Angeli's salt
Chemical compound
From Wikipedia, the free encyclopedia
Angeli's salt, sodium trioxodinitrate, is the inorganic compound with the formula Na2[N2O3]. It contains nitrogen in an unusual reduced state. It is a colorless, water-soluble solid, hence a salt. In research, this salt is used as a source of the metastable nitroxyl (HNO), which is a signalling molecule in nature.[3] It is also known by the name sodium trioxodinitrate(II) monohydrate.
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| Other names
Sodium α-oxyhyponitrite | |
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| Properties | |
| N2Na2O3 | |
| Molar mass | 121.991 g·mol−1 |
| Appearance | white solid |
| Very soluble[1] | |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Preparation and properties
As first reported by Angelo Angeli in 1896, the salt is prepared by combining hydroxylamine and an organic nitrate, as a source of nitronium (NO+
2):[4][5]
- NH2OH + RONO2 + 2 NaOR′ → ROH + 2 R′OH + Na2N2O3
The structure of the hydrate has been confirmed by X-ray crystallography. The anion is planar. Starting from the ONN end, the bond distances are 1.35 Å (N–O), 1.26 Å (N–N), 1.31 Å (N–Otrans), and 1.32 Å (N–Ocis). The negative charge is on the oxygen atoms at opposite ends of the molecule. The angles are 112.9° (Osingle–N–N), 118.4° (N–N–Otrans), and 122.5° (N–N–Ocis). This means that the nitrogen–nitrogen bond is a double bond, and that the cis oxygen is slightly repelled by the single oxygen.[1]
Decomposition
Aqueous solutions of trioxodinitrate are unstable below pH 10. The rate of decomposition increases between pH 10 and 8, is essentially stable between pH 8 and 4, then accelerates again below pH 4.[6]: 3795
Between pH 8 and 4, the main decomposition mechanism is tautomerization of monoprotonated trioxodinitrate (O−)2N+=N(OH) to O2N−NH(O−), which decomposes reversibly to nitrite (NO−2) and nitroxyl (HNO).[6]: 3798 [7] The resulting nitroxyl dimerizes rapidly to hyponitrous acid (HON=NOH), which dehydrates to nitrous oxide.[6]: 3796
- HN2O−3 ⇌ HNO + NO−2
- 2HNO → H2N2O2 → H2O + N2O
Below pH 4, the main decomposition mechanism is tautomerization of diprotonated trioxodinitrate (O−)2N+=N+H(OH) to reach the unstable transition state (O−)(H2O+)N+=N(O−). This loses water to yield an excited state of trans-dinitrogen dioxide that, upon de-excitation, fragments to two nitric oxide molecules.[6]: 3799–3800
- H2N2O3 → H2O + 2NO
Reactions
Reaction of Angeli's salt with secondary amines in the presence of a proton source results in extrusion of N2 via isodiazenes as proposed intermediates.[8]