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Azide

Azide is the anion with the formula N₃⁻. It is the conjugate base of hydrazoic acid. Azide is also a functional group in organic chemistry, RN₃^[1]. 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⁻.

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Inorganic azides

Azide forms both covalent and ionic compounds with metals. Sodium azide, NaN₃, is a salt that is widely used as the propellant in airbags. Covalent azides are numerous,^[2] an example being [Co(NH₃)₅N₃]Cl₂. A metal-organic azide is trimethylsilylazide, which is sometimes used as an anhydrous source of N₃⁻.

Azides in biochemistry

The azide anion is toxic, inhibiting the function of cytochrome c oxidase by binding irreversibly to the heme cofactor, in a process similar to that of cyanide. Azide salts are also used in studies of mutagenesis.

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 ^[3] ^[4].

In the azide alkyne Huisgen cycloaddition, organic azides react as 1,3-dipoles. Examples of organic azides are the chemical reagent phenyl azide and the antiviral drug zidovudine (AZT).

Another azide regular is tosyl azide here in reaction with norbornadiene in a nitrogen insertion reaction ^[5]:

Safety

Sodium azide is toxic (LD₅₀ oral (rats) = 27 mg/kg) and can be absorbed through the skin.
Heavy metal azides, such as lead azide are very explosive when heated or shaken.
Sodium azide decomposes explosively upon heating to above 275 °C.
Sodium azide 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.
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 by A. Hassner et al. ^[6].
Heavy-metal azides that are highly explosive under pressure or shock 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 (COX) inhibitors).
Solid iodoazide is explosive and should not be prepared in the absence of solvent.^[7].

References

^ Review: S. Bräse, C. Gil, K. Knepper, V. Zimmermann, Angew. Chem. 2005, 117, 5320-5374; Angew. Chem. Int. Ed. 2005, 44, 5188-5240.
^ Tornieporth-Oetting, I. C.; Klapoetke, T. M. "Covalent Inorganic Azides" Angewandte Chemie, International Edition in English (1995), volume 34, pages 511-20. AN 1995:483017
^ An efficient synthesis of γ-imino- and γ-amino-β-enamino esters Mangelinckx, S.; Van Vooren, P.; De Clerck, D.; Fülöp, F.; De Kimpea, N. Arkivoc JC-1560E 2005 Online Article
^ Reaction conditions: a) sodium azide 4 eq., acetone, 18 hrs reflux 92% chemical yield b) isopropyl amine, titanium tetrachloride, diethyl ether 14 hr reflux 83% yield. Azide 2 is formed in a nucleophilic aliphatic substitution reaction displacing chlorine in 1 by the azide anion. The ketone reacts with the amine to an imine which tautomerizes to the enamine in 4. In the next rearrangement reaction nitrogen is expulsed and a proton transferred to 6. The last step is another tautomerization with the formation of the enamine 7 as a mixture of cis and trans isomers
^ A Facile Synthesis of a Polyhydroxylated 2-Azabicyclo[3.2.1]octane Damon D. Reed and Stephen C. Bergmeier J. Org. Chem.; 2007; 72(3) pp 1024 - 1026; (Note) doi:10.1021/jo0619231
^ A. Hassner et al., Angew. Chem. Int. Ed. Engl., 25, 479 (1986), J. Org. Chem., 55, 2304 (1990).
^ L. Marinescu, J. Thinggaard, I. B. Thomsen, M. Bols, J. Org. Chem. 2003, 68, 9453 – 9455.

Categories: Functional groups | Azides

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