Nitrous acid
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| Names | |
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Preferred IUPAC name
Nitrous acid
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Systematic IUPAC name
Hydroxidooxidonitrogen
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| Identifiers | |
7782-77-6 
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| 3D model (Jmol) | Interactive image |
| 3DMet | B00022 |
| ChEBI |
CHEBI:25567
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| ChEMBL |
ChEMBL1161681
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| ChemSpider |
22936
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| ECHA InfoCard | 100.029.057 |
| EC Number | 231-963-7 |
| 983 | |
| KEGG |
C00088 
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| MeSH | Nitric+acid |
| PubChem | 24529 |
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| Properties | |
| HNO2 | |
| Molar mass | 47.013 g/mol |
| Appearance | Pale blue solution |
| Density | Approx. 1 g/ml |
| Melting point | Only known in solution |
| Acidity (pKa) | 3.398 |
| Hazards | |
| Flash point | Non-flammable |
| Related compounds | |
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Other anions
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Nitric acid |
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Other cations
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Sodium nitrite Potassium nitrite Ammonium nitrite |
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Related compounds
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Dinitrogen trioxide |
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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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 (what is ?) |
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| Infobox references | |
Nitrous acid (molecular formula HNO2) is a weak and monobasic acid known only in solution and in the form of nitrite salts.
Nitrous acid is used to make diazides from amines; this occurs by nucleophilic attack of the amine onto the nitrite, reprotonation by the surrounding solvent, and double-elimination of water. The diazide can then be liberated to give a carbene or carbenoid.
In the gas phase, the planar nitrous acid molecule can adopt both a cis and a trans form. The trans form predominates at room temperature, and IR measurements indicate it is more stable by around 2.3 kJ mol−1.
When cold, dilute solutions of nitrite ion, NO2− are carefully acidified, a light blue solution of nitrous acid is produced. Free nitrous acid is unstable and decomposes rapidly. It can be produced by dissolving dinitrogen trioxide in water according to the equation
In anything other than very dilute, cold solutions, nitrous acid rapidly decomposes into nitrogen dioxide, nitric oxide, and water:
Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous solution:
In warm or concentrated solutions, the overall reaction amounts to production of nitric acid, water, and nitric oxide:
Reduction of the acid gives different products, depending on the reducing agent:
With I− and Fe2+ ions, NO is formed:
With Sn2+ ions, N2O is formed:
With SO2 gas, NH2OH is formed:
With Zn in alkali solution, NH3 is formed:
With N2H5+, HN3, and subsequently, N2 gas is formed:
Oxidation by nitrous acid has a kinetic control over thermodynamic control, this is best illustrated that dilute nitrous acid is able to oxidize I− to I2, but dilute nitric acid cannot.
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