240154010.
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